Glaciers and Glacial Erosion GLY 2010 Summer 2013

Glaciers and Glacial Erosion – GLY 2010 - Summer 2013 -Lecture 20 • Ice Margin, Commonwealth Glacier, Antarctica 1

Glacier • Moving mass of ice that forms when snowfall exceeds snowmelt over a long period of time • Movement is downhill, due to gravity • Form at or above the snowline, the lowest altitude at which snow commonly forms in the mountains • As climates warm, the snowlines are expected to move higher in elevation, and glaciers to retreat 2

Types of Glaciers 3

Continental Ice Sheet • Large ice masses that blanket a sizable part of a continent • Ice may be kilometers thick, and movement is limited, occurring mainly in local areas or very slowly over time 4

Present Day Continental Ice Sheets • Greenland Antarctica currently are occupied by continental ice sheets 5

Snowfield and Ice Cap • Large mass of snow and ice on a flat surface, topped by recent snow • Ice caps show little movement • They occupy the tops of mountains • Outlet glaciers may flow downward from the ice cap 6

Snowfield and Icecap Above Byrd Glacier, Antarctica • Snow blankets and fills the valleys between the nunataks in the foreground 7

Fox Glacier, New Zealand • Outlet glacier fed by Ice Cap 8

Mountain Valleys • High in the mountains, snow may accumulate • Glaciers are formed in stages § Snow § Firn § Glacial Ice 9

Snow • Snow occurs in many forms, for example wet or dry • Snow accumulates with a great deal of air trapped inside (you need to pack snow to make a decent snowball) 10

Firn • During the first summer following snowfall, some of the snow melts • Meltwaters trickles downward, helping to compact the snow • As winter approaches, resulting mass freezes together to form firn 11

Formation of Glacier Ice 12

Glacial Ice • Repeated years of thaw/freeze cycles, and the weight of accumulated firn and snow, transform the lower layers to ice • There are twelve known structural forms of water ice, at least half of which occur in nature 13

Cirque Glacier • Ice movement erodes a depression near the head of the glacier • Glacier occupies the hole for a prolonged period, creating a bowl-shaped depression under the glacier • After the cirque glacier melts, the depression remains • Cirque may be filled with water, and is called a tarn lake, or it may be dry 14

Cirque Glacier Photo 15

Cirque • Photo by Dr. Michael Hambrey 16

Alpine or Valley Glacier • Confined by surrounding bedrock highlands • Generally move down steep to very steep surfaces • As cirque glaciers expand, they flow into pre-existing stream cut channels, enlarging and changing the shape of these valleys • Stream valleys have V-shaped profiles, whereas glacial valleys are U-shaped 17

Valley Glacier Photo Valley glacier flowing through mountains in Alaska 18

Piedmont Glaciers • One or more valley glaciers flowing from the confines of valley walls and spread out to form broad sheets • Piedmont literally means foot of the mountain • Malaspina Galcier (Alaska) is a classic large piedmont glacier that descends to tidewater from several mountain sources 19

Tidewater Glacier, Alaska • Tidewater glaciers flow into the sea, calving icebergs 20

Calving Glacier Video • Video was shot from a small ship July 1993 about one-half mile from the calving glacier by the instructor © David Warburton, 2006 21

Glacial Zones • • Accumulation Wastage (Ablation) Fracture Flowage 22

Glacial Anatomy 23

Zone of Accumulation • Snowfall exceeds snowmelt, on average over many years • Upper portion of the glacier, at all depths 24

Zone of Wastage (Ablation) • Snowmelt exceeds snowfall, on average over many years • Glacier will retreat unless gravitational movement of glacier downhill replaces glacial ice as fast as it melts • 85% of world’s glaciers are currently retreating 25

Glacial Cross-Section 26

Zone of Fracture • Upper surface of glacier is under little pressure • Near toe of glacier, glacier moves faster, and pulls ahead of the ice uphill • Tensional cracks develop (crevasses) • Ice behaves as a brittle solid 27

Zone of Flowage • Lower part of the glacier, except near the toe, where glacier is thin 28

Glacial Advance and Retreat 29

Crevasses, Exit Glacier, Alaska • Extensional crevasses • Note that they have the same shape as normal fault blocks 30

Inside a Crevasse • Photos by Kristina Ahlnas, University of Alaska • Glacial ice is so blue because the dense ice of the glacier absorbs every other color of the spectrum except blue--so blue is what we see! 31

Meltwater • As glaciers melt, water trickles through the glacier, getting under it • May form under-ice stream channels • Channels form tunnels under the ice • Tunnels emerge at snout of glacier 32

Ice Cave • Video of tidewater glacier, Prince William Sound, Alaska, July, 1993 © David Warburton, 2006 33

Work of Glaciers • Like rivers, glaciers alter the landscape • They may erode the landscape, but they also deposit large amounts of material 34

Glacial Erosion • • Abrasion Quarrying U-shaped valleys Hanging valleys Cirque Arěte Horn Roche moutonnée 35

Abrasion • Rock fragments imbedded in the base of the glacier scrap, and polish underlying rock, and in some cases create long striations (thin) or grooves (thicker) in the bedrock • Stria and grooves indicate the direction of glacier movement 36

Kelley's Island Grooves, Ohio • Grooves may be of glacial or fluvial origin • View is in direction of ice flow 37

Glacial Striations • Striations are smaller than grooves 38

Quarrying • Large masses of bedrock are lifted away from the bed after water from the glacier seeps into cracks and refreezes (frost wedging) • Rock is incorporated into the glacier 39

New Zealand Glacier • Block to the right has been partially quarried 40

U-Shaped Valleys • Glacial ice follows previously cut stream valleys • Stream valleys have V-shaped profiles • Glacial erosion changes the shape to a U 41

Glacial Valley Development I • Typical, meandering V-shaped river valley 42

Glacial Valley Development II • Running water erodes and deepens the V - shape 43

Glacial Valley Development III • Glacier fills the river valley • Channel is widened and straightened 44

Glacial Valley Development IV • Melting of glacier reveals a U – shaped valley 45

Glacial Trough • The U-shaped glacial trough seen here is in Glacier National Park, Montana 46

Hanging Valleys • Where tributary glaciers flow into the trunk glacier, they are often unable to erode as fast as the heavier trunk glacier • When the ice melts, a hanging valley is left 47

Yosemite Valley • Click to see glacial features outlined 48

Waterfalls in a Hanging Valley • Yosemite Falls occupies a glacial valley 49

Cirque • Bowl-shaped depression near the head of an alpine glacier • Cirques remain long after glaciers disappear • May fill with water to become tarn lakes 50

Tarn Lake • Lake Ann, North Cascades National Park, is a tarn lake, occupying a cirque 51

Arěte • Knife-edged ridge of land, formed by parallel erosion of two alpine glaciers • Another visible sign of previous glaciation 52

Horn • A three or four-sided mountain • Cut by glaciers flowing off an isolated mountain in several directions • Matterhorn in Switzerland is an excellent 53 example

Roche Moutonnée • From the French, meaning sheep rock • Glacial abrasion smoothes the slope facing the oncoming ice • Glacial quarrying plucks rocks from the opposite slope, steepening it 54

Liberty Cap, Yosemite National Park • Liberty Cap is a Roche moutonnée 55

Alpine Glacial Erosion 56

Glacial Deposition • • Till Drift Erratics Rock flour 57

Glacial Till • Deposits directly from glacial ice, with no sorting • Light rocks are cobbles and pebbles • Dark tan "matrix" is a mixture of sand, silt, and clay 58

Formation of a Till Deposit 59

Glacial Drift • Includes glacial till, material dumped by glacier when melting • Deposits from meltwater flowing out under a glacier • Meltwater deposits are moderately to well-sorted 60

Rock Flour • Finally ground sediment produced under a glacier • Meltwater streams carry the flour to lakes like this one in Alberta, Canada 61

Moraines • Masses of glacial drift left behind by a glacier • Types of Moraine § Terminal § Lateral § Medial 62

Terminal Moraine • Left at the end of a glaciers advance • May act as a natural earth-fill dam • Multiple terminal moraines may form a series of hills, running parallel to a ridge of mountains 63

Moraine Formation 64

Terminal Moraine Formation • Terminal moraine is a more common name for what they call end moraine 65

Cape Cod From Space • Cape Cod is a terminal moraine • It marks the farthest point that the glaciers reached during the most recent "ice age" in North America 66

Southern New England Moraines 67

Lateral Moraine Photo • Lateral moraines are visible to either side of the glacier Athabaska Glacier, Jasper National Park, Canada 68

Medial Moraine Formation 69

Medial Moraine Photo • Wrangell National Park, Alaska • Lateral moraines merging to form a complex of medial moraines 70

Glacier Junction, Southern Alps, New Zealand 71

Twentymile Glacier, Alaska • Two tributaries meet in a complex of medial moraines 72

Outwash, Tasman Glacier, New Zealand • Running water re-works the glacial gravel into the outwash • Note the car in the lower center of the image 73

Braided Outwash Channels, Toklat River, Alaska • Braided channels are constantly changing 74

Glacial Erratics • Large glacial erratic • Ice is capable of carrying all sizes of material 75

Kettle Lakes • Form when a large chunk of glacial ice is buried as glacier passes over • Later, it melts, and forms a lake • Bear and Nymph Lakes, Rocky Mountain NP 76

Drumlin Aerial View of drumlin field • Gently rounded hills formed by a massive ice sheet (thicker then a glacier) overriding a moraine • Massive ice reshapes the moraine into elongated hills 77

Drumlin Formation 78

Esker Formation • Meltwater streams, flowing within or under active glaciers deposit sand gravel in curving channels • When the glaciers melt the eskers are exposed as topographically positive features • Long, linear features, sometimes in a network 79

Esker Photo • Esker visible as a sinuous ridge (arrow) in this aerial photo 80

Kame • Formed when meltwater washes sediment into openings in a stagnant wasting glacier terminus 81

Glacial Features 82

Glacial Subsidence and Rebound • Continental ice sheets are so heavy they depress the rock under them (subsidence, due to isostasy) • When the ice sheets melt, the land begins to slowly rise • In the Canadian Shield region, the glacial rebound rate is about one foot per century • Rebound is sometimes visible along lake-shores, where older beaches and wave-cut terraces are now considerably higher then the current lakeshore 83

Glacial Isostasy 84

Crustal Depression • Crust bulges on either side of glacier (isostasy) 85

Bulging Produces Raised Beaches 86

Postglacial Effects 87

Creation of Lake Missoula 88

Ice Dam Breaks 89

Bretz Floods • The resulting floods, which recurred numerous times, are called Bretz floods after J Harlan Bretz, who first realized the significance of the gravel ridges meters high, and sometimes with wavelengths of a hundred meters, as Ripple Marks! 90

Giant Ripple Marks • Aerial view of giant ripple marks 91

Catastrophic Flood • Flooding occurred about 13, 000 to 15, 000 years ago 92

Dry Falls • Dry channels occur hundreds of feet above the present river channels, and former cataracts existed in many areas • The best known of these, Dry Falls, is a 3. 5 mile wide former cataract that dropped 400' over vertical cliffs of basalt 93

Scablands • Floods gouged and tore at the land removing surface cover • Left masses of basaltic lava as remnants, like scabs on a wound • Area is now called the channeled scablands 94

Palouse Falls, Washington • Note the very large channel behind the falls 95

Pluvial Lakes • During glacial times, the climate was cooler, and evaporation rates were much lower in arid and semi-arid regions • Many pluvial lakes formed from rainwater which did not evaporate • Pluvial comes from Latin pluvia meaning rain 96