When compared to the other planets of the inner solar system, the most
distinctive feature of Earth's geology is plate tectonics, because no
other planet displays it. Plate tectonics is a mechanism, resembling a
conveyor belt, that creates new crust along a system of rifts
circling the planet and destroys older crust along equally long
subduction zones. The Earth's crust is subdivided in slowly moving
'plates', bounded by rifts, subduction zones and faults. Along
these boundaries, the plates slide against, into, or away from each other:
at rifts, plates move apart; at subduction zones they come together. Much
of the seismic activity on our planet is the result of the movement of the
plates at their boundaries.
Figure 3.1: A global view of the oceanic crust, where the colors
from red to blue represent increasing age.
The plates move at a snail's pace of a few centimeters per year, from the
rifts to the subduction zones. At such slow speed, it takes about 200
million years for a plate to completely renew itself. In geological
terms, given the age of the Earth, this is a relatively short time. The
driving force behind the motion of the plates is a system of
convection currents taking place in the viscous semi-solid mantle
underneath the crust. The convection occurs because of the difference in
temperature between the core of the planet and the external crust.
Earth's continents lie on top of the tectonic plates, covering some
35% of the Earth's surface and moving as the plates move. It is tempting
to see the continents as the plates. Such a conception is
incorrect. The tectonic plates make up the Earth's crust, meaning that
not only do the continents rest above them, so too do the Earth's oceans,
separated from the crust by a thin layer of sediment. The average depth
of the oceans in these regions of the planet is about 4000 m. Such
regions afford little purchase to life and support rather limited
ecosystems. However, great expanses of the continents are also submerged
in the oceans. Called continental shelves, these regions are relatively
shallow, < 500m, and allow for the development of the most complex
marine habitats. Examples of continental shelves are the North Sea and
the portion of the Atlantic bordering North America.
The material that makes up the crust of the plates under the continents
and at deep ocean locations is basaltic in nature. When compared to the
rocks comprising the continents, basaltic rocks have a comparatively high
percentage of silicates, containing heavier metals such as iron (Fe) and
magnesium (Mg), thereby making the basaltic rocks denser than those found
in the continents. The basaltic rocks, however, are not as dense as those
found in the mantle. In this respect, the basalt of the tectonic plates
is intermediate in composition between mantle and continental rocks. The
continents are the 'foam' of Earth's crust, always remaining on top of the
plates. The plates, in turn, generally float on top of the mantle, except
in subduction zones, where the basaltic crust plunges hundreds of
kilometers into the mantle before melting and mixing with the surrounding
material.
Examples of Rifts
Though tectonic movement exists throughout the world, on land and under
water, the most obvious examples of the consequences of tectonic movement
of the plates are visible on land. Iceland, for example, is a part of the
mid-Atlantic ridge, a series of interconnected rifts stitched through the
Atlantic Ocean. Though these rifts are active throughout their length, it
is in Iceland that humans can best observe their actions. The eastern
part of Iceland is slowly drifting eastward and the western part westward,
as new crust is formed within the ridge, pushing each side of the ridge
outward. In Iceland's Thingvellir National Park, it is possible to stand
on a sort of no man's land supported by no tectonic plate and to look from
side to side and see the two canyonlike sides of the glacially spreading
plates. The centimeter by centimeter separation of the North American and
Eurasian plates now encompasses a distance of some twenty to thirty
meters.
Figure 3.2: The rift is the section in shadow emerging from the lower right hand
corner. The North American plate is pulling away to the left and the Eurasian
plate to the right.
Sometimes rifts run through continents. An example is the African Rift
Valley of East Africa. The northern edge of the rift is located roughly at
the Sea of Galilee, in Israel. From there, the rift develops southward
along the Dead Sea, the Red Sea, Ethiopia, Kenia, and into Tanzania. One
day the rift will split the African continent in two, though this process
will take millions of years.
Hot Spot Volcanism
Hot spot volcanism, which is common on Venus and Mars, is also
important on Earth as the only kind of volcanism not caused by plate
tectonics. On Earth, hot spot magnetism is manifested in the so-called
"shield" volcanoes, which exist far from continents and erupt an extremely
fluid lava that is basaltic in nature. Hot spot volcanism is the product
of magma corridors, called plumes, which push up through the mantle
and crust. The erupted lava is a mix of rock from the mantle and crust.
The fluidity of the magma results in eruptions that are rather quiescent
in comparison the explosive volcanoes that grow up around boundaries
between tectonic plates.
The shield volcanoes themselves are the products of successive eruptions,
each accumulating on top of the previous, and the entirety spreading in
broad circle with a diameter of many kilometers.
The Hawaii islands provide an excellent example of shield volcanoes. Yet
in this example we find an important difference between shield volcanoes
on other planets and those on Earth. On all three planets, hot spots
remain relatively stationary with respect to the mantle. However, as we
have discussed, one of the unique features of the Earth is that it's crust
moves. Therefore, the entire archipelago of Hawaii was created by a
single hot spot situated deep beneath the Pacific tectonic plate. As the
plate moves, new volcanoes form, spewing lava and creating land masses.
The oldest islands have already passed the hot spot, and all volcanic
activity on those islands has ceased. The hot spot is currently active at
the Kilauea volcano on the Big Island of Hawaii, although a new, submerged
volcano is building a new island just offshore. The Big Island itself was
born only a few million years ago.
Figure 3.3: A view of the main crater of the Kilauea volcano, on the
Big Island of Hawaii.
Yellowstone National Park, and the volcanic region that extends through
much of Idaho in the United States offers another example of hot spot
volcanism. Yellowstone National Park actually rests at the center part of
a large and ancient volcano, sitting on the plume of a hot spot. Contrary
to Hawaii, Yellowstone is in the middle of the North American continent,
and the kind of rocks one can find there range from basaltic to
continental, since the magma has to pass through the continent itself
before reaching the surface. While Yellowstone is dormant today, it was
the site of a large eruption less then one million years ago.
Volcanism and Mountain Building Associated with Plate Tectonics.
The volcanic activity not due to hot spots is correlated with the position
of adjacent tectonic plates, and is unique to Earth. The type of
volcanoes found at the edge of plates depends largely upon the kind of
magma they erupt, which is itself a function of the characteristics of the
plate boundaries.
In areas of subduction where no continent is present, chains of volcanic
islands form not near deep ocean trenches, which mark the location
where the crust of one plate is being subducted under another. In these
situations, most of the basaltic crust eventually melts and mixes with the
mantle; some of the melted crust makes its way back to the crust of the
other plate and forms volcanoes at the boundary. Examples of this type of
volcano can be seen in the South Pacific
In other variations of subduction zones, a continent might find itself at
the plate boundary, with nowhere to go. In these situations, the crust of
the continent is thrusted upward and downward but remains afloat, since it
is less dense than the mantle. Such is the case on the western edge of
South America: the crust is thrusted upward, forming the Andean mountain
chain. Indeed, mountains have 'roots' extending several kilometers lower
than the continental crust nearby. The deformation process, coupled with
the subduction of the heavier material at the boundary, gives rise to
widespread volcanism along the mountain chain.
In still another case that gives rise to volcanic activity, two
continents
collide. Unlike the previous case of subduction, in ths situation the
continents crumple with a force that often creates titanic mountains. The
Himalayas, for example, are the result of the collision between the
Indian Subcontinent and Asia. The collision process is still going on
today, as it has been for the last 25 million years.
Figure 3.4: Mountains formed by the Collision of Continents
Volcanism is also very active around rifts. As we mentioned, Iceland is
located at the mid-Atlantic ridge. The whole island is volcanic in origin,
and still possesses many active volcanoes. Like the volcanoes in Hawaii,
the lava erupted at the rifts is basaltic in nature, and therefore is
quite fluid. Icelandic volcanoes erupt quite frequently but without much
force; eruptions are only really destructive when they melt glaciers
sitting on top, or when a new crack in the island's crust inadvertently
appears in the vicinity of a town.
In locations where a continent is present at the meeting point between
plate boundaries, the associated volcanoes often lead to explosive
eruptions. Because magma made from continental crust is more viscous than
magma from basaltic crust, these types of volcanoes erupt a far more
viscous magma that contains greater quantities of gas. Rather than
erupting quiescently as the Hawaiian volcanoes do, continental volcanoes
often experience a gradual buildup of powerful internal pressure, which
eventually leads to an explosion. After erupting, such volcanoes enter a
dormant stage than can last thousands of years.
Because of their great explosive powers, continental volcanoes formed at
the meeting of two plates are the volcanoes most remembered in history.
Vesuvius, the volcano that destroyed the Roman town of Pompeii in the
1st century AD, is an example. The Vesuvius eruption destroyed Pompei and
spewed immense quantities of hot gas, ashes, and rocks, which all rolled
down to the Tyrrenian coast in a devastating "pyroclastic flow." The
avalanche of scorching hot material covered everything around for tens of
miles. The Island of Santorini, in the Aegean Sea of Greece, was also
nearly wiped out by an eruption about 3,500 years ago.
Figure 3.5: A view of the town of Fira, built around the caldera of the
volcano in Santorini, Greece. The caldera is flooded with the waters of the
Aegean Sea.
Eruptions of continental volcanoes are not rare, as demonstrated in the last
twenty years by the eruptions of Mt. S. Helens, Mt. Pinatubo,
Monserrat Is. and many others.
Water Erosion, Meteoric Cratering, and Mass Extinctions.
A final unique feature of Earth in relation to the other earth-like
planets is the heavy erosion it experiences at the hands of liquid
water and wind. It is possible that water erosion might have been a factor
during some phases of the history of Mars, but not any longer. The only
process still going on the red planet is wind erosion, and, given the
thinness of Mars' atmosphere, it is not a major agent. Conversely, the
Earth's surface has always been largely remodeled by erosion. Erosion is
a powerful reason why the Earth shows signs of so few meteoric craters,
while such geographic features are commonplace on every other solid body
in the solar system.
Figure 3.6: A view of the Grand Canyon, carved by the waters of the
Colorado River.
Let us compare Venus and Earth. Both planets are continually bombarded by
roughly the same number of asteroids in a given period of time. On
Venus, the craters can survive for several hundred million years, because
its surface does not exhibit plate tectonics and there is no water to
erode them away. As far as we know from the data on the surface of Venus,
wind erosion is not a major factor, either. On Earth, water erosion and
plate tectonics eliminate traces of most meteoric craters.
The scientific evidence accumulated in the last decade shows that the
impact of large asteroids against the Earth has functioned as one of the
most important factor in the evolution of life. For instance, scientists
have definitively concluded that the impact of an asteroid about 2
kilometers in diameter some 65 million years age wiped out the dinosaurs
and about one half of the living species on our planet, both in the seas
and on land. As a testament to the powers of erosion and plate tectonics,
the point of impact, on the Northern Yucatan Peninsula in Mexico, shows no
visible traces of the event.
