GEOGRAPHY
Geology and
Landforms
*The Earth is the fifth largest of the planets in the solar system,
ranking below all the gas giants, but above all the rocky planets and
the ice planets. Like all the planets except Mercury and Venus
(and the possible tenth planet), Earth has a moon, and ours is larger
in proportion to the planet than any other moon (except Pluto’s moon,
Charon), and this affects Earth significantly, most notably by causing
tides.
*Earth itself is not quite a perfect sphere; it is 24,900 miles around
the equator, but 24,818 miles around the poles. This is caused,
in part, by the rotation of the earth on its axis, which tends to
spread the mass of the earth towards the centre as it spins.
*About 70% of the Earth is the hydrosphere (the watery part) and 30%
the lithosphere (the land). The air is known as the atmosphere,
and it extends about 1000 miles from the surface, although almost all
the mass of the gases in it is within 6 or 7 miles of the
surface. All the life on Earth makes up the biosphere.
*Earth’s landmasses make up seven continents and thousands of
islands. List the seven traditional continents. Parts of
all the continents extend underwater as shallow areas of sea near the
coastlines. Such an areas is called a continental shelf.
*Earth’s natural features are called landforms. These include
such things as mountains, plateaus, plains, rivers, and valleys.
*Earth’s highest mountain is Mount Everest, whose top is 29,035 feet
(5½ miles) above sea level. The lowest point on land is
the shore of the Dead Sea on the border between Israel and
Jordan. It lies 1,349 feet below sea level. The deepest
point known to man is the bottom of the Mariana Trench, a vast
underwater chasm, near Guam in the Pacific Ocean. It is 35,827
feet (6¾ miles) deep.
*Landforms are created by the building up and breaking down of the
Earth’s surface.
*The Earth’s surface is built up by volcanic activity and tectonic
activity.
*Volcanic activity builds up the Earth simply by depositing lava and
cinders. If this happens with enough intensity or happens for
long enough, it can create mountains (like Mount Saint Helens) and even
islands (like Iceland and Hawaii).
*Tectonic activity refers to the movement of vast plates of the
lithosphere on top of an ocean of magma (hot liquid rock) that lies
below the Earth’s surface.
*The part of the Earth that we see is just the crust, a layer of rock
(with a very thin layer of life on top). The crust is as thin as
two miles thick at the bottom of the oceans, and as thick as 75 miles
in some mountain chains.
*The crust mainly consists of three types of rock: igneous (e.g.
granite, obsidian), sedimentary (e.g. sandstone, shale, limestone), and
metamorphic (e.g. marble).
*The crust sits on top of the mantle, a thick layer of hot, dense
rock. The elements that make up the mantle slowly shift around as
those closer to the centre of the Earth heat up and those near the
surface cool down. The mantle floats on the outer core of the
Earth, which due to the intense heat generated by the pressure of the
mantle on tope of it, is liquid rock.
*At the very centre of the Earth is the inner core, which, although hot
enough to be liquid (it is believed to be over 8,500 degrees Fahrenheit
in the core), is kept solid by the intense pressure on it.
*The crust of the Earth is broken into plates that slowly shift around
on a layer of magma in the mantle, as the mantle shifts around on the
core. Page 39 of the book shows the plates and their general
pattern of movement. Plates move at different velocities, The
African plate moves about 25 mm per year, whereas the Australian plate
moves about 60 mm per year.
*It is believed that all the continents may have once been part of a
supercontinent, known as Pangaea (Greek: ‘all lands’), that
slowly drifted apart over the past 500 million years or so.
*As the plates move, they slide and bump against each other, and
sometimes slip over or under one another. These plate tectonics
create many landforms.
*Sometimes two plates collide. Sometimes, one plate will be
forced deep under the other one into the Earth, where it will melt, and
the magma will be forced up in volcanic activity. This process is
called subduction, and it usually happens when a sea plate meets a land
plate; the Andes were created this way.
*Rather than forcing one plate under the other, the collision of plates
will sometimes simply force one or both to fold or break. Folding
helped form the Appalachian Mountains, and can still be seen in the
geology around here, where the layers of sedimentary rocks were bent.
*Sometimes, plates pushing against each other will cause the land to
break. This is called a fault. In reverse faulting, one
slab of land is forced over another. At other times, the pieces
will slide by each other, although even this causes massive earthquakes
(as along the San Andreas Fault). This is called transform
faulting.
*Other times, plates separate. This process is simply called
spreading. As it happens, magma comes up through the cracks
created in the spreading earth, and eventually forms underwater
mountain ranges and even islands. One such underwater mountain
range is the Mid-Atlantic Ridge; Iceland was formed by the same forces.
*Faulting, and other tectonic activity, can cause earthquakes and
volcanoes. Both are very common around the edge of the Pacific
plate (which is ringed with islands created by plate tectonics).
Because this area experiences so many earthquakes and volcanic
eruptions, it is sometimes known as the Ring of Fire.
*So volcanoes and plate tectonics help build up land, but landforms are
also created and shaped by things that wear down the land. These
are principally grouped as types of weathering and erosion.
*Weathering is not the same as erosion, although they are
similar. Weathering is the breakdown of rocks in place, either
through expansion and contraction of ice or plant roots in them in
them, or through chemical change, like water dissolving limestone.
*Erosion is the degradation of rock by things that come into contact
with it, such as rain, wind, glacial ice, or the pull of gravity on it.
*Wind erosion is worst in dry places where there is not much
vegetation. The wind picks up loose dirt and dust, and blows it
away. This can destroy an area agriculturally; in the 1920s and
1930s, parts of the US Midwest, which had been farmed too much, then
exposed to drought, dried up and blew away, in a phenomenon called the
Dust Bowl. On the other hand, such rich soil that is blown away
can collect elsewhere. Rich soil deposited in a new place by wind
is called loess.
*Glacial erosion brings weight, friction from carried rocks, and the
power of temperature change to bear in altering the landscape.
*Draw a glacier, with all the junk in it, especially the stuff at the
front.
*Glaciers carved out the Great Lakes and the many other lakes of
Minnesota and Manitoba. When they finally receded, glaciers also
left great ridges of stones called moraines. Glaciers also tend
to deposit a lot of rich soil, which is what has made the US Midwest
such a productive region.
*Liquid water, however, is probably the most significant cause of
erosion. It begins when rain flows downhill. Eventually it
wears out the weakest rock and soil, and forms stream beds. These
may eventually deepen into rivers.
*Eventually most rivers flood (usually every spring), and that deposits
sediment on the sides of the river.
*The best soil tends to collect in these valleys as it flows off the
mountains, making bottom land some of the richest in any region.
Therefore, most of the early human civilisations grew up around major
river valleys and their flood plains.