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Guide
to Reading 
You might think by now there would be nothing left
to say about water. Not so. This chapter is all about water, but
in its solid state, ice. More specifically, the chapter deals with
glaciers, which are accumulations of snow that recrystallize into
ice and begin to flow. They create an awesome but hostile environment.
If they are widespread, they can dominate conditions over a large
percentage of Earth for thousands of years. If they occupy smaller
areas, they can rework the scenery, by erosion and by deposition,
and create unique landscapes that were first correctly interpreted
by Louis Agassiz in the mid-1800s. The chapter begins by recognizing
his work and continues with a discussion of the nature and characteristics
of ice, its albedo, crystal form, reactions to temperature and pressure
changes, and similarities to metamorphic rocks.
A discussion of ice quite naturally leads to the
many types of glaciers (mountain or alpine, cirque, valley, mountain
ice cap, piedmont, and continental or ice sheet). The author makes
the point that while pieces of glaciers may end up in the sea by
the process of calving off icebergs, all true glaciers originate
and move on land. They begin as accumulations of snow that change
to firn and eventually to ice. (Masses of ice that originate as frozen
seawater are called ice shelves.) Once formed, glaciers move. They
may be wet-bottom or temperate glaciers and move by basal sliding,
or they may be dry-bottom or polar glaciers and move by internal
flow. They may break apart in their upper brittle zone as they move
over uneven ground and create crevasses. Their average speeds may
vary from 10 m to a few hundred meters per year, and they may show
occasional periods of exceptionally fast movement called surging.
The ultimate cause of glacial movement is the pull of gravity, which
may create lateral movement called gravitational spreading or simple
down-valley motion. To complicate the picture, even though glacial
ice always moves in response to gravity, it doesn’t always
appear to keep advancing in the same direction. If it loses enough
ice by melting, sublimation, or calving, its terminus (toe) may retreat,
and its zones of ablation and accumulation may grow or diminish.
Glaciers are powerful agents of erosion. They plow
through the landscape, incorporate and pluck (quarry) fragments from
the bedrock, abrade the land surface, leave chatter marks, gouge
out striations, and grind off rock flour to leave glacially polished
surfaces. In their head (upper) regions, valley glaciers erode mountain
peaks and valleys to create spectacular, jagged scenery composed
of cirques, tarns, arêtes, horns, U-shaped valleys, hanging
valleys, truncated spurs, roche moutonnées, and fjords.
Glaciers also cause major changes in the landscape
by their depositional activities. First-time observers in glacial
areas are often amazed at the amount of sediment and rock associated
with glaciers, at times more obvious than the ice. Sometimes rocks
even take over, and the glaciers become slowly moving jumbles of
rock, impregnated with ice, called rock glaciers. There are always
moraines—lateral, medial, end, terminal, ground, and recessional.
Many other depositional features may occur, including kames, glacial
drift (stratified and unstratified), till, lodgment till, erratics,
drop stones, glacial marine sediment, outwash, glacial lake bed sediment,
varves, drumlins, kettle holes, knob and kettle topography, and eskers.
Not surprisingly, glaciers greatly modify the climate.
Average temperature can be up to 13°C cooler, and strong glacial
winds (catabatic winds) pick up and transport and deposit fine-grained
sediment called loess, creating immediate dusty conditions and future
fertile farmland.
Ice loading causes glacial subsidence, and the removal
of ice causes glacial rebound. Sea level changes drastically when
glaciers tie up great quantities of water, and this affects life
in the area. Lowered sea levels in Pleistocene times exposed land
bridges that allowed life (including humans) to migrate extensively.
(For example, humans crossed the Bering Straits from Asia to Alaska
and eventually spread throughout North America.) Sea level is higher
now than it was in the Pleistocene, and it will get much higher if
all current ice sheets melt. If that happens, coastal areas will
be flooded, numerous new land lakes will form, and stream systems
will be altered. You read of warming times toward the end of the
Pleistocene that resulted in meltwater lakes, pluvial lakes, oversized
valleys, and catastrophic floods like the Great Missoula Flood. Even
areas around but not under the ice (periglacial areas) showed distinctive
features like permafrost, patterned ground, and stone rings.
Roughly the last third of the chapter is devoted
to ice ages, particularly the most recent, Pleistocene Ice Age, which
began about 3 million years ago and ended (if it really did) about
11,000 years ago. You read about the Laurentide, Keewatin, and Cordilleran
ice sheets that covered northern North America, and how ice sheets
covered roughly 30% of all land and greatly affected life on Earth.
Homo sapiens was one of the life forms that had to cope with the
harsh environment.
Why have ice ages happened? Milankovitch’s
ideas (which involve cyclical changes in Earth’s orbit); plate
tectonics phenomena such as shifting continents, mountain building,
and continental rifting; and changes in the amounts of atmospheric
carbon dioxide are all major factors to consider.
Are we still in an ice age, possibly an interglacial
period of the Pleistocene? Will another great ice age come in the
near future? How do scientists study these issues? In the 1800s Louis
Agassiz could only interpret the local rock record. Scientists today
can study tillites worldwide, apply radiometric dating to glacially
killed trees, study biologic communities like ocean plankton, and
interpret oxygen isotope ratios of marine shells. Data they have
collected have changed some long-held ideas and produced some surprising
theories.
Scientists now believe Earth has experienced four
or five major ice ages, perhaps during the Archean, and definitely
during the early Proterozoic, late Proterozoic, Permian, and Pleistocene.
They no longer think there were just four Pleistocene glaciations
in North America (Nebraskan, Kansan, Illinoian, and Wisconsin), but
instead they think there were at least thirty. The subject is very
complex, and new research has produced more questions than answers
about ice age issues. Equally qualified experts predict totally different
glacial futures for planet Earth, and neither side can absolutely
prove its viewpoint. Only time will tell.
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