Teacher Background

Past Ideas and Concepts to Review

William Bourne – 16^th Century Geographer: described the power of rivers to erode river banks, but did not recognize the carved their own valleys, and to reshape the landscape. Also, it was believed that the Earth was only 6000 years old!

Four hundred years ago it was believed that the planet was decaying and had since Noah’s Flood. The world was like a decaying body, and mountains were blemishes on a once perfect world. That is mountains and valleys were ruins left behind by catastrophes such as the great flood. There was not enough time for erosion to occur.

Robert Hooke (1635 – 1703) – suggested that landscapes were shaped and reshaped by “vicissitudes of change” or cycles of denudation (gradual erosion).

Hooke thought the Earth had gone through several of these cycles. This idea was ignored for 130 years.

• Cycle began with landscapes being worn down by rivers and waves.
• Eroded material is washed into the sea and deposited as sediment on the seabed.
• Sediments are cemented together or fused by heat to form new rock.
• Finally rocks are lifted up to form new landscapes, beginning the cycle again.

James Hutton (1726 –97) – published “Theory of the Earth” in 1795.

• Earth was much older than 6000 years, so there was time for denudation to take place.
• Denudation cycle – rediscovery of Hooke’s old idea. Process was not a cycle of destruction, but the formation of new landscapes. Creative process was a part of God’s plan.
• Evidence from granite intrusions. You could work out how much material had been worn away by figuring out the original rock structure.

Early 1800’s – scientists are going out into the field to make observations of these processes at work. They learned how to recognize the signs of weathering, etc. But there was a bitter dispute about the role of rivers by in landscape evolution.

Louis Agassiz (1807 – 73) – suggested that North America and Northern Europe were once covered by vast ice sheets. John Muir and his studies of Yosemite! Confirmed these theories. Show books and Bernstat pictures.

John Wesley Powell (1834 – 1918) and Grove Gilbert (1843- 1918) – Studies of the Colorado River in 1875 by Powell and the Henry Mountains in Utah in 1877 by Gilbert, showed how rivers could transform the landscape. Show Powell’s book!

Vocabulary

• Ablation: Ablation refers to all processes by which snow, ice, or water in any form are lost from a glacier - the loss of snow or ice by evaporation and melting.
• Ablation area: Ablation area is the lower region of a glacier where snow ablation exceeds snowfall.
• Accumulation area: Accumulation area is the upper region of a glacier where snow accumulation exceeds melting.
• Albedo: Albedo is the percentage of the incoming radiation that is reflected off a surface. An albedo of one indicates that 100 percent of the radiation is reflected. Fresh snow has a high albedo (0.7 to 0.9), indicating that 70 to 90 percent of the radiation received is reflected; glacier ice has a lower albedo of 0.2 to 0.4.
• Cirque: A glacially eroded basin shaped like half a bowl; a deep, steep-walled recess in a mountain, caused by glacial erosion.
• Cirque lake: A small body of water occupying a cirque depression, dammed by a rock lip, small moraine, or both.
• Crevasses: Crevasses are open fissures in glacier ice. Crevasses form where the speed of the ice is variable, such as in icefalls and at valley bends.
• Density: Density is the ratio of the mass of an object to its volume. Snow has a density averaging about 0.1, firn has a density of about 0.55, and glacier ice has a density of about 0.89. The density of unmineralized fresh water is 1.
• Equilibrium line: Equilibrium line is the boundary between the accumulation area and the ablation area.
• Firn: Firn is old snow that has been recrystalized into a more dense substance. Snowflakes are compressed under the weight of the overlying snowpack. Individual crystal near the melting point have slick liquid edges allowing them to glide along other crystal planes and to readjust the space between them. Where the crystals touch they bond together, squeezing the air between them to the surface or into bubbles. During summer we might see the crystal metamorphosis occur more rapidly because of water percolation between the crystals. By summer's end the result is firn -- a compacted snow with the appearance of wet sugar, but with a hardness that makes it resistant to all but the most dedicated snow shovelers! Firn has a density greater than 0.55.
• Glacial advance: Glacial advance is the net movement of glacier terminus downvalley. Advance occurs when the rate of glacier flow downvalley is greater than its rate of ablation. Advances are characterized by a convex-shaped terminus.
• Glacial drift: Glacial drift is the loose and unsorted rock debris distributed by glaciers and glacial meltwaters. Rocks may be dropped in place by the melting ice; they may be rolled to the ice margins, or they may be deposited by meltwater streams. Collectively, these deposits are called glacial drift. Till refers to the debris deposited directly by the glacier. Rock debris rolls off the glacier edges and builds piles of loose unconsolidated rocks called glacier moraine. Lateral moraines form along the side of a glacier and curl into a terminal moraine.
• Glacial flour: Glacial flour is the fine-grained sediment carried by glacial rivers that results from the abrasion of rock at the glacier bed. Its presence turns lake water aqua blue or brown, depending on its parent rock type.
• Glacial polish: Glacial polish is the leveling and smoothing of rock by fine-grained debris at the glacier bed. Coarser rocks may gouge scratches called striations.
• Glacial retreat: Glacial retreat is the net movement of the glacier terminus upvalley. Retreat results when the glacier is ablating at a rate faster than its movement downvalley. Retreating termini are usually concave in shape.
• Glacial till: An unsorted, unstratified mixture of fine and coarse rock debris deposited by a glacier. Also called: Till.
• Glacier: A glacier is a body of ice showing evidence of movement as reported by the presence of ice flowline, crevasses, and recent geologic evidence. Glaciers exist where, over a period of years, snow remains after summer's end.
• Glacier outburst flood: A sudden release of melt water from a glacier or glacier-dammed lake sometimes resulting in a catastrophic flood, formed by melting of a channel or by subglacial volcanic activity.
• Great Ice Age: The Pleistocene Epoch
• Hydrothermal alteration: Hydrothermal alteration is the alteration of rocks or minerals due to the reactions of geothermally heated water with minerals. The process weathers and weakens the rocks such that they may become unstable.
• Icefalls: Icefalls are somewhat analogous to waterfalls in rivers. The flow of the ice down a steep gradient often results in crevasses and seracs.
• Kinematic waves: Refers to a wave of ice moving downglacier propagated by its increased thickness. The wave of ice may move at two to six times the velocity of surrounding thinner ice.
• Lahar: A lahar is a mudflow or debris flow originating on a volcano.
• Lateral moraine: A moraine formed at the side of a glacier. Piles of loose unsorted rocks along the side margins of the glacier. The rocks may be pushed there by the moving ice or dumped from the glacier's rounded surface.
• Mass balance: Mass balance describes the net gain or loss of snow and ice through a given year. It is usually expressed in terms of water gain or loss.
• Medial moraines: Medial moraines form where two mountain glaciers bearing lateral moraines unite. They appear as dark streaks of rock along the glacier centerline.
• Moberg hills: A moberg hill is a rounded landform composed chiefly of palagonitized hyaloclastite. Hyaloclastite is very fine-grained to coarsely fragmented rock, consisting of a high percentage of glass relative to crystalline rock fragments, generally of basaltic composition, and commonly poorly sorted. It is produced where lava flows or intrudes into water, ice, or water-rich sediment.
• Moraine: Rock debris deposited by a glacier.
• Neoglaciation: Neoglaciation refers to the advances made by mountain glaciers since the great Pleistocene ice age. In the Cascades the advances have occurred since 6,600 years before present.
• Ogives: Ogives are arc-shaped features occasionally found across the glacier surface below icefalls. They may be ridges and swales in the ice or bands of darker or lighter ice. One theory of their formation suggests that the ice is stretched and sometimes dirtied when exposed in the icefall during the high velocities of summer; it is compressed during the winter so that bands of different ice thickness form.
• Perfectly plastic solid: A solid that does not deform until it reaches a critical value of stress, after which it will yield infinitely. Some glaciologists say that ice is a perfectly plastic substance. (That is, brittle and capable of cracking like a solid, yet deformable and capable of flowing at other stresses.)
• Pleistocene: The period of earth's history, roughly two million years ago to about ten thousand years ago, characterized by the advance and recession of continental ice sheets.
• Roche moutonnee: A roche moutonnee is a small asymetrically-shaped hill formed by glacial erosion. The upper sides are rounded and smoothed and the lower sides are rough and broken due to quarrying by the glacier.
• Seracs: Seracs are the pinnacles of ice formed where the glacier surface is torn by sets of crevasses.
• Striations: Striations are the scratches etched into the rock at the bed of a glacier. Their presence indicates grinding of sand and rock particles into the bed under considerable pressure. In some places find-grained debris polishes the bedrock to a lustrous surface finish called glacial polish.
• Suncup: A suncup is a small depression on a snow or firn surface formed by melting and evaporation resulting from direct exposure to the sun.
• Tarn: A small mountain lake or pool, especially one that occupies an ice-gouged basin on the floor of a cirque.
• Terminal moraine: A moraine formed at the downvalley end of a glacier. Piles of loose unconsolidated rock at the glacier's downvalley end. The rocks may be pushed there by the forward motion of the glacier or dumped from the glacier's rounded surface.
• Terminus: The downvalley end of a glacier. It is sometimes referred to as the glacier snout.
• Till: The unsorted rock debris deposited directly by the glacier without the extreme reworking by meltwater. Also called: Glacial till.
• Trimlines: The sharp vegetative boundaries delimiting the upper margin of a former glaciation. The age differences of the ground surface are often visible because of different ages of the vegetation.

• How are mountains formed?
• What is erosion?
• What causes erosion?
• What is a glacier?
• How do glaciers move? Why do they move?

Tell Students:

One of John Muir’s greatest achievements, in a life filled with remarkable accomplishments, was his early recognition and announcement of the important part played by glaciation in the origin of Yosemite Valley. He first visited the valley in April, 1868, and returned in November, 1869, to live there for the next several years. By the following August he had definitely concluded that a great system of glaciers converging in the Yosemite was responsible for the creation of this mighty gorge with its superb cliffs and falls.

It is not strange that a youth with John Muir’s inquiring and scientific mind¹ and enthusiasm for the outdoors should have been drawn to the study of this creation problem. To see Yosemite Valley is to wonder how it was made. No one can view the vertical cliffs of El Capitan and Half Dome, with Yosemite and Bridalveil fall plunging out of the sky over other equally sheer walls, with Nevada and Vernal falls leaping down over gigantic steps of solid granite in the Merced River’s stately descent to the valley and then wander over that broad expanse of parklike floor without speculating how this exceptional aggregation of impressive scenic features came into existence.

Weathering

Demonstration 1: The Power of Ice

Materials: 2 – 5 pound weights, strong glass bowl, saucer, water

• Fill a strong bowl with water, and then cover it with a saucer. Place a heavy weight on top.
• Place in a freezer and leave for 24 hours. As the water turns to ice it expands, forcing up the weight.

Demonstration 2: Cracking Up

Materials: modeling clay, plastic wrap, water

Procedures:

• Moisten some modeling clay and roll into two balls. Wrap both in plastic wrap and put one in the freezer.
• After 24 hours, unwrap the two balls, and compare them. Wet and refreeze if the cracking is not clear.

High in the mountains, temperatures often fall below freezing at night, and then rise above freezing during the day. When rock is frozen and thawed out – not once or twice but thousands or even millions of times the results can be dramatic!!!

Activity 1: Weathering and Surface Area

A piece of rock that is full of cracks and joints weathers much faster than one that is smooth and solid, not only because its joints are week points but also because it has a larger surface area.

Take a cube of sugar and try to dissolve in a lukewarm glass of water. Repeat for a packet of sugar, making sure the water is filled to the same level. Give a stir, and watch what happens.

Activity 2: Glacial Flow

Tell Students:

Because it takes an enormous amount of mass to make a real glacier creep downhill, scientists often rely on substitute materials to make a model of fluid flow in glaciers. In this activity, you'll make a highly viscous suspension of cornstarch and water to simulate a glacier, and track the way that it flows down a "valley."

Materials:

• plastic shoe box
• one 16-oz box of cornstarch
• one to two cups of water
• one 2-qt mixing bowl
• 5 wooden toothpicks
• 5?6 large pebbles
• one 5" x 7" inch index card
• pencil
  1. Mix the cornstarch and water together in the bowl to form a suspension with the consistency of toothpaste. (It should not be runny or wet.)

2. Lay the pencil flat on the table and place one end of the shoe box on top of it to give the box a slight tilt. Begin pouring the cornstarch mixture into the box at the raised end and observe what happens.

3. After the mixture has flowed through the entire box, scrape it up with your hand and pile it in the raised end of the box. Use the index cards to make a "dam" across the shoe box valley to hold the mixture back. Lay the five toothpicks across the front of the mixture so that they are one inch apart and parallel to each other. Remove the dam and observe the way the toothpicks move as the glacier flows.

4. After you have tracked the flow of the glacier with the toothpicks, repeat the experiment, but this time place a few large pebbles on the bottom of the shoe box to make obstructions in the valley. Allow the glacier to flow again and observe what happens when it interacts with the obstructions.
   

Ask Students:

• When the cornstarch mixture initially flowed through the box, what shape did the front take?
• How does this relate to valley glaciers?
• When you released the mixture from behind the index card "dam," what pattern did the toothpick markers make? What do you think caused this?
• What happens to the flow of the glacier when it hits the obstructions in the valley?
• Do you notice anything different in the top of the glacier as it flows over the rocks??

Activity 3: Solid Flow

Materials: plastic trough, tray. Water and water bottle, sand

Procedures:

• Fill gutter or tray with a deep, even layer of sand.
• Hold one end of the gutter over the tray, and then raise the other end until the sand starts to flow.
• Refill the gutter with sand, and then sprinkle the sand with water. This will help bind the sand together.
• Test the flow again.
• Repeat again, this time really drowning the sand with water.

Tell Students: Sand is rock that has been weathered into very small grains. Between the grains are small holes, called pores. In dry sand, these pores are filled with air. In wet sand, the pores are filed with water. The water, or lack of it, affects the way the sand grains behave. Dry sand forms slopes, and in many ways acts like a solid surface. Very wet sand behaves more like a liquid. The “pore pressure” of the water pushes the grains apart, so that they flow around each other.

Demonstration 3: Creeping Ice

Materials:

• Weight
• Cards
• Tray

Procedures:

• Stack the cards in a pile on the tray. Put the weight on top. The weight represents a thick layer of ice, the cards the flattened crystals beneath.
• Slowly tip one end of the tray upward. The weight will gradually start to slip downhill as the cards slide over each other.
• Eventually the weight will slide off the tray. Notice the cards nearest the tray have moved the least – just like the deepest ice in a glacier.

Activity 4: Scoured by Ice

Materials:

• Ice cubes
• Sand
• Softwood board

Procedures:

• Take an ice cube and let a melt just a little. Dip it into sand.
• Moving your hand in a circle, rube the sandy ice cube on the wooden board. Keep dipping the cube in the sand so that it stays sandy as you rub.
• After a few minutes, look at the surface of the board. The ice will have scraped the sand against the wood – just as a glacier scrapes against bedrock.

Demonstration 4: Sliding Away

• An ice cube melts quickly under a heavy weight, just as the deepest glacier ice can melt under the weight of the glacier, which then slides over the water. This is called “basal slip”.

Activity 5: Making a Moraine

Materials:

• Plastic bag
• Fork
• Water
• Sand
• Tray

Procedures:

• Put the sand in the tray and level it out. Fill the bag with water, and then tie it tight. Now put the bag on the sand and push it along sideways.
• Now take away the bag, and you will see a “moraine” at the end and along the sides where your “glacier” was. Like a real moraine, it will be curved.
• Now pull the bag back, give it a sharp prod with the fork. Water will flow out of the bugs, just like a glacier melting as it retreats.
• As the water flows out, some of moraine will be washed away, leaving a channel. The same thing happens when a real glacier melts.

Ask Students:

• What types of landforms do glaciers create?
• How do glaciers change the shape of the land?
• What effect have glaciers had in California other than landform changes?