Ice Ages

15.3

1. Ice Agesa.An ice age, or glacial period, is a long period of

climatic cooling during which continental ice sheets cover large areas of Earth’s surface.

b.Interglacial periods are the times of warmer temperatures between the colder glacial periods.

i. Scientists have found evidence of several major glacial periods dating from 600 million years ago to as recent as 114,000 years ago, and the ice sheets started to retreat only 11,000 years ago.

Ice Ages

2. Climate

a.Scientists believe that ice ages begin with a long-term decrease in the earth’s average temperature. i.This is a drop of only 5 degrees Celsius. ii.While the temperature was colder during the

last ice age, there is evidence of smaller warming and cooling periods during the ice age.

Climate

3. Glacial Periods

a. During the most recent ice age, approximately one-third of the earth’s land surface was covered with glaciers.

Glacial Periods

Last Ice Age Ice

Coverage

i.There were many ice sheets during the last

ice age. Some ice sheets covered Canada, which were as far south as Missouri and the Ohio River. Some covered the mountainous regions of Alaska, the mountains of the western United States and flowed outward from the Rockies, Cascades, and Sierra Nevada mountain ranges.

Glacial Periods

ii. This was enough ice to decrease the

sea level by 140 meters compared to today.

iii. This major covering of glacial ice formed many of today’s land features.

Glacial Periods

Current Sea Level

Sea Level During Last Ice Age

4. Causes of Ice Ages

a.Many theories have been developed to state a cause for the ice ages, and all have explained gradual cooling and glacial retreat during interglacial periods.

b.The Milankovitch theory is a widely accepted theory among scientists.

Causes of ice Ages

i. This theory proposes that small, regular

changes in the earth’s orbit and tilt of the earth’s axis cause the ice ages.

ii. Three kinds of periodic changes occur in the way the earth moves around the sun.

Causes of Ice Ages

1.One change is in the shape of the earth’s orbit.

a.The orbit varies from nearly circular to more elongated and back to circular about every 100 thousand years.

Causes of Ice Ages

Change in Shape

1.A second change is in the tilt of the earth’s axis.a.Over a period of about 41,000

years, the tilt of the axis varies between 21.5 degrees and 24.5 degrees.

Tilt of Earth’s Axis

Tilt of Earth’s Axis

3. A third periodic change results from the

circular motion, or precession, of the earth’s axis. a.Precession causes the axis to change its

position.b.The axis of the earth traces a complete circle

in space every 26,000 years.

Causes of Ice Ages

Precession

Milankovitch Cycles

iii. Milankovitch calculated how these three

changes periodically change the distribution of solar energy reaching the earth’s surface.

iv. Evidence to support this theory include shells of sea animals and their relation to temperature. 1.Scientists have found that the record of ice ages

found in marine sediments closely follows the cycle of cooling and warming predicted by the Milankovitch theory.

Causes of Ice Ages

c. There are many other theories than the

Milankovitch theory. However, these theories do not give as in depth explanations.i. Some state reasons being changes in solar energy

caused by varying amounts of heat from the sun, volcanic dust blocking the sun’s rays, and tectonic plate motion changing positions of the continents which prevent warm ocean currents from reaching regions near the poles.

Causes of Ice Ages