geosc 10: geology of the national parks tearing down mountains ii: glaciers, glaciation, & ice...

GEOSC 10: Geology of the National Parks

Tearing Down Mountains II:Glaciers, Glaciation, & Ice Ages

Presented by Dr. Sridhar AnandakrishnanThe Pennsylvania State University

GEOSC 10 - Geology of the National Parks

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 groundremains frozen: Permafrost


Glacier Movement

•Glaciers move and deform because of gravity

GreaterForce

Less Force

Net 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?!).

geosc 10: geology of the national parks tearing down mountains ii: glaciers, glaciation, & ice...

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