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Glacial and Arid Landscapes

Chapter 4 Lecture

Natalie Bursztyn

Utah State University

Foundations of Earth Science

Eighth Edition

Explain the role of glaciers in the hydrologic and rock cycles.

Describe the different types of glaciers and their present-day distribution.

Focus Questions 4.1

A glacier is a thick mass of ice formed over 100s or 1000s of years

Originates by accumulation, compaction, and recrystallization of snow

Glaciers move slowly because of gravity

Accumulate, transport, and deposit sediment

Glaciers and the Earth System

Many landscapes were shaped by glaciers during the last Ice Age

Alps, Cape Cod, Yosemite Valley, Long Island, the Great Lakes, fiords of Norway and Alaska…

Glaciers play an important role in both the hydrologic cycle and the rock cycle

Precipitation can be trapped in glaciers for thousands of years

Ice is an agent of mechanical weathering

Glaciers and the Earth System

Valley or alpine glaciers occur in valleys in high mountains

Relatively small

Advance slowly (a few cm per day)

Flow down valley from an accumulation center

Generally, width is narrow relative to length

Valley (or Alpine) Glaciers

Ice sheets are found at the poles

Flow out in all directions from a center of snow accumulation

Large-scale, obscure underlying terrain

Greenland and Antarctica

Ice Sheets

Extensive ice sheets during the Last Glacial Maximum (~18,000 years ago)

Also covered North America, Europe, and Siberia

Ice sheets have advanced and retreated several times over the last 2.6 million years

Ice Sheets

The Arctic Ocean is covered by sea ice (frozen seawater)

Floats

Ranges from a few cm to 4 m thick

Expands and contracts with the seasons

Ice shelves form when glacial ice flows into the ocean

Large, relatively flat

Attached to land and flow outward away from coast

Become thinner seaward

Ice Sheets

Ice caps

Cover uplands and high plateaus

Smaller than ice sheets but bury underlying terrain

Piedmont glaciers

Form in broad lowlands at the base of mountains

Form when glaciers emerge from the confining walls of a valley

Outlet glaciers

Extend out from ice caps and ice sheets

Other Types of Glaciers

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

Focus Question 4.2

Glaciers move in two ways

Plastic flow within the ice

Bonds between layers of ice are not as strong as bonds within a layer

Layers remain intact but slide over one another

The entire body of ice slips along the ground

Uppermost 50 m of ice is the zone of fracture

Low pressure so ice behaves as a brittle solid

Tension creates cracks called crevasses

How Glaciers Move

Glacial movement is slow

<2 m/year to >800 m/year

Occasional rapid advances (surges)

Flow is greatest at the center

Drag along valley walls and floor slows flow at edges

Observing and Measuring Movement

Glaciers form when winter snowfall is greater than summer snowmelt

Net accumulation of snow

Snow accumulation and ice formation occur in the zone of accumulation

Area where there is a net loss to the glacier is the zone of wastage

Glaciers also lose ice because of calving

Generates icebergs

Budget of a Glacier: Accumulation Versus Wastage

Glacial budget

Balance or lack of balance between accumulation and wastage

Accumulation > wastage = glacial advance

Accumulation = wastage = stationary terminus

Accumulation < wastage = glacial retreat

Even if front is retreating, ice is always flowing

Budget of a Glacier: Accumulation Versus Wastage

Glaciers are very sensitive to temperature change

Almost all glaciers are retreating at unprecedented rates

Budget of a Glacier: Accumulation Versus Wastage

Discuss the processes of glacial erosion and the major features created by these processes.

Focus Question 4.3

Glaciers erode and transport tremendous volumes of rock

Debris cannot settle out like sediment carried by water or wind

Capable of carrying very large pieces of debris

Glacial Erosion

Glaciers erode land in two primary ways:

Plucking

Flowing ice lifts fractured blocks of bedrock from the surface

Meltwater penetrates cracks and expands when it refreezes

Rocks break loose and are carried away by the glacier

Abrasion

Ice grinds bedrock and polishes the surface

Rock flour is finely ground bedrock

Glacial striations form when large rock fragments scrape scratches and grooves in the bedrock

Linear features provides evidence for direction of flow

How Glaciers Erode

Rate of glacial erosion depends on

Rate of glacial movement

Thickness of ice

Shape, abundance, and hardness of rock fragments carried in the ice

The erodability of the surface beneath the glacier

How Glaciers Erode

Glacial landforms created by valley (alpine) glaciers are more pronounced than those created by ice sheets

Ice widens, deepens, and straightens valleys into U-shaped glacial troughs

Tributary glaciers create hanging valleys

Cirques are bowl-shaped depressions at the head of a glacial valley

Arêtes are sharp ridges and horns are pyramid-like peaks associated with enlarged cirques

Fiords are deep, steep-sided inlets of the sea

Landforms Created by Glacial Erosion

Glacial Erosion

Distinguish between the two basic types of glacial deposits.

Briefly describe the features associated with each type.

Focus Questions 4.4

Material picked up by glaciers is eventually deposited when they melt

Glacial drift

Any sediment of glacial origin

Till

Material deposited directly by ice when it melts

Stratified drift

Sorted and deposited by glacial meltwater

Glacial erratics

Boulders different from bedrock below found in the till or lying on the surface

Types of Glacial Drift

Moraines are layers or ridges of till

Lateral moraines form along the sides of the valley

Medial moraines form between two advancing glaciers

Dark stripe of debris within the glacier

End moraines form at the terminus of a glacier

Deposited while glacial balance in equilibrium

Ground moraines are gently rolling layers of till deposited as the terminus retreats

Moraines, Outwash Plains, and Kettles

End moraines from the last Ice Age are prominent in the Midwest and Northeast

Kettle Moraine near Milwaukee, Long Island, and Cape Cod

Moraines, Outwash Plains, and Kettles

Braided meltwater streams form a broad ramp of stratified drift

Outwash plains associated with ice sheets

A valley train is confined to a mountain valley

Kettles are basins or depressions in the outwash plain formed by buried ice that eventually melts

Typically <2 km in diameter and <10 m deep

Moraines, Outwash Plains, and Kettles

Drumlins

Streamlined asymmetrical hills made of till

Steep side faces direction of ice advance and gentle side indicates direction of ice flow

Occur in clusters (drumlin fields)

Eskers

Sinuous ridges of sand and gravel made by streams flowing in tunnels underneath the ice

Kames

Steep-sided hills of stratified drift

Glacial Deposits

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

Focus Question 4.5

Forced migration of animals

Alterations in stream courses

Rebounding of land

Ice sheets dam meltwater and create lakes

Proglacial lakes

World-wide change in sea level

Up to 100 m lower during the Ice Age

Pluvial lakes formed during cooler, wetter climates

Other Effects of Ice Age Glaciers

Discuss the extent of glaciation and climate variability during the Quaternary Ice Age.

Focus Question 4.6

Last ice age began between 2 and 3 million years ago during the Quaternary period

Antarctic Ice Sheet formed at least 30 million years ago

Ice sheets and alpine glaciers were far more extensive than they are today

Almost 30% of Earth’s land was glacially influenced

Extent of Ice Age Glaciation

Describe the general distribution and causes of Earth’s dry lands.

Describe the role that water plays in modifying desert landscapes.

Focus Questions 4.7

30% of Earth’s land surface is arid

Affected by many geologic processes

Mountain building, running water, wind

Deserts

Dry climate

Yearly precipitation less than the potential loss of water by evaporation

Desert (arid)

Steppe (semiarid)

Marginal and more humid variant of desert

Transition zone that surrounds the desert

Concentrated in subtropics and middle latitudes

Distributions and Causes of Dry Lands

African, Arabian, and Australian deserts are a result of prevailing winds

Subtropical highs in the lower latitudes

Subsiding air is compressed and warmed

Creates clear skies and ongoing dryness

Middle-latitude deserts and steppes occur in the deep interiors of large landmasses

Little precipitation because of distance to oceans

Example of how geologic processes (mountain building) can affect climate

Distributions and Causes of Dry Lands

Ephemeral streams only carry water during specific rainfall events

Little vegetation to mediate runoff

Flash floods are common

Responsible for most erosion in deserts

Wind primarily transports sediment

The Role of Water in Arid Climates

[insert Figure 4.29 here]

Discuss the stages of landscape evolution in the Basin and Range region of the western United States.

Focus Question 4.8

Regions with internal drainage have ephemeral streams that do not flow out of the basin in to the ocean

E.g., Basin and Range region in western U.S.

Characterized by over 200 small fault-block mountain ranges separated by basins

Water causes erosion following uplift

Basin and Range: The Evolution of a Mountainous Desert Landscape

Occasional heavy rain loads rivers with sediment

Alluvial fans deposited at mouth of a canyon

A bajada is created when several alluvial fans from adjacent canyons merge

A playa lake forms when rainfall is sufficient to cover the basin floor

Salt flats can form when water evaporates

Continued erosion gradually diminishes local relief

Eventually only bedrock knobs called inselbergs remain

Basin and Range: The Evolution of a Mountainous Desert Landscape

Describe the ways in which wind transports sediment and the features created by wind erosion.

Distinguish between two basic types of wind deposits.

Focus Questions 4.9

Moving air can pick up and transport loose material

Similar to a river

Velocity of wind increases with height above surface

Transports fine particles in suspension and heavier particles as bed load

Different from a river

Less capable of transporting coarse material

Not confined to a channel

Relatively insignificant erosional agent

Wind Erosion

Deflation

Lifting and removal of loose material

Clay and silt only

Saltation

Rolling or skipping of larger particles along the surface

Blowouts

Shallow depressions caused by deflation

Desert pavement

Stony veneer left behind after deflation removes finer material

Wind Erosion

Wind can also erode via abrasion

Occurs in dry regions and along some beaches

Windblown sand polishes exposed rock surfaces

Generally <1 m above the surface

Wind Erosion

Generally two distinctive types:

Extensive blankets of silt from suspended load called loess

Mounds and ridges of sand from bed load called dunes

Wind Deposits

Loess is windblown silt

Tends to erode in vertical cliffs

Lacks bedding

Deserts and glacial deposits of stratified drift are primary sources of silt

Wind Deposits

Sand accumulates in mounds and ridges where the path of wind is obstructed

Many dunes have asymmetrical profiles

Leeward (sheltered) slope is steep and windward slope is gently inclined

Sand accumulates on the slip face (leeward side) because wind velocity is reduced just beyond the crest of the dune

Dunes migrate slowly in windward direction

Inclined layers in the windward direction are called cross bedding

Wind Deposits