Geology Chapter 22 Glaciers We will cover: 1.Formation and Glacier Types 2.Glacier Movement 3.Glacial features 4.Ice Ages

Formation and Glacier Types Glacier- slowly flowing sheets or rivers of ice that exist year round must move under its own weight *Ice is a mineral Where they form: Areas ALWAYS covered by snow, Can form at any latitude as long as elevation is high enough Snow line- lowest elevation at which the layer of permanent snow occurs in summer

Formation: 1.Snow falls (90% air) 2.Crystals pack together due to melting (due to pressure) and sublimination. 3.When it contains 25% air it is called firn- similar to a packed snowball 4.Pressure continues to melt firn at contact points between crystals and new crystals form- forms interlocking ice crystals- now glacial ice (about 20% air)

Firn

Glacial Facts May take 10’s of years or 1000’s of years to form Considered by some to be metamorphic rock Appears blue because ice absorbs red light

Types of glaciers: 1.Continental glaciers (Ice sheets) - vast layers of ice that spread over thousands of km2 of continental crust currently only Greenland & Antarctica 2. Mountain (Alpine) glaciers- those in mountainous regions Largest are in Alaska Many types (see next slide)

Types of Mountain Glaciers a. Cirque glaciers- fill bowl shaped depressions called cirques b. Valley glaciers- flow down valleys c. Mountain ice caps- submerge crest of mountain d. Piedmont glaciers- end of a valley glacier where ice fans out onto an adjacent plain

Assignment 22A: 1.Do glaciers currently occur in New York state? 2.Does Africa have any glaciers? How? 3.Does it snow below the snow line? 4.Is the North pole covered by glaciers? Why or why not? 5.What are the two major types of glaciers? 6.Why is ice a mineral? Why is glacial ice a metamorphic rock?

22B Glacial Movement Glaciers move in 2 ways: 1. Basal sliding- meltwater accumulates at the base of a glacier, lessening friction- entire glacier slides Temperate glaciers 2. Internal flow- ice changes shape internally Crystals deform plastically, disappear and reform and slide past one another Main source of flow for polar glaciers- very cold areas

Glacial Movement Glaciers travel faster at the top and center- less friction Crevasse- a large crack in the ice surface Not deeper than 60m- too much pressure

Glacial Advance and Retreat Snowfall adds to glacier, ablation (removal of ice by sublimination, melting and calving) subtracts from the glacier When calving occurs near the ocean, icebergs are created Glaciers advance when snowfall is greater than ablation, retreat occurs when ablation is greater than snowfall

Assignment 22 B 1.Do glaciers normally retreat above the snowline? 2.Draw an ice cap, piedmont, valley and cirque glacier (p 670). 3.Do glaciers deform elastically or plastically as they move? 4.What continents do glaciers occur on? 5.After internal flow how are crystals shaped?

Type of Glacial Erosion 1.Glacial plowing- ice moves loose sediment, like a bulldozer 2.Glacial incorporation- ice surrounds and incorporates debris 3.Glacial plucking (or glacial quarrying)- a glacier breaks off and carries away bedrock 4.Glacial abrasion- clasts of rock become embedded in the glacial ice, which grinds away at the substrate- produces rock flour- a fine sediment, polished surfaces and glacial striations- grooves in the bedrock

Glacial Plucking

Glacial Abrasion

Erosional features (alpine glaciers): 1.Cirque- bowl shaped depression on the side of a mountain 2.Arête- a knife like ridge of rock separating two cirques 3.Horn- a pointed mountain peak with at least 3 cirques surrounding it (ex: Switzerland’s Matterhorn) 4.U-shaped valleys- entire valley is eroded causing steep sided (unlike a V-shaped valley usually produced by a river)

Cirque

Arête

Horn

U-shaped Valleys

More Erosional features (alpine glaciers): 5. Hanging valleys- side valley from a tributary glacier- ends suddenly into a steep drop 6. Fjords- glacial valleys that have flooded after glacier melted and sea level rises to fill valley

Hanging Valley

Fjord

Depositional features (alpine glaciers): 1. Moraine- a ridge of sediment deposited on the edge of a glacier A. Lateral moraine- on the side of the valley B. Medial moraine- forms where two valley glaciers meet and their lateral moraines combine down the center of the trunk glacier C. Terminal moraine- forms at the toe (end) of a glacier

Moraine

More Depositional features (alpine glaciers): 2. Kame- ridge of stratified (layered, sorted by size) sediment formed by a stream flowing along the side of a glacier 3. Drumlins- streamlined elongate hills formed when a glacier advances over a terminal moraine- steeper slope point up glacier, shallower in direction of movement 4. Kettles- depressions formed when a piece of the glacier breaks off, is buried and then melts- often form lakes

Kame

Drumlin

Kettle

More Depositional features (alpine glaciers): 5. Esker- snake-like ridge formed by streams of meltwater in tunnels in the glacier depositing their load 6. Glacial drift- all sediment transported by a glacier Several types- see next slide

Esker

Types of Glacial Drift 1.till- unsorted material deposited underneath glacier or at the toe 2.erratics- boulders that have been dropped by a glacier 3.Glacial marine- sediments deposited in the sea from icebergs melting and dropping sediment (pebbles and larger are called drop stones)

Till

Erratics

More Types of Glacial Drift 4. Glacial outwash- till at the end of a glacier that has been eroded by meltwater and deposited down stream. Forms an outwash plain. 5. Lake-bed sediment- deposited in meltwater lakes. Varve- a pair of sedimentary layers, one of silt- due to spring floods, one of clay formed when lake is still in winter. 6. Loess- windblown sediment (usually silt, not limited to glaciated areas)

Outwash Stream

Varve

Loess

Features of Continental Glaciers 1.Glacial subsidence- surface is compressed and sinks due to ice loading 2.Glacial rebound- land gradually rises after glacier melts. 3.Polished surfaces, lack of regolith (bare bedrock), striations, erratics

Striations

Assignment 22C 1.What is the main difference between glacial till and glacial outwash? 2.What is the main difference between a kame and an esker? 3.What type of moraines do piedmont glaciers generally make? 4.In general, how do mountain glacier features differ from continental glacier features?

Ice Ages Earth’s history has been marked with many ice ages, the most recent was about 1.3 mya to 11,000 years ago. It is called the Pleistocene ice age (Named for the Pleistocene epoch time period) During this period much of North America (as far south as Pennsylvania) was covered by a continental glacier called the Laurentide ice sheet Interglacial periods- times between glaciations

Pleistocene Ice Age Mountain ice caps and valley glaciers grew in the Rocky, Sierra Nevada and Cascade mountains Areas south of the ice sheet experienced much colder temperatures Glaciers also covered much of Northern Eurasia The term Holocene is used to refer to the time since the last ice age

Long Term Causes of Ice Ages 1. Plate tectonics a. much of the continents must have been at high latitudes b. low sea level- controlled by sea floor spreading rates c. air currents cannot carry heat to high latitudes- controlled by positions of continents 2. Concentration of carbon dioxide (CO 2 )- greater concentrations cause global warming

Short Term Causes of Ice Ages 1.Orbital eccentricity- Earth’s orbit around the sun becomes more circular then more elliptical over time. Called the eccentricity cycle, occurs every 100,000 years 2.Tilt of Earth’s axis- tilt of earth varies between 22.5º to 24.5º- our current tilt is 23.5º 3.Procession of Earth’s axis- actual axis on Earth varies- currently our axis is at the North pole

Milankovitch Milankovitch noticed these factors affect the Earth’s insolation- exposure to the sun’s rays Determined that these factors combine to produce glaciations every 100,000; 40,000; and 20,000 years- these are called Milankovitch cycles

Positive Feedback and Ice Ages Ice ages cause positive feedback to continue the ice age- due to snow reflecting more solar radiation (albedo), interruption of the global heat conveyor (less evaporation, saltier ocean water, not as strong ocean currents moving heat) and fewer plants causing less CO2 accumulation

Assignment 22 D 1.What type of glaciers dominated northern North America during the last ice age? 2.What happens to the snow line during an ice age? 3.What do ice ages do to sea level? 4.Does a mirror have a high or low albedo? Asphalt?