Transcript Chapter 17: Glaciers - Prairie Science

Chapter 17: Glaciers
Prairie School
Earth Science
Glaciers: Moving Ice
When you take a handful of snow and squeeze it, you are creating a
product that is similar to that of glaciers.
Glacier: fast moving, large mass of ice.
Snowfield: Where snow starts to accumulate on the snowline.
Snowline: the elevation where snow remains throughout the year.
Places where the temperature is below freezing of water.
Glaciers
• As the temperature rises, there are periods of melting and
recrystallization. This makes the snow grainy ice (firn)
• Within the deep layers of the snow and firn, the pressure from the
layers on top start to flatten the ice and push the air out from
between the grains.
• This leads to a glacier that starts to move when this buildup of firn
and snow starts to move because of gravity and its own weight.
Two Types of Glaciers
• Alpine Glacier: a narrow, wedged
shaped mass of ice that forms in a
mountainous region and is
confined to a small area because
of surrounding topography.
– Ex: Himalaya mountains, Andes, Alps
• Continental Glacier: massive
sheets of ice that may cover
millions of square kilometers, and
may be thousands of meters thick.
They are not confined by
surrounding topography.
– Also called “ice sheets,” exist only in
Greenland and Antarctica.
– When these melt, could cause and
increase in sea level.
Movement of Glaciers
• Sometimes called “rivers of ice. ”However, glaciers move very differently than
rivers.
• Few centimeters to 1km per year
• Large crevasses (cracks up to 50 meters deep!) form from unequal ice movement
• Basal Slip: Glaciers “slip” over a thin layer of water and sediment, which is
between the ice and the ground
– Increase in the pressure from weight of ice decreases the melting point of ice.
– Ice melts where the glacier touches the ground.
– Water mixes with sediment and creates a slippery surface, leading to basal slip
• Internal Plastic Flow: Pressure deforms grains of ice under the glacier. As the
grains deform, they slide over each other and cause the glacier to flow slowly.
– The interior of a glacier moves this way, unlike basal slip where the entire glacier moves.
– Edges of a glacier moves move slowly than the center because of an increase in friction.
Glacial Erosion
• There are many different landforms
created from glacial erosion and
deposition.
• Glacial Erosion: Like rivers, glaciers
are agents of erosion.
– Rocks carried by the glacier have a
smooth appearance to them
– As the glaciers move through the narrow,
V- shaped valley, erosion takes place and
the walls get steeper. This also pulls
rocks from the floor of the glacier.
– Cirque: bowl shaped depression
– Arête: a sharp jagged ridge
– When several arêtes join together, they
form a pyramid peak called a horn.
Glacial Erosion
• U shaped Valleys: the original V shape eventually forms a U shape
after glacial erosion.
• Geologists can look at an area and decide if there was once a
glacier, based on the U shaped valleys.
• Erosion by Continental Glaciers: leveled landforms, producing
smooth and rounded landscape. This is similar to the way that
bulldozers level landscape.
– Scratches on the rock surface from glacier movement may also be seen.
Glacial Deposition
• Deposition happens when a glacier starts to melt (reaching lower
elevation or increase in temperature)
• As the glacier melts, starts to deposit all the material it was
carrying.
• General term for glacial deposits: glacial drift
• Large rocks: erratics
• Unsorted grouping of glacial drift from a melting glacier is called
a till.
• Sorted grouping of sediments is called a stratified drift-deposited
in layers by streams or melt water from glacier.
Glacial Deposition
• Landforms that results when a glacier
deposits till are called moraines.
• Lateral, Medial and terminal moraines
• The unsorted material left behind
beneath a glacier is called ground
moraine.
• Outwash plain: a deposit of stratified
drift that lies in front of a terminal
moraine and is crossed with melt water
streams.
• Kettles: depressions in outwash plains,
fill with water, form kettle lakes
• Esker: A long, winding ridge of gravel
and coarse sand deposited by glacial
Glacial Lakes
• Lake basins can form where glaciers erode
surfaces and leave depressions in the
bedrock.
• Can also form from uneven surface of
ground moraine deposited by glaciers.
• Can form where terminal and lateral
moraines block existing streams
– Example: Great Lakes
– Glacial erosion widened and deepened existing
river valleys and moraines to the south blocked off
the ends of these valleys.
– As ice sheets melted, the water was trapped in the
valleys by the moraines and lakes formed
Ice Ages
• The last ice age happened 4 million years ago and the earliest
known is estimated to have happened 800 million years ago.
• Ice age is a long period of climatic cooling where continents are
glaciated repeatedly.
– Last glacial period, 1/3 of Earth surface covered with glacier, water levels
140 meters lower (extended coastlines), Canada and Alaska under ice
• A drop in the average temperature around 5 degrees Celsius may
be enough to start an ice age.
• Cooler times=glacier advancement
• Warmer times=retreating glaciers (interglacial period)
• Currently we are in an interglacial period
Causes of Glaciation
• Milankovitch theory: cyclical changes in Earth’s orbit and the tilt of
the Earth’s axis occur over thousands of years and this causes
climate change.
• Every 100,000 years the shape of the Earth's orbit changes to
become more circular path.
• Every 41,000 years the tilt changes between 22.2 degrees and
24.5 degrees
• Procession: A gradual change or “wobble” in the orientation of the
Earth’s axis affects the relationship between the Earth’s
eccentricity and tilt.
• These three factors are proposed to affect the distribution of
solar energy that reaches earth’s surface.