Ch. 18 - Glaciers and Glaciation

Class: GEOL-101

Notes:

18.1 Glaciers: A Part of Two Basic Cycles

Explain the role of glaciers in the hydrologic and rock cycles. Describe the different types of glaciers, their characteristics, and their present-day distribution.

Glaciers

Presentation content:
A glacier is a thick mass of ice that forms, over hundreds and thousands of years, by the accumulation, compaction, and recrystallization of snow

• Glaciers are parts of two basic Earth cycles:
  • Hydrologic cycle
  • Rock cycle

Where do we find glaciers?

• Ice Sheets
  • Ice sheets exist on a larger scale than valley glaciers, currently exist at both poles
    • Greenland and Antarctica
• Valley (Alpine) Glaciers
  • Glaciers that exist in valleys of mountainous areas are called valley or alpine glaciers

Notes from the lecture:

• As snow continues to be buried it crystallizes into ice.
• Responsible for breaking up and creating sedimentary rocks also groundwater is locked up in glaciers
• We typically find them in ice sheets (large scale features) and valley (alpine) glaciers in mountain areas

Ice Sheets (Greenland and Antartica)

Presentation content:

• The Arctic Ocean is covered with sea ice (frozen seawater), not glacial ice
  • Sea ice is up to 4 meters thick while glaciers are hundreds to thousands of meters thick
  • Sea ice expands and contracts with the seasons
• Glaciers form on land (continental ice sheets)
  • Greenland (60º – 80º N. latitude)
    • Ice sheet covers 1.7 square million kilometers, avg. ~1500 meters thick
  • Antarctica in the southern hemisphere
    • Ice sheet covers 13.9 square million kilometers
  • Ice flows out in all directions from one or more snow accumulation centers

Notes from the lecture:

• Greenland is really the last one in the north pole

Flow

Presentation content:

• Along parts of Antarctica, glacial ice flows into the sea, creating ice shelves
  • In shallow water, the ice touches bottom and is grounded
  • In deep water, the ice shelf floats
• Thickest on landward side and thin seaward
  • Sustained by ice flow from the adjacent ice sheet
• Some ice shelves are unstable and starting to break apart
  • Breakup of ice shelves attributed to the trend related to accelerated climate change

• They look small here but they are very large blocks (km)

Notes from the lecture:

• In deep water glaciers float, in shallow water it sits on the floor
• Throughout geologic time ice shelves are weaker and weaker and are breaking up

Valley (Alpine) Glaciers

Presentation content:

• Glaciers that exist in valleys of mountainous areas are called valley or alpine glaciers
• Piedmont glaciers form when one or more alpine glacier emerges from the valley and spreads out in a broad lobe, occupying broad lowlands at the base of steep mountains

Notes from the lecture:

• Eventually they open up "flat lines"

18.2 Formation and Movement of Glacial Ice

Describe how glaciers move, the rates at which they move, and the significance of the glacial budget.

Glacier formation

Presentation content:

• Glaciers form in areas where more snow falls in winter than melts during the summer
  • Snow above the snowline does not melt in the summer
• Glacial Ice Formation
  • Air infiltrates snow
    • Extremities of crystals evaporate
    • Snowflakes become smaller, thicker, and more spherical
  • Air is forced out
    • Snow is recrystallized into a much denser mass of small grains called firn
    • Once the thickness of the ice and snow exceeds 50 meters, firn fuses into a solid mass of interlocking ice crystals—glacial ice

Notes from the lecture:

• As snow crystals fall, they accumulate and the edges start to melt and start to recrystallize, they get smaller but thicker
• They become spherical particles of ice, they start fusing together and start creating sedimentary masses of ice
• As the thickness continues to increase, they compact together and pressure starts to fuse them together.

Movement of a Glacier

Presentation content:

• Glacial ice moves as a flow
• The solid flows in two ways:
  • Plastic flow involves movement within the ice. Under pressure, ice behaves as a plastic material
  • Along the ground, the entire ice mass slides along the ground as basal slip. Meltwater acts as lubricant

• Ice behaves as a brittle solid in the upper 50 meter of the glacier and is called the zone of fracture
  • Crevasses (cracks in the ice) are present in the zone of fracture but sealed off by plastic flow at depth
• In contrast with the lower plastic portion of the glacier behaves plastically.

Notes from the lecture:

• We typically refer to them as flowing
• In deeper part of the ice sheet the ice behaves plastic
• But surface is a little bit more brittle
  • You got rigid topography of fractures
• Basal slip: locally melts the bottom of the ice sheet, which acts as a lubricant to slide more easily

Rates of Glacial Movement

Presentation content:

• Movement of glacial ice is not obvious
• Like a river, glacial ice does not all move at the same rate
  • Flow is fastest in the center of the glacier
  • Valley walls and floor slow the base and sides, causing drag
• Glacial velocity ranges from extremely slow to several meters per day
  • Some glaciers exhibit extremely rapid movements called surges
  • Rates of movement have been measured using markers in the past, and time-lapse photography more recently

Notes from the lecture:

• They take a series of different timeline photos
• Rates increases along the walls because of friction along pathways
• They can move very slow or relatively fast
• Partially due to the melting of the glaciers
• With satellite imaginery we can measure this a little bit better
• Ice shelfs grow very rapidly
  • A few thousand meters per year but as you move inland you get very slow rate

Budget of a Glacier

Presentation content:

• Glacial zones

  • The zone of accumulation is the area where a glacier forms
  • The zone of wastage is the area where there is a net loss of glacial ice

• Loss of ice by a glacier is called ablation

  • Melting
  • Calving
    • the breaking off of large pieces of ice
    • creates icebergs where the glacier has reached the sea

• The glacial budget is the balance, or lack of balance, between accumulation and loss of ice

• If accumulation exceeds loss, the glacial front advances

• If ablation increases and/or accumulation decreases, the ice front will retreat

Notes from the lecture:

• Zone of accumulation: more in cold days and allows for essentially the snow to accumulate

• As you move down-gradient you get to the zone of wastage, there is more seasonality here

• Essentially at this margin is where the glacier starts to melt away.

  

• Glaciers break down by the process of ablation

  • Icebergs have sedimentary rocks accumulated in them and as they melt they drop these rocks into the bottom of the ocean.

• End marine reflects the furthest extent of the glacier

  • Ice extended all the way through there
  • We use them as an indicator of the furthest extent of the ice

18.3 Glacial Erosion

Discuss the processes of glacial erosion. Identify and describe major topographic features created by glacial erosion.

Glacial Erosion

Presentation content:

• Glaciers are capable of great erosion and sediment transport
• Glaciers erode the land primarily in two ways:
  • As a glacier flows over a bedrock, it loosens and lifts blocks in a process called plucking
  • Rocks in the glacier also act like sandpaper to smooth and polish a rock surface in a process called abrasion
• Glacial abrasion produces:
  • Rock flour (pulverized rock)
  • Glacial striations (grooves in the bedrock)
• Glacial erosion is controlled by:
  • Rate of movement
  • Thickness of the ice
  • Types of rock fragments trapped in the ice
  • The erodibility of the surface below the glacier

Notes from the lecture:

• As they undergo abrasion they melt and degrade sediments
• Is basically polishing off the surface
• Rock flour is very very fine rocks (pulverized to nothing)
  • This is the reason of the turquoise color of the lakes
• Thickness of the ice is how much ice is accumulated (in inches sometimes)

Landforms

Presentation content:

• Landforms created by valley glaciers and ice sheets are very different
  • While ice sheets subdue most topography, valley glaciers create sharp and angular topography
    • Valley glaciers widen and deepen valleys, creating U-shaped glacial troughs
    • Glaciers tend to straighten valleys, removing sharp curves and creating truncated spurs
    • Glaciers in a main (trunk) valley typically erode more than tributary glaciers, creating hanging valleys

Notes from the lecture:

• Glacial troughs usually are U or V shaped
• truncated spurs
• Hanging valleys act as little water falls that all feed and main glacial trough

Glaciated Valleys

Presentation content:

• A pater noster lake forms after parts of the bedrock (lifted and plucked by the glacier) fill with water
• A cirque (a bowl-shaped depression) is typically found at the head of a glacial valley
• After the glacier has melted away, the cirque basin is sometimes occupied by a small lake called a tarn

Notes from the lecture:

• The cirque is the point of initial formation

Arêtes and Horns

Presentation content:

• Some features form from the continued glacial erosion of cirques
  • An arête is a sharp-edged ridge
  • A horn is a pyramid-like peak

Notes from the lecture:

• They polish it off into a basically pyramid-shaped feature

Roches Mountonees

Presentation content:

• An asymmetrical knob of bedrock produced by continued glacial erosion
• Glacial abrasion smoothens the gentle slope facing the oncoming glacier and plucking steepens the opposite side as the ice sheet rides over it

Notes from the lecture:

• Typically these form as a result of some sort of faulting
• Often covered with striations

Fiords

Presentation content:

• Deep, steep-sided inlets of the sea
  • Drowned glacial troughs that form when sea level rises
  • Depths may exceed 1000 meters

Notes from the lecture:

• Very similar to glaciated valleys
• Glacial valleys that have since then flooded
• Glaciers are extending through this valley

18.4 Glacial Deposits

Deposition

Presentation content:

• As glaciers melt, the rocks and sediments in the glaciers are deposited
  • Glacial drift refers to all sediments of glacial origin
    • Till is material that is deposited directly by the ice
    • Sediments laid down by glacial meltwater are called stratified drift

Notes from the lecture:

• As glaciers start to melt, they will drop all of the things they had accumulated (sediment, rocks, etc.)
• Heavily striated
• Stratified drift:
  • As glaciers start to melt sediments start to settle out as the glacier is incorporated

Landforms Made of Till

Presentation content:

• Lateral and Medial Moraines
  • A lateral moraine is an accumulation of debris on the side of the glacial till
  • A medial moraine is created when two alpine glaciers converge
• End/Terminal and Ground Moraines
  • A glacier is similar to a conveyor belt—regardless of the movement, sediments are constantly moved forward and dropped at the terminus
    • An end moraine is an accumulation of debris that forms at the terminus of a glacier
• Drumlins
  • Drumlins are streamlined asymmetrical hills composed of till and formed from ice sheets
  • Range in height from about 15 to 50 meters and may be up to 1 kilometer long
    • The steep side of the hill faces the direction from which the ice advanced
    • The gentler, longer slope points in the direction the ice moved

Notes from the lecture:

• Medial Moraines form from the convergence of two glaciers

• Lateral Moraines: accumulation around the sides of it

  

  

Basic landforms composed of stratified drift:

Presentation content:

• Ice-Contact Deposits
  • Meltwater flows over, within, and at the base of motionless ice deposits, stratified drifts that remain once the ice melts away
  • A kame is steep-sided mound formed from ice-contact stratified drift
  • An esker is a narrow, sinuous ridge composed largely of sand and gravel
• Outwash Plains and Valley Trains
  • Glacial meltwater, choked with sediment, flows onto a flat surface, drops its load, builds a broad, ramp like surface, and creates braided streams
    • Outwash plains are associated with ice sheets
    • Valley trains are associated with mountain valleys (alpine glacier)
      • Often are pockmarked with basins or depressions known as kettles

Notes from the lecture:

• An esker looks almost like a river but of sediment

  

• Remember glacier is melting and dropping sediments

• Remember glaciers are episodic, they may sometimes advance or degrade back

18.5 Other Effects of Ice Age Glaciers

Describe and explain several important effects of Ice Age glaciers other than erosional and depositional landforms.

Crustal Subsidence and Rebound

Presentation content:

• Ice sheets cause downwarping of the crust
• After the glacier melts, the crust gradually rebounds

Notes from the lecture:

• Calculations of thickness of ice in a location in the past are based on that rebound
  • How much ice would be needed to create that amount of rebound

Sea-Level Changes

Presentation content:

• During the last glacial maximum, sea level was 100 meters lower than present level
• If the Antarctic Ice Sheet melted, sea level would rise 60 or 70 meters

Notes from the lecture:

• Why is the shoreline of the Western US didn't change much?
  • It's got a convergent plate boundary

Changes to Rivers and Valleys

Presentation content:

• The advance and retreat of the North American ice sheets changed the routes of rivers and modified the size and shape of many valleys
• Upper Mississippi Drainage Basin
  • Prior to the Ice Age, a significant part of the Missouri River drained north toward Hudson Bay
• New York’s Finger Lakes
  • Prior to the Ice Age, they were a series of river valleys, glacial erosion transformed themi nto deep, steep-walled lakes

Notes from the lecture:

• Since the last 10000 years the Mississippi Drainage basin is much larger

Ice Dams Create Proglacial Lakes

Presentation content:

• Ice sheets and alpine glaciers can act as dams to create proglacial lakes
  • Examples: Lake Agassiz, Lake Missoula
  • The failure of ice dams can release large volumes of water very quickly

Notes from the lecture:

• Ice extended down, locking the drainage of that dam.
• Eventually the ice melts and all that water drains away
• This changed the topography of Eastern Washington
• This is one line of evidence for for example the formation of the Grand Canyon: Massive Floods.

18.6 The Ice Age

Briefly discuss the development of glacial theory and summarize current ideas on the causes of ice ages.

The Glacial Theory and the Ice Age

Presentation content:

• Glaciers were once more extensive than they are today
  • Looking at glacial deposits and using the principle of uniformitarianism
• Glacial/interglacial cycles occur every 100,000 years
  • The Northern Hemisphere Ice Ages began between 2 and 3 million years ago
    • ~20 of these cycles spanned the Ice Age
  • The Antarctic ice sheet formed at least 30 million years ago

Notes from the lecture:

• "Cyclisity driven by glaciation"
• Based on the position of the earth throughout ages

Causes of Ice Ages

Presentation content:

• The Quaternary Ice Age is not the only ice age in Earth’s history

  • Tillite is a sedimentary rock formed from glacial till
  • Rock evidence of earlier ice ages

• Any successful theory about the causes of ice ages must include:

  • Causes of the onset of glacial conditions
  • Causes of alteration between glacial and interglacial stages

• Variations in Earth’s Orbit

  • Changes in Earth’s orbit can vary the amount of solar radiation received
    • Variations in the shape of Earth’s orbit around the Sun (eccentricity)
    • Changes in the angle of Earth’s axis (obliquity)
    • The wobbling of Earth’s axis (precession)

• Other Factors

  • Changes in Earth’s atmosphere
  • Changes in ocean circulation
  • Changes in the reflectivity of Earth’s surface
  • Changes in continental configuration and location of plates

Notes from the lecture:

• Oval (orbit of earth) changes over time
  • Narrower elipse: more solar input
• Obliquity:
  • Changes in the angle of earth axis
  • This is why during the summer we receive more solar radiation
  • Happening on an annual scale
  • There is variability on that tilt
  • 23-24.5 degrees
• Precession
  • The wobble of the earth
  • Happens on a 26,000 years scale
• "Is the compounding of multiple of these cycles" what drives those big glacial events
  • All 3 working together to reduce the amount of solar radiation received by the Earth