Geography Reference
In-Depth Information
Where the Indian plate meets the Eurasian plate, the
two plates are converging. The Himalaya Mountains on
this plate boundary were built through convergence. The
Himalayas are still lifting, and earthquakes are relatively
common in the region as a result. When an oceanic plate
converges with a continental plate, it is called a subduction
zone. In a subduction zone, the denser oceanic plate sub-
ducts under the continental plate, creating a trench along
the boundary as well as volcanoes and strong earthquakes.
Most of the strongest earthquakes that occur, including
Haiti, Chile, and Japan in 2010 and 2011, happen along
subduction zones.
Where the North American plate meets the Pacifi c
plate, in and near California, the two tectonic plates are
moving past each other, which is called a transform plate
boundary. Earthquakes are also common in this region.
However, volcanoes are not. Compare Figure 13.2 with
Figure 13.5, and look for the correlation between subduc-
tion zones and active volcanoes. Focus on the transform
boundary in and near California. Earthquakes are com-
mon but volcanoes are not. This is because unlike diver-
gent boundaries where a magma source is creating new
crust, often through volcanoes, or a subduction zone
where one plate is being crushed to molten rock under
the heat and pressure of another plate, magma is not com-
mon in transform boundaries. Rather, the plates move
past each other, pressure builds up, and that pressure is
released in earthquakes.
huge quantities. A very long time passed before oxygen
became a substantial gas in the atmosphere. Around
1500 million years ago, green algae started to spread
across Earth's ocean surfaces, and as their colonies grew,
their
photosynthesis
(the conversion of carbon dioxide
and water into carbohydrates and oxygen through the
absorption of sunlight) raised the atmosphere's oxygen
content. About 800 million years ago, the oxygen con-
tent in the atmosphere was about one-twentieth of its
present strength, or just 1 percent of the total. But that
was enough to support the emergence of the fi rst single-
celled animals, the protozoa.
Fire and Ice
Today major volcanic eruptions happen infrequently
enough that they make the news. Krakatoa (1883),
Mount St. Helens (1980), Pinatubo (1991), and Merapi
(2010) took many lives, damaged property, and, in the
case of Pinatubo, even changed global climate slightly. In
2010 a relatively mild eruption of the Icelandic volcano
Eyjafjallajökull spewed enough ash in the air to disrupt
air traffi c across the northern Atlantic for more than a
week. Over the past three decades, ongoing eruptions of
the Kilauea volcano have altered the coastline of the Big
Island of Hawai'i (Fig. 13. 4).
Yet such events are relatively minor compared to
one billion years ago, when Earth's crust was still imma-
ture and subject to huge bursts of volcanic activity. Such
episodes poured incalculable volumes of gases and ash
into the atmosphere, causing
mass depletions
(loss
of diversity through a failure to produce new species)
and contributing to the three
mass extinctions
(mass
destruction of most species) known to have occurred over
the past 500 million years.
The Earth's most recent experience with mass vol-
canism took place between 180 and 160 million years ago,
when the supercontinent Pangaea began to fracture. Lava
poured from fi ssures and vents as South America separated
from Africa and India moved northeast. Skies were black-
ened, the atmosphere choked with ash. Animals responded
as they always have in time of crisis: by migrating, frag-
menting into smaller groups, and speeding up their adap-
tive, evolutionary response. Physical geographers hypoth-
esize that the earliest phase of Pangaea's fragmentation was
also the most violent, that the plate separations that started
it all were driven by built-up, extreme heat below the super-
continent, but that the motion of the plates has since slowed
down. The
Pacifi c Ring of Fire
—an ocean-girdling zone
of crustal instability, volcanism, and earthquakes—is but a
trace of the paroxysm that marked the onset of Pangaea's
breakup (Fig. 13.5). Yet, as we saw with the tsunami in
Japan in 2011, tectonic events have cost millions of humans
their lives and altered the course of history.
Ocean and Atmosphere
Earth is often called the Blue Planet because more than
70 percent of its surface is covered by water and views
from space are dominated by blue hues and swirls of
white clouds. We do not know with any certainty how
Earth acquired its watery cloak or exactly when. Some
scientists hypothesize that the water was originally
trapped inside Earth during its formation and rose to
the surface during the time when heavier constituents
sank to form the core. Others calculate that most of the
water that did reach the surface in this way would have
been evaporated into space by the searing heat then
prevailing, suggesting that another source must be iden-
tifi ed. This has led to the comet hypothesis, which pro-
poses that icy comets bombarded Earth for more than
a billion years while its atmosphere was still thin, accu-
mulating fresh water from space that fi lled the basins in
the formative crust.
Neither do we know precisely how the atmosphere
formed. Originally, the atmosphere was loaded with the
gas carbon dioxide (CO
2
), and if you could have looked
up at the sky it would have been bright red because CO
2
scatters red light. Eventually, however, the primitive
ocean, still heated from below, began to absorb CO
2
in
