GLACIERS AND GLACIATION - Chapter 11

GLACIERS:

are thick accumulations of ice.
originate on land from the accumulation, compaction, and recrystallization of snow (mineral ice!).
display evidence of movement: ice flows "plastically" and is capable of erosion.
currently cover 10% of earth.

GLACIATION: the term applied to periods of time when glaciers advanced and retreated on different portions of the Earth's surface.

PLEISTOCENE EPOCH - "ICE AGES" or a period of several glaciation episodes in Earth's "recent" geologic past that began roughly 2 million and "ended" roughly 10,000 years ago. "Ice Age"-style glaciation is still in progress in Antarctica, Greenland, and in many other arctic and mountainous regions of the globe.

Causes of Ice Ages remains in debate. Many complex Earth "systems" are involved in the advance and retreat of glaciers including global temperatures, atmospheric patterns, marine circulation patterns, and more. Extraterrestrial factors may also important. Factors relating to the amount of INSOLATION (INcoming SOLar radiATION) are predicted by the "Milankovitch Theory" a theory involving the study of "solar cycles." Factors that influence the amount of "insolation" in polar regions include:

1) shape of earth's orbit
2) changes in the angle of earth's tilt (axis)
3) wobbling axis (precession)

The combination of these cycles produce "rhythms" in the amount of insolation. Other earth processes are affected by to these cycles, including, sea level variations and climatic cycles.

Other possible "trigger" for glaciation may massive volcanism episodes or asteroid collisions?

A "typical" solar cycle results in an Ice Age that lasts approximately 200,000 years (+/- 150,000 years); This is followed by a roughly 20,000 year "warm period." (The last "ice age" ended about 18,000 years ago.)

There have been at least 4 "major" glaciation cycles in the last 1-2 million years, with as many as 16 or more lesser cycles.

The impact of human activity (including the "Greenhouse Effect") on climatic cycles is unknown.

When glaciers "form" sea level goes down. When glaciers "melt" sea level rises. Estimates vary, but the maximum fall of sea level may have been as much as 450 feet (150 meters). When the earth was much warmer before the "ice ages" sea level was anywhere between 50 to 300 feet HIGHER that today.

TYPES OF GLACIERS

ALPINE GLACIERS: also called VALLEY GLACIERS because they flow down valleys between mountains.
ICE SHEETS - glaciers that cover large regions (such as Greenland and Antarctica)
ICE CAPS - Plateaus covered with ice, such as islands in the arctic, parts of Iceland, and British Columbia.
PIEDMONT GLACIERS - where many Alpine Glaciers join together to cover a large area of an adjacent plain (such as in parts of southern Alaska)

FLOW OF GLACIERS

50-60 meter of ice accumulates it will start to flow plastically at base.

CREVASSES form on the surface of glaciers where the flow of ice is hindered by rough terrane on the bedrock. Crevasses can open up suddenly (which sometimes results in the accumulation of DEAD mountain climbers and/or GEOLOGY PROFESSORS!)

RATE OF FLOW measured in feet per year; fast 10-100s feet/day.

SURGE - rapid sudden movement from major snow falls.

PARTS OF A GLACIER

ZONE OF ACCUMULATION - where snow accumulates faster than melts
SNOW LINE - accumulation = melting
ZONE OF WASTAGE - end of glacier where it melts as fast as it advances.

CALVING - where glaciers meet oceans or lakes, icebergs form

EROSION BY GLACIERS

PLUCKING - water freezes in cracks, moving ice breaks it off.
ABRASION - rocks imbedded in the bottom of a glacier grind and polish the bedrock surface.
ROCK FLOUR - fine rock powder caused by abrasion, this fine powder makes melt water white in color.
GLACIAL STRIATIONS - scratches on barren bedrock left by moving ice and rocks. (evidence even on Hawaii, common all over the east).

GLACIAL DEPOSITS

ERRATICS - big blocks or rock left behind by glaciers

DRIFT - glacial deposits (TWO KINDS)

TILL - mixed silt, sand, gravel and boulders (unsorted!); left behind when a glacier melts.
OUTWASH - glacial sediment "sorted" by running water melting from a glacier.

MORAINES - hills of rock debris left by a glacier.

LATERAL MORAINES - along the edges of glaciers
MEDIAL MORAINES - where two glaciers join (also called lateral moraines)
END MORAINES - hills at the end of a glacier (Long Island)
GROUND MORAINE - rough terrane left when glaciers melt
OUTWASH PLAIN - Barren plain below where a glacier melts (Outwash plains are typically cold, gravel-covered, windy, with lots of braided streams choked with sediment.)

LOESS - wind blown silt from an outwash plain (forms dunes); example, upper Missouri River Valley (IOWA) is covered with loess; it weathers into rich soil good for agriculture.

KETTLES - large blocks ice buried by till melt and form ponds.

KAMES - stratified drift deposited by a glacial stream in the form of a small hill (where a creek drains of the end of a glacier).

ESKERS - streams under ice deposit snake-like gravel ridges.

DRUMLINS - gravel piles smoothed over by moving glaciers.

GLACIAL FEATURES IN MOUNTAIN REGIONS

U-SHAPED VALLEY - GLACIAL TROUGH
HANGING VALLEYS - isolate valleys with waterfalls
FJORDS - a drowned valleys (where Hudson) when glaciers melted and sea level rose and flooded the valley after the glaciers melted.
CIRQUE - bowls carved out where ice accumulates
TARN (cirque lake)
HORNS - spikes formed by the joining of cirques
ARETE - a jagged ridge between two valley glaciers

GREAT LAKES - These North American lakes were carved by advancing continental glaciers; many ancient larger lakes drained when ice age glaciers melted causing incredible floods - and may have caused sea level to rise almost over night!

Click here to return to the CLASS Web Page.