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The west Antarctic ice sheet is the world's only remaining marine ice sheet, an ice sheet anchored to bedrock below sea level and with margins that are floating. Other marine ice sheets existed in the Northern Hemisphere during the last glacial maximum but all disintegrated and melted away during the current warm period. Marine ice sheets are important because their existence and future behavior depend not only on atmospheric conditions and ice movement, but also on sea level changes. As the sea level rises, more of the ice at the edge of the sheet floats, and the forces that hold the ice sheet together are reduced, causing ice to flow more rapidly to the oceans. This positive feed-back loop (sea-level rise, leading to reduced forces holding the ice together, leading to increased ice flow into the ocean, leading to sea-level rise) could lead to rapid disintegration or collapse of a marine ice sheet. Scientists want to learn more about the west Antarctic marine ice sheet to understand how it operates and to determine if it is vulnerable to future disintegration or collapse, which would affect global sea level.
Scientists have long recognized that global sea levels were 5 to 6 meters higher during the last interglacial warm period, about 125,000 years ago, than they are today. It is also estimated that the west Antarctic ice sheet contains ice which, if it were incorporated into the oceans, would cause global sea levels to rise about 6 meters. It has been suggested that the West Antarctic ice sheet might be responsible for the higher sea level during the last interglacial period. Future global warming could result in the disappearance of the West Antarctic ice sheet and a substantial rise in global sea level.
The west Antarctic ice sheet is drained by surprisingly large, fast, river-like currents of ice flowing through the ice sheet. These ice streams, as they are called, are up to 100 kilometers wide and several hundred kilometers in length, moving at rates as fast as several hundred meters per year. Studies have shown that the ice streams are underlain by a water-saturated, unconsolidated sediment layer a few meters thick. This sediment acts to lubricate the ice streams so that they flow rapidly over their beds. The size and speed of the streams lead to the possibility of rapid collapse of the ice sheet.
The West Antarctic Ice Sheet Program is an interdisciplinary program to study this ice sheet, understand the processes important to its behavior, and develop models to predict its future behavior. Recent studies suggest a possible connection between the subglacial geology and the presence of the ice streams. In fact, measurements of ice thickness, magnetics, and gravity, made using a specially equipped aircraft, have shown that there is evidence for high heat flow and active volcanism near the ice streams. These elements could explain the high degree of subglacial melting and lubrication of the sediment under the ice streams. This work demonstrates the importance of subglacial geology for a complete understanding of the stability of the west Antarctic ice sheet, its ice streams, and its potential for raising global sea level.
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Facies are groupings of rocks based on similar features. The field trip will focus on making observations of the beach and defining facies. Rocks (and sediments) can be grouped by a suite of different characteristics, for example grain size, sedimentary structures, grain composition, grain rounding, etc. The goal of assigning rocks or sediments to facies is to provide a useful categorization scheme.
Turbidites provide a good summary of the ideas we have been talking about, e.g. facies and sedimentary structures related to flows. Turbidites are deposited from slurries of sediment and water in any standing body of water (lakes, oceans).
1) Turbidity flows start with slope failure in soft sediment. Slopes become oversteepened where sedimentation rates are very high, such at the mouths of rivers. Because flow speeds are very low in standing water, the sediment does not get washed downslope. Rather, it builds up until there is a subaqueous slope failure. Earthquakes can trigger these slides
2) Sediment and water mix creating a “fluid” that is denser than the surrounding water because of the entrained sediment. Thus, it flows downhill even if the slope is very low (1°).
3) The base of the flow is commonly erosional on steep slopes, so more sediment is entrained in the flow.
4) Enough sediment is entrained that erosion stops. Deposition begins as the slope gets shallower or the flow starts to slow down. Initially, the coarsest grains are deposited (remember the Hjulstrom diagram) and then finer grains, so the sediment is “graded”. However, the sediment is usually poorly sorted because the flow is a slurry of water and sediment so hydraulic sorting is reduced. (Facies = Bouma a)
5) Sediment concentration decreases with deposition, so one gets more hydraulic sorting. The flow is very fast so the sediment has upper plane bedding. (Facies = Bouma b)
6) As the flow slows more, grain size decreases and ripples start to form. Dunes are not usually found for two reasons: a) often only fine sand and finer grains are left in the flow by this point; and b) dunes do not have time to develop. (Facies = Bouma c)
7) Eventually, the flow slows to the point that bedload transport stops and deposition is mostly settling of silt and then clay. The progressive settling of coarser and then finer grains produces a faint lamination, but it is not as strong as the planar laminations in Bouma b. (Facies = Bouma d)
8) Mud settles out producing shale. This can look identical to background settling of clays brought into the lake/ocean as suspended sediment. (Facies = Bouma e)
Bouma divisions a-d can take hours or a day or so to be deposited. However, division e, which is usually the thinnest, commonly accumulates over months or longer (e.g. hundreds of years) depending on how frequent turbidites are in the area.
Watch these movies of turbidites in flumes:
And here are some photos of turbidites: http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Turbidites.html
Changes in Character Downslope - The parts of turbidites that are deposited change downslope and usually only a few of the subdivisions are preserved. In the most proximal (upslope) environments, divisions a and b are most common. In the more distal areas, all of the coarser sediment has already been deposited upstream, so divisions d and e are most common. Generally, there are also channels which fan out producing variations in rock types that change in space and through time.
For HW #3, you will draw a stratigraphic column of a turbidite and then describe the facies characteristic of different environments.
Turbidite Facies Models - Over the decades, sedimentologists have described and interpreted sedimentary rocks and defined generalized facies and facies associations that are characteristic of different depositional environments. These generalized facies and associations are called Facies Models. Each depositional environment or system has its own facies model. This is a VERY powerful tool for interpreting ancient environments.
Extra on Turbidites - Turbidite facies analysis and the resulting facies model led to the discovery of a new process. Sedimentologists had characterized turbidites all over the world. They all had the same flow characteristics consisting of a very strong erosive flow, deposition of a normally graded bed which was massive, followed by upper plane bedding, rippled finer sands, coarsely laminated silts, then shales. Comparisons with known flows showed that this sequence of deposits must come from a strong initial flow that slowed through time to still water. And this repeated again and again. The associated facies and the succession of different facies in these sequences suggested that the deposits had to be in deep water. For example, the fossils were all characteristic of deep water, shales were abundant and only settle from still water (shallow or deep), and they were sometimes associated with deep water storm deposits. Thus, the sedimentologists proposed slope failure and turbid currents flowing downslope and called them turbidity currents. A process like this had not been observed in modern depositional environments, so the idea was controversial. Many geologists did not believe that you could generate strong enough currents underwater to get those flow characteristics. Eventually in 1964, two geologists Heezen and Drake realized that an event in 1929 provided strong evidence for turbidity currents. In 1929, without satellites, under water telegraph cables were strung from Newfoundland to Europe. In November, about 30 cables broke in order from farthest north and shallowest to farther south and deeper water. At the time, people did not know why they broke, but Heezen and Drake suggested that a turbidity current was triggered by an earthquake and the cables broke as the turbidity current passed over them (they are strong flows!). Because they were continuously used for communication, the time each cable broke was very well known. Heezen and Drake calculated that the front of the flow traveled at 250 km/h (36,000 cm/s) when it first formed and then slowed to around 20 km/h (7000 cm/s) by the time the last cables broke 500 km from the source. This was a fast, strong flow and may be typical of turbidites. These speeds are above the upper end of the Hjulstrom diagram and are very erosive. It is only after the turbidite slows down even more that you get deposition. The characteristics of the flow seen by the breaking cables fit the flow characteristics proposed by the sedimentologists, and now turbidity currents and the facies model developed for turbidites are widely accepted and often treated as an ideal example of rocks that closely reflect flow characteristics. Turbidites and their interpretation are almost an ideal example of a good Facies Model.
Extra on Dense Sediment Flows
Sometimes with slope failures on land or under water, much more sediment can be put into motion than the flow would normally erode. Depending on the amount of water mixed with the sediment, the flow characteristics are different. When abundant water is present, the sediment can form a thick slurry with a higher density than sediment-free water, commonly leading to a higher Re and more turbulent flow (Re=u*l*r/µ). Also, collisions between grains become extremely important. Both of these tend to keep the sediment moving. Grain-to-grain collisions also have an important effect on grain sorting. The collisions tend to make sorting much less efficient and the sediment that gets deposited tends to consist of whichever grains make it to the base of the flow and are not kicked back up again. Usually, the largest grains are part of this first deposit because they weigh more, but small grains are also present. As the amount of sediment decreases, the flow becomes more like typical water flows. Turbidites are subaqueous flows that start out with a very high sediment load and decrease in time to more normal flows. They have characteristic sedimentary structures associated with them that reflect these changes.
If there is very little water associated with a clay-rich sediment flow, the flow can be very viscous due to the charge attraction among clay particles. The high viscosity makes the flow laminar (Re=u*l*r/µ). Debris flows with lots of cohesive mud are like this. In laminar flows, there is no mixing of the water or grains (or ice) and there is no sorting of grain sizes. Thus, the sediment remains mixed up with large grains, sometimes boulders, “floating” in mud. They flow down hill pulled by gravity until the flow seizes up and stops. This can be due to too low a slope or loss of water. Underwater debris flows can also be diluted by water that gets incorporated at the edges of the flow and become less viscous and more turbulent.
There also are dry sediment flows in which air is present between grains. For example, rock avalanches and some pyroclastic flows from volcanoes lack water. For these to move significant distances, large amounts of energy from either gravity or explosions are necessary to keep the sediment in motion.
Here is a 6 minute summary of my turbidite lecture: http://www.youtube.com/watch?v=G05juwK2OTI
A nice, hour long lecture on turbidites in the Monterrey Bay canyon, CA, can be found at: http://online.wr.usgs.gov/calendar/2010/jun10.html The actual lecture starts about 5 minutes into the video.
Stratigraphy and Time:
Stratigraphy is the study of sedimentary rocks in space and time.
Stratigraphy is the basis of interpreting what happened in the past. We use facies to interpret depositional environments from the rocks. Changes in facies both vertically and horizontally allow us to interpret changes in ancient landscapes and processes.
Example: Beach Facies. Beach environments grade laterally into each other. The off shore areas grade into the swash zone of the foreshore. The foreshore grades into the berm and backshore (if present). Eolian (wind) dunes or erosional cliffs can be present landward of the beach. Rock facies similarly grade into each other because they were deposited in different depositional environments. If the depositional environments stay in exactly the same place through time, a stratigraphic column in each place would consist of a uniform facies, but each stratigraphic column would have a different style of rock (facies). However, depositional environments tend to migrate back and forth as sea level rises or falls, basins fill in with sediment, etc. Thus, facies in stratigraphic columns tend to change upward. They also vary laterally. See figure 19.8 on pg. 308 of Nichols.
Changes in sea level and depositional environment lead to variations in stratigraphic columns both laterally and vertically. If you compare different stratigraphic columns, there are several ways you might "correlate" them. If you correlate different rock types, e.g. lithostratigraphy, you are marking regions with similar characteristics, but the sediments in each unit were not necessarily deposited at the same time. In contrast, if you correlate rocks that were deposited at the same time, e.g. chronostratigraphy, each unit often consists of more than one facies. This is obvious when you look at the distribution of depositional environments now. Different areas are accumulating different types of sediment at the same time.
Lithostratigraphic correlations are easy because you can directly observe rock type. These correlations are very useful for studies of reservoir properties, where one might want to identify a porous sand that acts as a water or hydrocarbon reservoir. However, these correlations do not help you interpret ancient depositional environments because they do not represent an ancient landscape. Chronostratigraphic correlations tell you the most about depositional environments and their distribution through time, but they can be VERY difficult because you have to have a time marker that tells you what deposits were synchronous. Sometimes volcanic ash beds or other depositional events allow you to directly observe which rocks were deposited at the same time, but these events are rare. Often, chronostratigraphic correlations require detailed facies analyses and an understanding of how depositional environments change through time.
Walther’s Law is key for understanding the differences between lithostratigraphy and chronostratigraphy. Walther’s Law states that environments that are adjacent to each other are represented as vertical successions of facies in the rock record if there is no break in sedimentation (no unconformity). If sea level is rising relative to the shore line, the different depositional environments are migrating inland. This leads to different facies accumulating progressively inland as well. The most landward deposits are river deposits and alluvial plain deposits, followed by marsh and then marine deposits. Vertically, you see the facies representing those depositional environments in the same order. At any given time, rocks are being deposited in all of the different environments.
Chronostratigraphy - Chronostratigraphy enhances the interpretation of the stratigraphic record in terms of Earth history. Even when one has a detailed map of the distribution of depositional environments, it is difficult to say exactly how to correlate section in terms of time. In real rocks, there are a number of tools that you can use to get correlations of various accuracy. These include: fossils (biostratigraphy); magnetic properties (magnetostratigraphy); absolute ages of interbedded volcanic ash beds and basalt flows; some chemical properties such as elemental isotope ratios in carbonates; geological instantaneous depositional events such as huge storms, meteorite impacts, etc.; and unconformities due to sea level falls and the geometry of sedimentary deposits (sequence stratigraphy). We will get back to all of these in more detail throughout the quarter, particularly near the end.
Distribution of Rock and Time - One might think that sections can be correlated based on assuming that the same amount of sediment gets deposited in all places in the same amount of time. This is a BAD assumption, although many researchers are forced to use it. It is important to understand that the preserved rock does not represent all of time. What I mean is that time is not evenly represented by rock thickness. For example, with turbidites, the sandstones may have been deposited in a couple hours to a day at most, whereas the shales (Bouma E) represent 100’s to 1000’s of years of fine grains settling out. Thus, most of the "time" is represented in the much thinner shales. In addition, there is erosion at the base of some of the turbidites. Thus, there is a significant amount of time that is only represented by an erosional surface which produces a gap in the rock record. Generally, sedimentation is thought of as a continuous processes. This is NOT true. Sedimentation is episodic and there are unconformities in the stratigraphic record spanning all time ranges from minutes to millions of years. Gaps of minutes might occur in a river if there is a burst of strong flow that is erosive rather than depositional. Gaps of hours occur at low tides when the intertidal zone is exposed. Gaps of years to thousands of years can occur in land environments where there is no source of sediment or the topography is too high to collect sediment. Gaps of millions of years also occur in terrestrial environments, especially if there is erosion. The longer time gaps usually represent regional changes in deposition and can be very useful for correlating rocks chronostratigraphically. Also, different depositional environments accumulate sediment at different rates: thickness does not equal time!
Here is a short video summary of the distribution of time in rocks: http://www.youtube.com/watch?v=9ch-6HiOAW4
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Glaciers can form anywhere that snow and ice persist year-round. They grow slowly over thousands of years, as snowflakes from one snowfall are compacted and eventually turn to ice under the weight of subsequent snowfalls.
Although ice is generally perceived to be rigid, glaciers are never stationary. Because the weight of their accumulated snow and ice is so great, gravity actually forces the glacier to spread outward downslope. Glacial movement is responsible for the formation of ice shelves, which extend from the land to the sea surface. It also plays an important role in shaping landscapes. For example, in glaciated terrain, river valleys are eroded by glacial motion, becoming wider and deeper from the flow of glaciers through them.
Glacial ice serves many functions that affect the entire Earth system, including the regulation of global moisture, temperature, and ocean salinity. Because glaciers reflect sunlight, the evaporation rate and surface temperature where they exist are low. As glaciers melt, however, more heat is absorbed by water and land, causing more surface water to evaporate into the atmosphere and surface temperatures to rise.
Explaining the relationship between glacial ice and the third factor, ocean salinity, is more complicated. Sea ice is ice that extends seaward from continental ice sheets each winter and melts each summer. As sea ice extends, pure water is crystallized out of the ocean and salt is left behind, increasing the salinity of the ocean water in the immediate vicinity and making it denser than it would otherwise be. The temperature and density gradient that exists in the ocean waters from the equator to the poles drives ocean circulation. Should icebergs and glaciers melt and spill large quantities of fresh water into the ocean, circulation patterns could change, which could dramatically change global climate.
Global warming's effect on ice could have potentially serious consequences for human societies. In mountainous regions, accelerated glacial melting could produce flooding, followed by a drought that would affect hundreds of millions of people who rely on stable glaciers to supply their water. In polar regions, significant melting of the ice sheets would put the populations of mammals and birds that inhabit the ice sheets at risk. A visual comparison of present-day coastline positions with those from the peak glacial advance 20,000 years ago begins to show how much water is contained in glaciers and the impact accelerated global warming could have, raising the sea level and causing flooding of continental coastlines.
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In the previous page, we saw how atoms could achieve a complete shell of electrons by losing or gaining one or more electrons, to form ions. There is another way atoms can satisfy the octet rule: they can share electrons. For example, two hydrogen atoms can share their electrons, as shown below. Because each of the shared electrons then "belongs" to both atoms, both atoms then a fulled shell, with two electrons. The pair of shared electrons is symbolised by the heavy line between the atoms.
In terms of charge-charge interactions, what happens is that the shared electrons are located between the two bonded atoms. The force attracting them to both nuclei is stronger than the repulsive force between nuclei.
The methane (CH4) molecule illustrates a more complex example. Each of the 4 electrons in the outermost ("valence") shell of carbon is shared with one hydrogen. In turn, each of the hydrogens also shares one electron with carbon. Overall, carbon "owns" 10 electrons - satisfying the octet rule - and each hydrogen has 2. This is shown here:
When a molecule of methane is studied experimentally, it is found that the four hydrogens spread out evenly around the carbon atom, leading to the three-dimensional structure shown here:
As you would expect given that the electrons are shared, if we plot the region where the electrons sit, this is not localised on one atom, as it was for the ionic compounds, but is all over the molecule:
As we have seen above, atoms can share electrons with others to form chemical bonds. This can also take place between two carbon atoms, to form a molecule such as ethane (C2H6):
When we add two more carbon atoms and 4 more hydrogens, to make butane (C4H10), an interesting situation arises: There are two different ways of bonding the carbons together, to form two different molecules, or isomers!!! These are shown below. For one of the isomers, the first carbon is bonded to three hydrogens, and to the second carbon, which is itself bonded to another two hydrogens and to the third carbon, which is itself bonded to the fourth carbon. In the other isomer, one of the carbons forms a bond to all three carbon atoms:
Larger compounds can also be formed, and they will have even more isomers! For example, this compound with 8 carbons is called isooctane, and is one of the main components of petrol for cars:
Can you check that the formula for this compound is C8H18? Can you sketch another compound with the same formula?
Because covalent bonds can be formed in many different ways, it is possible to write down, and to make, many different molecules. Many of these are natural compounds, made by living animals or plants within their cells. This example shows one such molecule, cholesterol (C27H46O), which can contribute to heart disease in people whose diet is too rich in fats:
Note that in this structure, two neighbouring carbon atoms appear to form only three bonds, which would go against the octet rule. In fact, these atoms bond by sharing two electrons each (a total of four electrons). In this way, they complete their electron shell like the others. This situation is referred to as a double bond, and is shown in the pop-up window as a thicker stick between those two atoms (Can you find this bond? Check that all other carbon atoms do form four bonds).
Other compounds are synthetic, they are made by chemists. Chemists can also make the natural compounds, starting from only simple things like methane and water. The "natural" molecules made in this way are identical to the "real" natural compounds! Other synthetic molecules do not exist in nature. They can have desirable properties, for example, many medicines are made in this way. An example of a "small" medicine molecule is aspirin, C9H8O4, shown below. In this molecule, two bonds between carbon and oxygen are double bonds, and are shown as thicker sticks in the model.
The covalent bonds between atoms in a given molecule are very strong, as strong as ionic bonds. However, unlike ionic bonds, there is a limit to the number of covalent bonds to other atoms that a given atom can form. For example, carbon can make four bonds - not more. Oxygen can form two bonds. As a result, once each atom has made all the bonds it can make, as in all the molecules shown above, the atoms can no longer interact with other ones. For this reason, two covalent molecules barely stick together. Light molecules are therefore gases, such as methane or ethane, above, hydrogen, H2, nitrogen, N2 (the main component of the air we breathe, etc. Heavier molecules, such as e.g. the isooctane molecule, are liquids at room temperature, and others still, such as cholesterol, are solids.
As well as the solids just referred to, formed by piling lots of covalent molecules together, and relying on their slight "stickiness" to hold the solid together, one can also form solids entirely bound together by covalent bonds. An excellent example is diamond, which is pure carbon, with each carbon atom bonding to four others, to form a huge "molecule" containing many millions of millions of atoms. This shows a part of a diamond molecule:
In diamond, all the carbon atoms share one electron with each of their four neighbouring carbon atoms. There is another form in which pure carbon can be formed: graphite. This is the main component of the "lead" in pencils. Here, instead of each carbon having four neighbours, it only has three. Each carbon shares one electron with two of its neighbours, and 2 electrons with the third neighbour. In this way, one C-C bond out of three is a double bond. The atoms all bond together in planes, and the planes stack on top of each other as shown:
In graphite, the C-C bonds in the planes are very strong, but the force between the different planes is quite weak, and they can slip over one another. This explains the "soft" feel of graphite, and the fact that it is used as a lubricant, for example in motor oil.
In solids like diamond and graphite, the different atoms all bond to one another to form one very large molecule. The atoms are bonded to each other in all directions in diamond, and in two directions (within the planes) in graphite, with no bonding in the other direction. Some important covalent molecules involve atoms bonding to each other repeatedly along just one direction, with no bonds in the others. These are called polymers, and one simple example if polyethene (also called polythene, or polyethylene). The structure of polythene is shown here (the dangling bonds at each end indicate how the bonding should really continue for thousands of atoms on each side):
Polythene is what most plastic bags are made of. Other polymers include molecules such as nylon, teflon (these, like polythene, are man-made), or cellulose (the stuff that makes wood hard), a biological polymer.
Covalent bonds involve sharing electrons between atoms. The shared electrons "belong" to both atoms in the bond. Each atom forms the right number of bonds, such that they have filled shells. There is lots of flexibility in terms of which atom bonds to which other ones. This means that many isomeric molecules can be formed, and Nature as well as chemists are skilled at designing and making new molecules with desirable properties. In most cases, only a small number of atoms are bonded together to make a molecule, and there is no bonding between atoms in one molecule and other atoms in other molecules. This means that molecules are only very slightly "sticky" between themselves, and covalent compounds are either gases, or liquids, or sometimes solids. In some cases, bonding occurs to form large molecules with thousands or millions of atoms, and these can be solids.
Click Here to return to the main structure and bonding webpage.
Click Here to return to the previous page (Ionic Bonding).
Click Here to go on to the next page (Other Types of Bonding).
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Apiculture Factsheet #111
Bee Behaviour During Foraging
Insect pollinators, including honeybees, evolved together with
flowering plants for millions of years. Plants developed floral
parts with increasingly specialized features to attract visiting
insects who, in turn, would distribute pollen grains and optimize
the plant’s reproductive capabilities. Simultaneously, these
wasp-like insects underwent physiological adaptations to take
advantage of the nutritional benefits offered by flowering plants.
Physical adaptations were augmented by changes in foraging and
nesting behaviour that proved mutually beneficial to flora and fauna.
Some of the physical adaptations of the honeybee include:
Each compound eye is spherical in shape and comprised of
some 6,300 cone-shaped facets or eyes. Bees can easily distinguish
high contrast shapes and patterns. The visual spectrum of the
honeybee has shifted towards shorter wavelengths, enabling it to
detect ultra-violet, while red, with its longer wavelength, appears as
a dull grey. Bees are particularly sensitive to blue, yellow and
blue-green colours even though bees can detect light intensity only
1/20 as well as humans.
The sensitivity to ultra-violet and polarized light enables the
honeybee to observe the sun under cloudy conditions. Its
spherical-shaped eyes allow the honeybee to measure angles
accurately between the relative positions of the sun, the food
source and the nest. These field observations are then interpreted
and communicated to other bees inside the hive through a 'dance'.
Scout bees can direct their fellow worker bees to the location of a
food source, negating the need for each individual to search. Unlike
most other insect pollinators, the adaptation of communication has
enabled honeybees to utilize floral resources of a large area. As a
result, honeybee colonies can attain a biomass at the height of
season far greater than any other pollinating insect. Bumble bees
and all solitary bees do not communicate and hence, each individual
relies on its own foraging success. The foraging range of these
pollinators is limited to a comparatively small area.
The honeybee’s olfactory sense is estimated to be 40 times
better than man’s and plays a critical role in locating food sources
and communication in and outside the nest. Some 5,000 - 6,000 olfactory
detectors are on each antenna.
Taste is detected through the mouthparts and forelegs.
Bees have a limited range of taste and many substances detected by
humans are tasteless to bees. Within the narrow range of substances
they can taste, bees display high sensitivity. Sugar solutions as
low as 2% can be detected although for foraging purposes, bees are
not interested unless the sugar concentration is 30% or more.
Sense of Time
Bees are known to be time sensitive.
Communication inside the nest expressing the location of a site
relative to the sun has been observed over time, even when the sun's
position progressed below the horizon. Awareness of time is
important in determining the time of nectar secretion and the
commencement of foraging.
Economics of Foraging
Foraging requires energy and the
honeybee's evaluation as to where, what and how long to forage is
all related to the economics of energy consumption and the net gain
of food to the colony. For example, foraging bees may not access a
high quality food source because its distance requires energy
expenditure exceeding the energy value of the food source. Generally
bees fly only as far as necessary to secure an acceptable food
source from which there is a net-gain. Factors that influence
foraging behaviour and determine profitability:
- weather e.g. wind,
temperature, and sunlight,
- distance of the food source from the
hive (including differences in elevation),
- food quality
(concentration of sugar, protein content of the pollen),
- quantity of nectar or pollen.
Bees are known to fly as far as 12 km (8 miles),
but usually foraging is limited to food sources within 3 km.
Approximately 75% of the bees from a colony forage within one
kilometre while young field bees only fly within the first few
Foraging bees tend to limit their visits
to a single species of plant during each trip. This behavioural
adaptation is critically important for plants since it assures the
transfer of pollen from one plant to another plant of the same
species. In commercial crops, foraging constancy is essential for
optimizing seed set and fruit development.
Individual foragers will acquire a sample through scouting in the
morning and tend to fly to the same source as long as it remains
profitable. Bees will shift to another plant species if the nectar
or pollen fails. Even then, memory will cause these foragers to
return several times and re-check. In areas with great floral
diversity and small plantings, a higher percentage of foraging bees
will visit different kinds of plants during the same trip. This
would account for the mixed pollen loads of returning bees.
Speed of Work
Bees visit up to about 40 flowers per minute
depending on floral type, nectar availability and weather
conditions. Floral visitation rate by honeybees of some important
- apricots 10 sec
- apples 68 sec
- cherries 82 sec
- raspberry 116 sec
- black currant 134 sec
The longer the time period, the greater the nectar availability. It
takes twice as much time to collect a load of nectar compared with a
load of pollen.
Honeybees are foraging generalists and capable of utilizing a
wide range of floral sources. On the other hand, many insect
pollinators are specialists and only visit certain floral sources.
Foraging specialization by the insect coincides with higher
efficiency of utilizing the food source, which means
improved pollination for the plant. For example, bumblebees evolved in bog
environments of temperate zones where the principal nectar and
pollen sources are characterized by long colliery floral tubes and
bloom during cool and wet spring conditions. Furthermore, pollens of
these sources are generally moist and sticky. Bumble bees have
developed a long proboscis (tongue), are highly pubescent (hairy) to
forage under inclement weather, and are capable of
"buzzing" while on the flower to cause the release of
pollens. As such, bumblebees have proven highly efficient in crops
such as blueberry, cranberry and blackberry.
- Below 8 C - no foraging
- 8 C - 16 C -
- 16 C - 32 C - optimal conditions
- Above 32 C -
reduction in foraging, increase in water collection.
Speed of Flight
- Loaded bee - approx. 25 km/h (15 mph) on
- Empty bee leaving hive - 20 km/h (12.5 mph) on average.
Increased wind reduces foraging activity. At a wind speed of 40
km/h (25 mph) foraging will stop.
Number of Trips per Day
The number of trips will depend on
various conditions including weather, forage availability, strength
of colony, etc. In general, 5-15 trips are made while a water
collector may make as many as 100 trips per day.
The nectar flow is the period when bees forage and collect nectar
to sustain the colony. The nectar flow is the period when there is
such an abundance in nectar production that the bees gather a surplus
beyond the immediate needs of the colony, which is converted to
honey and stored in the combs. To optimize the nectar resource of an
area, the beekeeper must be thoroughly familiar with the vegetation,
its condition and blooming time.
Locating the Nectar
Nectar and pollen sources are located by any foraging bee and not
limited to scout bees. After finding a valuable food source, the bee
will return to the colony and communicate its finding to other bees
through a 'bee dance'. Carl von Frisch first described this form of
communication, expressing direction, distance and food quality.
The bee's specialized tongue, called the proboscis, is a suction
pump. The nectar passes through the esophagus into the nectar sac
where a valve prevents the nectar from passing into the digestive
stomach or ventriculus. The nectar sac is essentially a widening of
the esophagus and functions as a collecting chamber of liquid foods
during transportation. The weight of a full nectar sac may be as
much as 90% of the body weight of the bee.
During the return trip to the hive, saliva is added to the nectar
which contains the enzyme invertase. Invertase reduces complex
sugars into simple sugars, which is part of the conversion from
nectar into honey. Should the bee require more energy for the flight
home, the valve between the nectar sac and the ventriculus will open
allowing nectar to pass into the digestive stomach.
A field bee carrying only nectar will fly with the rear legs wide
Handling Nectar on Return to the Hive
After return to the hive, the forager passes the nectar on to
'house' bees. She opens her mandibles with her proboscis retracted,
and a drop of liquid appears at the base of the glossa while the
house bee extends her proboscis fully, and sucks up the drop. The
speed of food transmission and processing is determined by various
factors, including temperature, the age of the bees, colony
strength, its food reserves and the total colony intake of nectar
During a strong nectar flow the partly ripened honey is stored in
the cells of the comb immediately, or after only a few transfers
from bee to bee. During a moderate or weak flow the food is passed
on to and by many bees before it is stored. The greater the number
of bees in the chain, the richer the ripe honey will be in their
secretions and hence in enzymes.
Partially processed nectar or raw honey contains too much water.
Water is removed through evaporation during the ripening process,
which involves two phases.
A bee, actively involved in processing nectar, pumps out the
contents of her nectar sac into a flat drop on the underside of the
proboscis which she then draws up again. This back-and-forth action
is repeated rapidly for 15-20 minutes. The liquid is thereby exposed
to the warm air of the hive, causing evaporation. In this way, the
bees produce half-ripened honey containing about 50-60% (maximum
70%) of dry substance.
The second, passive phase of honey ripening involves the deposit
of half-ripened honey in small droplets on the cell walls, or in a
thin film on the cell floor. As a rule, 1/4 to 1/3 of the cell is
filled; but during a strong flow, or if there is lack of space, 1/2
or 3/4 of each cell is filled straight away. Normally, when the
honey is nearly ripe, the bees move it again, and the cells are then
filled to 3/4 of their capacity. The final ripening takes 1 - 3
days, depending on the water content when the honey is first put
into the cells, the level to which the cells are filled, the amount
of air movement achieved, and temperature and relative humidity.
Under good conditions the % of water in the honey will be reduced to
below 20% in about 4 days.
The rate of evaporation from cells 1/4 full is three times that
of cells filled 3/4 full. When adequate comb space is available few
cells are more than half full. As moisture is evaporated, bees fill
cells, leaving empty cells to receive more green nectar. It is
important to have adequate empty comb space during the nectar flow
to prevent crowding.
Pollen Collecting and Storage
Pollen is dislodged from the anther of the flower and adheres to
the branched hairs of the bee. The tongue and mandibles (jaws) are
often used to lick and bite the anther. Pollen becomes stuck to the
mouthparts and is moistened. While the bee is resting or hovering in
the air she removes the pollen from her body and transfers it to the
corbicula (pollen basket) of her rear legs.
The process involves all of the bee's three pairs of legs. The
wet pollen is removed from the mouthparts, head and antenna by the
forelegs. Small amounts of nectar are used to moisten the pollen
mixture. The second pair of legs (mid legs) comb pollen from the
underside of the thorax and receive it from the forelegs. The inside
of the basi-tarsi of the rear legs contain combs which remove the
pollen from the brushes of the mid legs. By rapidly rubbing the hind
legs, pollen is gradually moved up to the opening between the basi-tarsus
and tibia of the rear leg. The rake of the opposite leg will then
force the pollen into the corbicula or "pollen basket". A
pollen load contains up to 10% nectar, which is necessary for
In the hive, pollen is removed from the rear legs by a spike on
the mid legs and is placed in cells. Often the head is used to pack
the pollen in cells. Honey is added to maintain pollen quality. This
final product is called bee bread.
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On this day in 1896, Utah became the 45th state, after numerous attempts over nearly 50 years to achieve statehood.
Between 1847 and 1868, about 60,000 Mormon emigrants traveled across the country, most on foot in handcart companies.
When the initial company arrived in what is now the Salt Lake Valley, their leader, Brigham Young, declared, "This is the place." The area they began to settle was soon claimed as U.S. territory as part of the Treaty of Guadalupe Hidalgo, which ended the Mexican-American War.
The Mormons, or members of The Church of Jesus Christ of Latter-day Saints, had traveled westward in search of religious and political freedom, and many felt statehood was the path to ensure those freedoms.
The steps for creating a new state are outlined in Article IV, Section 3 of the Constitution.
In 1849 those living in the Utah Territory petitioned to become part of the Union as the state of Deseret. Congress denied their request, primarily because the population was too small and the boundaries were too large (it covered not only present-day Utah but most of Nevada and Arizona and parts of several other western states). In 1850, Congress created the Utah Territory, which was smaller than the proposed Deseret.
Statehood petitions were also submitted in 1856, 1862, 1867, 1872, and 1882. But the Mormons, the majority residents of the territory, faced several challenges.
Most significant was lawmakers' and others' hostility toward the church's practice of polygamy, which the Republican Party had denounced as one of the "twin relics of barbarism" along with slavery. (The church opposed slavery.)
Polygamy was forbidden by a series of federal laws. In addition, a Supreme Court decision in 1879, Reynolds v. U.S., ruled against a Mormon husband who called plural marriage his religious duty as protected by the First Amendment. Chief Justice Morrison R. Waite wrote:
“Can a man excuse his practices…because of his religious belief? To permit this would be to make the professed doctrines of religious belief superior to the law of the land.”
In addition, the politics of the territory, which included a Liberal Party made of mostly non-Mormon voters and a People's Party of mostly Mormon voters, was not clearly aligned with national politics. Also, many non-Mormons disliked Mormons' dominance in local politics.
The issue of partisan politics was addressed as local leaders encouraged voters to divide along national party lines.
As for the issue of polygamy, in 1890 church president Wilford Woodruff issued a statement officially disavowing the practice of plural marriage by church members.
The statement finally cleared the way for Utah's statehood. The enabling act for admission was passed in 1894 and required that the state constitution ban polygamy.
On January 4, 1896, President Grover Cleveland proclaimed Utah a state of the Union.
Holly Munson is assistant editor of Constitution Daily and programs coordinator at the National Constitution Center.
Recent Constitution Daily Stories
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For years scientists at the University of Texas have been planning an expedition to drill directly into Chicxulub crater, what many believe is "ground zero" for the mass extinction event 66 million years ago. Now, we are weeks away from drilling to determine what lies beneath the surface of an asteroid impact crater large enough to drive dinosaurs and most life on the planet to extinction.
Scientists will analyze the chemistry, mineralogy, biology and physical mechanics of the rock within this massive crater to help understand how life came back from the extinction event.
As a matter of scale, the asteroid that formed Chicxulub crater was at least 6 miles in diameter and left a crater on Earth more than 110 miles wide. It is estimated that this impact killed more than 75% of all species currently living on Earth.
In Search Of The Extinction Event That Killed The Dinosaurs
The Cretaceous-Paleogene extinction event, also known as K-Pg and formerly known as the K-T extinction, was one of the largest mass extinction events in our Earth's history. Plants and animals largely died from this impact, including all non-avian dinosaurs. Those species that did survive were forced to rapidly adapt to a much different environment where food, prey, habitats, ecological niches, were all vastly different. This adaptation led to many species we see today.
How are we sure that the dinosaur extinction is a result of an asteroid impact? This was a fundamental question in geology and paleontology for quite some time. With growing evidence, scientists are able to link the geologic time of extinction to worldwide changes in the sedimentological record. For instance, the clays seen around the world during the K-Pg event show very high iridium levels, a substance that is very rare on Earth's Crust and yet abundant on asteroid. This coincides with a drastic change in the fossil record, where paleontologists no longer see evidence of dinosaur fossils and an abrupt change in the palynology around the globe.
This abrupt change in the fossil record and unusually high iridium levels led to the conclusion that an impact was likely the cause of dinosaur extinction. To see an extinction of this magnitude, however, requires a large impact of which are limited examples on the Earth's modern crust. The Chicxulub crater has all the telltale signs of a major impact. The crater contains high iridium, shocked quartz (a result of very high pressures on quartz minerals), and of course a telltale impact crater seen today. Age dating of the Chicxulub crater put it at 66 million years ago, a perfect match to the mass extinction event. The coincident impact and extinction event have led the scientific community to a majority consensus on the causation of the K-Pg extinction event that killed the dinosaurs.
What The Chicxulub Crater Can Tell Us About Life
A team of scientists will begin shortly on the International Ocean Discovery Program (IODP) plan to drill into the center of the Chicxulub crater. Scientists will core rock to be analyzed through the interval associated with the asteroid impact.
The impact displaced approximately 48,000 cubic miles of sediment, which is enough sediment to fill almost 17 Lake Superiors. This displacement of sediment caused earthquakes, formed tsunamis and transformed Earth's landscape.
During the expedition, the team will core 1500 m into the crater to collect rock samples for further analysis. They will attempt to answer some key questions about the impact and how life responded afterward.
As the scientists drill deeper and deeper into the Earth they will discover changes in rock indicative of the impact. Fossils and signs of life will become less and less abundant and less diverse as they approach 800m below the surface. Below 800m is the impact layer that represents the massive deposition of sediment immediately following the impact. Finally, below this will be the peak ring, with fractured granites that were blasted from the basement rock. This peak ring interval will provide a treasure trove of information on the impact.
"You can assume that at ground zero of this impact we are dealing with a sterile ocean, and over time life renewed itself. We might learn something for the future," professor Sean Gulick from the University of Texas told CNN.
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The origin of the ice age has greatly perplexed uniformitarian scientists. Much cooler summers and copious snowfall are required, but they are inversely related, since cooler air is drier. It is unlikely cooler temperatures could induce a change in atmospheric circulation that would provide the needed moisture. As a result, well over 60 theories have been proposed. Charlesworth states: 1
"Pleistocene phenomena have produced an absolute riot of theories ranging 'from the remotely possible to the mutually contradictory and the palpably inadequate.'"
A uniformitarian ice age seems meteorologically impossible. The necessary temperature drop in Northern Canada has been established by a sophisticated energy balance model over a snow cover. Summers must be 10 degrees to 12 degrees C cooler than today, even with twice the normal winter snowfall. 2
The Milankovitch mechanism, or the old astronomical theory, has recently been proposed as the solution to the problem. Computer climate simulations have shown that it could initiate an ice age, or at least glacial/interglacial fluctuations. However, an in-depth examination does not support this. The astronomical theory is based on small changes in solar radiation, caused by periodic shifts in the earth's orbital geometry. It had been assumed too weak to cause ice ages by meteorologists, until the oscillations were "statistically" correlated with oxygen isotope fluctuations in deep-sea cores. The latter cycles are believed related mostly to glacial ice volume, and partially to ocean paleotemperature, although the exact relationship has been controversial. The predominant period from cores was correlated to the 100,000-year period of the earth's eccentricity, which changes the solar radiation at most 0.17% 3 This is an infinitesimal effect. Many other serious problems plague the astronomical theory. 4, 5 Although models can test causal hypotheses, Bryson says they ". . . are not sufficiently advanced, nor is our knowledge of the required inputs, to allow for climatic reconstruction. . . ." 6
The climate change following the Genesis Flood provides a likely catastrophic mechanism for an ice age. The Flood was a tremendous tectonic and volcanic event. Large amounts of volcanic aerosols would remain in the atmosphere following the Flood, generating a large temperature drop over land by reflecting much solar radiation back to space. Volcanic aerosols would likely be replenished in the atmosphere for hundreds of years following the Flood, due to high post-Flood volcanism, which is indicated in Pleistocene sediments. 7 The moisture would be provided by strong evaporation from a much warmer ocean, following the Flood. The warm ocean is a consequence of a warmer pre-Flood climate and the release of hot subterranean water during the eruption of "all the fountains of the great deep" (Genesis 7:11). The added quantity of water must have been large to cover all the pre-Flood mountains, which were lower than today. Evaporation over the ocean is proportional to how cool, dry, and unstable the air is, and how fast the wind blows. 8 Indirectly, it is proportional to sea surface temperature. A 10 degree C air-sea temperature difference, with a relative humidity of 50%, will evaporate seven times more water at a sea surface temperature of 30 degrees C than at 0 degrees C. Thus, the areas of greatest evaporation would be at higher latitudes and off the east coast of Northern Hemisphere continents. Focusing on northeast North America, the combination of cool land and warm ocean would cause the high level winds and a main storm track to be parallel to the east coast, by the thermal wind equation. 9 Storm after storm would develop near the eastern shoreline, similar to modern-day Northeasters, over the continent. Once a snow cover is established, more solar radiation is reflected back to space, reinforcing the cooling over land, and compensating the volcanic lulls.
The ice sheet will grow as long as the large supply of moisture is available, which depends upon the warmth of the ocean. Thus, the time to reach maximum ice volume will depend upon the cooling time of the ocean. This can be found from the heat balance equation for the ocean, with reasonable assumptions of post-Flood climatology and initial and final average ocean temperatures. However, the heat lost from the ocean would be added to the atmosphere, which would slow the oceanic cooling with cool summers and warm winters. The time to reach maximum ice volume must also consider the heat balance of the post-Flood atmosphere, which would strongly depend upon the severity of volcanic activity. Considering ranges of volcanism and the possible variations in the terms of the balance equations, the time for glacial maximum ranges from 250 to 1300 years. 10
The average ice depth at glacial maximum is proportional to the total evaporation from the warm ocean at mid and high latitudes, and the transport of moisture from lower latitudes. Since most snow in winter storms falls in the colder portion of the storm, twice the precipitation was assumed to fall over the cold land than over the ocean. Some of the moisture, re-evaporated from non-glaciated land, would end up as snow on the ice sheet, but this effect should be mostly balanced by summer runoff. The average depth of ice was calculated at roughly half uniformitarian estimates. The latter are really unknown. As Bloom states, "Unfortunately, few facts about its thickness are known . . . we must turn to analogy and theory. . . ." 11
The time to melt an ice sheet at mid-latitudes is surprisingly short, once the copious moisture source is gone. It depends upon the energy balance over a snow or ice cover. 12 Several additional factors would have enhanced melting. Crevassing would increase the absorption of solar radiation, by providing more surface area. 13 The climate would be colder and drier than at present, with strong dusty storms that would tend to track along the ice sheet boundary. The extensive loess sheets south of and within the periphery of the past ice sheet attest to this. Dust settling on the ice would greatly increase the solar absorption and melting. A mountain snowfield in Japan was observed to absorb 85% of the solar radiation after 4000 ppm of pollution dust had settled on its surface. 14
Earth scientists believe there were many ice ages—perhaps more than 30—in regular succession during the late Cenozoic based on oxygen isotope fluctuations in deep-sea cores. 15 However, the ocean results have many difficulties, and sharply conflict with the long-held four ice-age continental scheme. Before the early 20th century, the number of ice ages was much debated. Some scientists believed in only one ice age, but the sediments are complex and have evidence of anywhere from one to four, or possibly more till sheets, separated by non-glacial deposits. Four ice ages became established mainly from gravel terraces in the Alps, and reinforced by soil stratigraphy. Much has been learned about glacial behavior and sedimentation since then. The Alps terraces are now viewed as possibly ". . . a result of repeated tectonic uplift cycles—not widespread climatic changes per se." 16 Variously weathered "interglacial soils" between till sheets are complex, and practically always have the top organic horizon missing. It is difficult to know whether they are really soils. 17 Besides, the rate of modern soil formation is unknown, and depends upon many complex factors, like the amount of warmth, moisture, and time. 18 Therefore, the number of glaciations is still an open question.
There are strong indications that there was only one ice age. As discussed previously, the requirements for an ice age are very stringent. The problem grows to impossibility, when more than one is considered. Practically all the ice-age sediments are from the last, and these deposits are very thin over interior areas, and not overly thick at the periphery. Till can sometimes be laid down rapidly, especially in end moraines. Thus the main characteristics of the till favor one ice age. Pleistocene fossils are rare in glaciated areas, which is mysterious, if there were many interglacials. Practically all the megafaunal extinctions were after the last—a difficult problem if there was more than one.
One dynamic ice age could explain the features of the till along the periphery by large fluctuations and surges, which would cause stacked till sheets. 19 Organic remains can be trapped by these oscillations. 20 Large fluctuations may be caused by variable continental cooling, depending upon volcanic activity. In addition, most of the snow and ice should accumulate at the periphery, closest to the main storm tracks. Large surface slopes and warm basal temperatures at the edge are conducive to rapid glacial movement. 21
In summary, the mystery of the ice age can be best explained by one catastrophic ice age as a consequence of the Genesis Flood.
1 Charlesworth, J.K., 1957, The Quaternary Era, Vol. 2, London, Edward Arnold, p. 1532.
2 Williams, L.D., 1979, "An Energy Balance Model of Potential Glacierization of Northern Canada," Arctic and Alpine Research, v. 11, n. 4, pp. 443-456.
3. Fong, P., 1982, "Latent Heat of Melting and Its Importance for Glaciation Cycles," Climatic Change, v. 4, p. 199.
4 Oard, M.J., 1984, "Ice Ages: The Mystery Solved? Part 2: The Manipulation of Deep-Sea Cores,"Creation Research Society Quarterly, v. 21, n. 3, pp. 125-137.
5 Oard, M.J., 1985, "Ice Ages: The mystery Solved? Part 3: Paleomagnetic Stratigraphy and Data Manipulation,"Creation Research Society Quarterly, v. 21, n. 4, pp. 170-181.
6 Bryson, R.A., 1985, "On Climatic Analogs in Paleoclimatic Reconstruction," Quaternary Research, v. 23, n. 3, p. 275.
7 Charlesworth, Op. Cit., p. 601.
8 Bunker, A.F., 1976, "A Computation of Surface Energy Flux and Annual Air-Sea Interaction Cycles of the North Atlantic Ocean," Monthly Weather Review, v. 104, n. 9, p. 1122.
9 Holton, J.R., 1972, An Introduction to Dynamic Meteorology, New York, Academic Press, pp. 48-51.
10 Oard, M.D., "An Ice Age Within the Biblical Time Frame," Proceedings of the First International Conference on Creationism, Pittsburgh (in press).
11 Bloom, A.L., 1971, "Glacial-Eustatic and Isostatic Controls of Sea Level," in K.K. Turekian, ed., Late Cenozoic Glacial Ages, New Haven, Yale University Press, p. 367.
12 Patterson, W.S.B., 1969, The Physics of Glaciers, New York, Pergamon, pp. 45-62.
13 Hughes, T., 1986, "The Jakobshanvs Effect:" Geophysical Research Letters, v. 13, n. 1, pp. 46-48.
14 Warren, S.G. and W.J. Wiscombe, 1980, "A Model for the Spectral Albedo of Snow. II. Snow Containing Atmospheric Aerosols," Journal of the Atmospheric Sciences, v. 37, n. 12, p. 2736.
15 Kennett, J.P. 1982, Marine Geology, New Jersey, Prentice-Hall, p. 747.
16 Eyles, N., W.R. Dearman and T.D. Douglas, 1983, "Glacial Landsystems in Britain and North America" in N. Eyles, ed., Glacial Geology, New York, Pergamon, p. 217.
17 Valentine, K. and J. Dalrymple, 1976, "Quarternary Buried Paleosols: A Critical Review," Quarternary Research, v. 6, n. 2, pp. 209-222.
18 Boardman, J., 1985, "Comparison of Soils in Midwestern United States and Western Europe with the Interglacial Record," Quaternary Research, v. 23, n. 1, pp. 62-75.
19 Paul, M.A., 1983, "The Supraglacial Landsystem," in N. Eyles, ed., Glacial Geology, New York, Pergamon, pp. 71-90.
20 Eyles, Dearman and Douglas, Op. Cit., p. 222.
21 Patterson, Op. Cit., p. 63-167.
* Mr. Oard is a meteorologist with the U.S. Weather Bureau, Montana.
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Learn something new every day More Info... by email
Glaciers are gigantic, solid “rivers” of ice. They have been around for much of earth’s history, and are responsible for large geographic features on its surface, including the Great Lakes. Glaciers travel miles from their point of origin and deposit debris in wide swaths of land.
The question is, how does a solid like ice move like that? There are rockslides, but they are sudden and caused merely by erosion. Rockslides do not flow for miles in the way that glaciers do. So what is the cause of this glacial motion?
There are several mechanisms at play. The primary one has to do with the relationship between temperature and pressure. The melting point of most substances increases as the pressure increases – atoms pushed more closely together become more stable. This is not the case with ice. For ice, the melting point drops as pressure increases.
The ice at the bottom of glaciers is under enormous pressure. Some glaciers are over a mile deep. Through a combination of these extreme pressures and latent heat coming from the earth itself, some of the ice melts and gives the glacier above it a slick surface to slide down.
However, this melting process is not reliable. It varies depending on pressure and temperature variations. Therefore, glaciers only move slowly, between an inch and a couple of feet per day. The large variance in glaciers' flow speeds is due to the equally large variance in pressures and temperatures within the glacier.
Another mechanism is motions of the ice crystals within the glacier itself. A glacier is faster at its center, where there is the least friction with surrounding rock. Little ice particles, even in solid form, move tiny millionths of an inch in response to slight pressure changes and small inclines. The aggregate influence of all these little motions adds up to a significant global effect that propels the glacier forward.
Sometimes, glaciers move forward at an unprecedented pace, called a surge. For instance, in 1953, the Himalayan Kutiah Glacier moved seven miles in three months. Scientists are still not entirely sure of the cause of these surges, but they may occur when delicate structural arrangements within the glacier reach a “tipping point” and cause a cascade of collapses and a corresponding flow.
@ Alchemy - You described the movement of glaciers across the bedrock, but glacial movement can also be examined as a natural force much like the ebb and flow of the tides. In this context glacial movement is referred to in terms of accumulation and ablation.
Accumulation is the addition of snow that turns to ice on a glacier; resulting in the glacier flowing down slope. The distance the glacier expands every year is the accumulation. Ablation is just the opposite. When glaciers actually melt, and retract this is referred to as ablation.
This natural cycle of accumulation and ablation has been thrown completely out of sync in the past century due to climate change. Some of the climate change is natural, but some is also the result of human influences on the environment. In many glaciers ablation is occurring at a faster rate than accumulation; resulting in the overall decrease in ice cover.
Glaciers have two main types of movement. The Basal sliding that the article describes is one way that glaciers move; although, this type of movement is more common at the toe of a glacier. This type of movement leaves behind characteristic grooves and valleys running parallel to the glaciers flow (Cirque glaciers actually leave behind bowls). Glaciers also leave their mark on the terminal landscape of the glacier. These can range from alluvial and till deposits to moraines, eskers, and kettles.
The other type of glacial movement is internal flow. The article did not touch on this type of glacial movement, but it is very significant. At the head of the glacier, friction holds the glacier to the bedrock, but internal stress causes the glacier to creep along its internal planes. This causes the glacier to move from its interior, with the oldest ice remaining at the depths of the glaciers highest elevation.
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