Transcript Glacier

Lecture Outlines
Physical Geology, 14/e
Plummer, Carlson & Hammersley
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glaciers & Glaciation
Physical Geology 14/e, Chapter 12
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glaciers & Earth’s Systems
Glacier – a large, long-lasting mass of
ice, formed on land, that moves downhill
under its own weight
•part of Earth’s hydrosphere
•along with sea ice, glaciers are known as the
cryosphere
• ~ 75% of the world’s supply of fresh water
is locked up in glacial ice
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Formation of Glaciers
Develop as snow is compacted and
recrystallized, first into firn and then glacial ice
Can only form where more snow accumulates
during the winter than melts away during the
spring and summer
• alpine glaciation occurs in mountainous
regions as valley glaciers
• continental glaciation covers large land
masses in Earth’s polar regions in the form of
ice sheets
• glaciation occurs in areas cold enough to allow
accumulated snow to persist from year to year
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Anatomy of a Glacier
Advancing glacier – gains more
snow than it loses, has a positive budget
Receding glacier – loses more snow
than it gains, has a negative budget
• zone of accumulation – snow added
• zone of ablation – melting and
calving of icebergs
• equilibrium line– separates
accumulation and ablation zones, will
advance or retreat depending on
climate
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Movement of Glaciers
Valley glaciers and ice sheets move
downslope under the force of
gravity
Movement occurs by basal sliding
and plastic flow of the lower part of
the glacier, and passive “riding
along” of an overlying rigid zone
• crevasses – fractures formed in the
upper rigid zone during glacier flow
Due to friction, glacier flow is
fastest at the top center of a glacier
and slowest along its margins
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glacial Erosion
Glaciers erode underlying rock by
plucking of rock fragments and
abrasion as they are dragged along
• basal abrasion polishes and
striates the underlying rock
surface and produces abundant
fine rock powder known as
rock flour
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Erosional Landscapes
U-shaped valleys
Hanging valleys – smaller tributary
glacial valleys left stranded above more
quickly eroded central valleys
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Erosional Landscapes
Cirques – steep-sided, half-bowl-shaped
recesses carved into mountains at the heads
of glacial valleys
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Erosional Landscapes
Arêtes – sharp ridges separating glacial
valleys
Horns – sharp peaks remaining after cirques
have cut back into a mountain on 3+ sides
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Erosional Landscapes
Till – general name for unsorted,
unlayered glacial sediment
• deposits of till left behind at the
sides and end of a glacier are called
lateral, medial and end moraines,
respectively
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glacial Deposition
Lateral moraines – elongate, low
mounds of till along sides of valley
glaciers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glacial Deposition
Medial moraines – lateral moraines
trapped between adjacent ice streams
End moraines – ridges of till piled up
along the front end of a glacier
Recessional moraines – successive
end moraines left behind by a retreating
glacier
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glacial Deposition
Glacial outwash – sediment
deposited by large amounts of liquid
water flowing over, beneath and
away from the ice at the end of a
glacier
• sediment-laden streams
emerging from ends of glaciers
have braided channel
drainage patterns
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glacial Deposition
Outwash landforms include
drumlins, eskers, kettles and
kames
•
annual sediment deposition in
glacial lakes produces varves,
which can be counted like tree
rings
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glacial Ages
In the early 1800s, past extensive glaciation of Europe was first
hypothesized
• hypothesis initially considered outrageous, but further observations by
Louis Agassiz (initially a major opponent of the hypothesis) in Swiss
Alps found much supporting evidence
• Agassiz traveled widely in Europe and North America, finding more
and more supporting evidence, eventually leading to the theory of
glacial ages
Theory of glacial ages states that at times in the past, colder climates
prevailed during which much more of the land surface of Earth
was glaciated than at present
• most recent glacial age was at its peak only ~18,000 years ago
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Direct Effects of Past Glaciation
Large-scale glaciation of North America
during the most recent ice age produced
the following effects:
• most of the soil and sedimentary
rocks were scraped off underlying
crystalline rock in northern and
eastern Canada, and lake basins were
gouged out of the bedrock
• extensive sets of recessional
moraines were left behind by
retreating ice sheets in the upper
midwestern U.S. and Canada
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Indirect Effects of Past Glaciation
Large pluvial lakes (formed in a period of
abundant rainfall) existed in closed basins in
Utah, Nevada and eastern California
• Lake Bonneville
• Lake Missoula
Giant gravel ripples formed during
draining of Lake Missoula
Sea level was significantly lowered by large
amounts of water locked up into ice sheets,
allowing stream channels and glaciers to erode
valleys below present-day sea level
• fiords are coastal inlets formed by drowning
of glacially carved valleys by rising sea level
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Evidence for older Glaciation
Tillites – lithified glacial till, have distinctive
textures that suggest emplacement of sediments
by glaciers
• unsorted rock particles including angular,
faceted and striated boulders
• in some areas, old tillites directly overlie
polished and striated crystalline rocks
• formed during late Paleozoic era in portions
of the southern continents indicate that these
landmasses were once joined
• strong evidence supporting theory of plate
tectonics
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mars on a Glacier
Meteorites - extraterrestrial rocks, fragments of material from space that
have landed on Earth’s surface
• difficult to distinguish from terrestrial rocks
• small number of meteorites appear to have come from the Moon and
Mars
• a large number of meteorites have been concentrated where the
Antarctic ice sheet ablates up against the Transantarctic Mtns.
• several of these appear to have come from Mars, including one that
might bear signs of past life on Mars
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
End of Chapter 12
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.