Tearing Down Mountains II: Glaciers, Glaciation, & Ice Ages Presented by Dr. Sridhar Anandakrishnan The Pennsylvania State University

Glaciers and Ice Ages What glaciers are Why ice ages (and what evidence for them) Erosion by glaciers

Glaciers A mass of snow and ice that deforms and moves Glaciers form where snowfall exceeds melting, known as the accumulation zone Usually in high, cold mountains and cold North/South But also possible if there is huge amount of snowfall, like in the Olympic Mountains If it is cold, but little snowfall, the ground remains frozen: Permafrost

Glacier Movement Glaciers move and deform because of gravity Net Force Greater Force Less Force

Glacier Movement Glaciers move from where their surface is high to where their surface is low They can even move in an “uphill” direction If you pour pancake batter onto a griddle, it can lap up onto the raised edges...

Glacier Cross Section

Glacier Cycle Ice will flow from high to low spots Remember, high and low refer to the glacier surface Ice will deform most intensely at the bottom, and the upper ice rides on it... so the top of glacier is fast Sometimes the bottom of the glacier is wet and/or soft and the glacier slides

Ablation Zone Ice flow from accumulation zone to ablation zone Ablation = loss of ice by melt or by breaking off of big icebergs (known as calving) Ice always flows down the surface slope Ice flows even though solid because it is “hot” Ice at -30C is close to its melting point (0C) Ice at -30C is far from absolute zero (-273C) Iron at room temp is far from its melting point...

Glacier Erosion Glaciers with lots of water at the bottom are good at eroding Plucking is when the glacier breaks loose small rocks and carries them Abrading is when the glacier drags those small rocks over other rocks and acts like sandpaper Any water flow under the glacier carries away the loose stuff

Smooth on one side (where the glacier came from). Rough on the other side Roche Moutonee (rock sheep)

Rocks are smoothed and scratched by the glacier...

Alpine Landforms U-shaped valleys Hanging valleys Rounded bowls (cirques) Sharp ridges and sharp mountains

Glacier Erosion Glaciers really good at eroding Great lakes, Finger lakes... U-shaped valleys Alpine terrain (cirques, arete, horns...) Streams make a very different landscape Sharp, V-shaped valleys Usually much steeper

Hanging Valley When 2 glaciers meet, the bigger one digs a deeper hole So the smaller one is left perched way up high on the valley wall left behind by the bigger glacier

Alpine Landforms U-shaped valleys Hanging valleys Rounded bowls (cirques) Sharp ridges and sharp mountains

Big Ice 1/10th of land is covered by ice. Mainly Antarctica (South, penguins, etc.) and Greenland (North, polar bears, Inuit, etc..) In the past, 1/3rd of land was ice covered. How do we know that? And why did it happen? And could it happen again?

Lots of Ice 20,000 Years Ago Ice pushed down the Earth in Canada & Scandinavia It is slowly rising after the removal of that weight, 21,000 years ago That much water for the ice must have come from the oceans... ...so the oceans must have had less water, and been lower

How Much? How Long? Ocean water has a lot of oxygen and hydrogen (which make up water - H2O) Most are “normal,” but some have a different atomic weight and are called isotopes Protons - number determine the element Neutrons - number determine the weight Isotopes have fewer (or too many) neutrons

Isotope Ratio There is a certain ratio of number of light to heavy O isotopes (about 1 in 500 is heavy) When water evaporates, the lighter isotopes evaporate more easily When all that water evaporated to make the big ice in Canada, relatively more of the light isotopes evaporate and get trapped on the ice sheet The ratio of light to heavy isotopes changes

The Record The layers of shells alternate between isotopically heavy and light Heavy = light isotopes have evaporated = light isotopes are frozen into ice sheets. And vice versa. For last 800,000 years, we have had 90,000 years of cold, with big ice sheets followed by 10,000 years of warm with small ice sheets

Why? Heat of the sun! The amount of sunlight varies according to: Shape of earth’s orbit (eccentricity, changes on a 100,000 year cycle) Amount of tilt of earth’s axis (obliquity, changes on a 41,000 year cycle) Sirection of tilt of earth’s axis (precession, changes on a 19,000 year cycle)

Ocean Critters Record Changes The oxygen in shells also changes depending on the amount of ice in the Big Ice sheets Those shells fall to the bottom of the ocean and stay there until we dig them up We can find a shell from 100,000 years ago and tell how big the ice sheets were

Milankovich Predicted by Serbian mathematician, Milutin Milankovich in 1920s, long before these isotope records were available! The amount of sunlight in far Northern hemisphere seems to control ice ages. Lots of sunlight in Canada means warm, no ice. Changes in sunlight relatively small, but have big effects. Still don’t understand all the changes. Some very rapid (10s of years?!).