glaciers & glaciation

Types of glaciersHow do glaciers

move?Glacial landforms

Causes of glaciation

types of glaciers: valley or alpine glaciers found in mountainous

regions

flow in valleys that were previously occupied by rivers

Frequently merge with other valley glaciers

Lengths vary from less than a kilometer to tens of kilometers

Highly effective as erosional agents

Continental ice sheetsWhat’s it like to be on a continental ice sheet?

Here’s a typical view.

These ice sheets develop in part due to the low levels of solar radiation reaching the Earth’s polar regions. (It’s COLD!)

Continental ice sheets cover about 10% of Earth’s land area.

In general, the ice is thickest near the center of the ice sheets and flows outward in all directions from there.

Where ice flows over ocean water ice shelves are formed.

Shelves become thinner seaward and often break off forming icebergs.

Ice shelves are sustained by ice from the mainland as well as freezing seawater from below.

ice sheets on Antarctica & Greenland

What would happen if the ice melted?

Glaciers and ice sheets contain 2% of the world’s total volume of water (fresh plus salt).

Ice sheets contain over 2/3 of the world’s fresh water.If they melted, sea level would rise 60-70 meters!

Consider what would happen to coastal regions and the major metropolitan areas located near them.

What does that mean in the United States?

It all starts with a few snowflakesHere are some glacialogists collecting

samples. The snowflakes shown are photographed shortly after they fell and accumulated on the glacier.

Step 2: conversion of snowflakes into firn

Is this an SEM image of firn or ice worm eggs

Cross sectional view showing layers of snow and firn in a snow field.

Illustration on left shows density of snow, on right density of firn.

What do you think a picture showing the density of ice would look like?

What does glacial ice look like?Is glacial ice a mineral?

Solid?

Naturally occurring?

Inorganic?

Definite composition?

Definite internal structure?

Glacial ice is interlocking crystals of solid H2O that form once the ice and snow thickness reaches 50 m

Note the growth patterns in the glacial ice above

What makes glaciers and ice sheets flow?

Is a glacier homogenous throughout it’s depth?

Upper 40 meters forms the rigid zone

Crevasses form here as glacier stretches to flow over bumps in a valley

What is the maximum depth of a crevasse?

Characteristics of the plastic zoneThe plastic zone begins at a depth of 40 mBonds between layers of ice are weaker than those within a layer, thus the layers flow over each otherBasal slip also contributes to the movement of a glacierMelt water lubricates the bedrock allowing the glacier to slip down the valley

How fast do glaciers flow?Flow rates vary considerably from one glacier to another

Ex. Byrd Glacier, Antarctica has flow rates of about 2 m per day

Flow is often not smooth and continuous

Surges occur, rates of up to 100x normal may occur

Glacial budgetsZone of accumulation is separated from zone of ablation by equilibrium line or snowline zone of accumulation amount of snowfall is greater than melting on an annual basisZone of ablation (wastage) melting is greater than snowfall on an annual basisGlacial budget is the balance between accumulation and ablation

The roar of the thunderous calving of a

glacierGlaciers and ice sheets waste in two ways, melting and calving

Calving is the breaking off of large pieces of ice at the glaciers terminus

Calved pieces form icebergs where the glacier reaches the sea or a lake

How do glaciers alter landscapes?

Glaciers alter landscapes significantly in two ways

Glacial erosion creates many new features

Glacial deposition also creates new landscapes

Let’s look at some of the features created by glaciers

How do glaciers erode? pluckingPlucking occurs when glaciers flow over fractured bedrockLoose rock is then incorporated into the iceFrost wedging increases the effectiveness of pluckingGlaciers aren’t too particular about the size of the sediment transportedIn the illustrations we see a boulder about to be plucked and a slope that shows evidence of plucking

How do glaciers erode? abrasion

Ice containing sediment acts as sandpaper as it flows over bedrock

Striations are long grooves or scratches created as ice flows over bedrock

Landforms associated with glacial erosion

glacial trough

arête & truncated spur

hanging valley

Erosional features

horn, cirque, arête

pater noster lakes

Erosional features

tarn lake in a cirque

col, cirque, arêtes

Erosional features

fjord

roche moutonnee

horn

Glacial driftAll sediments of glacial origin are collectively known as glacial drift

Two categories of drift

Till – sediment directly deposited by glacier

Stratified drift – sediment deposited by glacial melt water

Glacial till & glacial erraticsDeposited directly by glacial ice

Poorly sorted

Grains often show evidence of abrasion

Erratics – boulder sized grains deposited directly by glacial ice

valley/alpine glaciers: moraine

deposits Glacial ice erodes the valley walls incorporating large volumes of rock debris to the edges of the ice

When the ice melts, this debris is deposited along the valley walls forming lateral moraines

medial morainesWhen two tributary glaciers merge, their lateral moraines merge to form a central medial moraine

By counting medial moraines, glaciologists can determine the number of tributary glaciers that have merged to create a larger glacier

terminal, end or recessional moraines

A ridge of till that is deposited at the terminus of a glacier or ice sheet during a time when the glacier is in equilibrium (ablation equals accumulation)Terminal or end moraines are only different from recessional moraines in that they mark the furthest extent of a glacier’s terminus

Terminal, end or recessional moraines

ground morainesA relatively flat layer of till deposited as the terminus of the ice recedesGround moraines tend to level the surface, filing in depressions and clogging stream channels They are common with either valley/alpine glaciers or ice sheets

drumlinsHills of glacial till deposited by a glacier or ice sheet

Range in height from 15 to 50 meters

May be up to 1 km long

Oblong in shape

Steeper side faces the direction the ice advanced from, gentler slope points direction ice flowed toward

Landforms of stratified driftGlacial meltwater sorts and deposits sediment

Commonly sand & gravel sized grains

Rock flour is transported as suspended load

Outwash plains are associated with ice sheets

Valley trains are associated with valley glaciers

Broad, flat surface covered with stratified drift deposits

Kettle lakes, kames, eskersKettle lakes form when blocks of ice remain lodged in drift and eventually melt in the depression they formed

Eskers are ridges deposited beneath glacial ice by sub-glacial streams