Worlds of David Darling > Children's
Encyclopedia of Science > Could You Ever Dig a Hole to China? > 2. Lands
COULD YOU EVER DIG A HOLE TO CHINA?
a book in the Could You Ever? series by David Darling
2. Lands Adrift
Explorers who set out to dig a hole deep into the Earth must first go through
the crust. For such a challenging project, it is important to know as much
as possible about how the crust is made and how it varies from place to
As long ago as the 1600s, something very strange had been noticed about
the world's continents. They look like the pieces of a jigsaw puzzle that
may once have fitted snugly together! The east coast of South America, for
instance, closely matches the west coast of Africa. Could all the continents
have been joined as one great land mass many millions of years ago?
This theory, or idea, began to seem even more correct when huge coal deposits
were found in America that exactly matched deposits in Europe. Then scientists
found another convincing piece of evidence. They discovered fossils of ancient
plants and animals on opposite sides of the Atlantic Ocean at just the places
where the continents could best be fitted together. It seems that, in the
remote past, the land surface of the Earth may have been a single, giant
continent. Over many millions of years, this huge continent broke up and
drifted apart. In 1915, a German scientist Alfred Wegener, argued strongly
that this was the case. But there was a problem with his theory. No one
could explain how Wegener's supercontinent, called Pangaea, could have been
split up into pieces and scattered around the globe.
In 1928, an English scientist, Arthur Holmes, suggested that there might
be currents in a liquid layer of the upper mantle. Holmes’s theory led some
scientists to think that the continents might drift about like rafts on
a partly molten sea of rock.
Later, an American, Harry Hess, took this idea a step farther. He claimed
that the currents in the upper mantle could force up molten rock, known
as MAGMA, through great cracks in the sea floor. As the magma cooled, said
Hess, it would form a strip of dark, heavy rock, called BASALT. That rock
would gradually spread as more magma flowed through the crack. The great
ridges that were known to run the length of the world's oceans were, he
said, the site of this activity.
|New material is added to crustal plates at
mid-ocean ridges where they are moving apart. Ocean plates plunge
down into the mantle where they ram into continental plates.
These startling ideas of CONTINENTAL DRIFT and SEA-FLOOR SPREADING were
not fully accepted until the late 1960s. Then, within the space of a few
years, they were linked together in a grand new theory known as PLATE TECTONICS.
Tectonics comes from a Greek word meaning "builder."
According to this theory, the outer layer of the Earth consists of at least
15 interlocking plates of rock. These slowly glide over the hot, toffee-like
rock of the mantle below. In doing so, they collide, move apart, and slide
past each other, carrying the oceans and continents with them.
|The world's 15 major crustal
Plates are constantly being added to, says the theory; by new rock that
wells up at the cracks in the middle of the oceans. The plates that meet
in the Mid-Atlantic Ridge, for example, are gradually moving apart by about
an inch a year. Since the Earth's crust has to stay the same size, then
plates must be destroyed at an equal rate in other parts of the world.
This destruction happens where an ocean plate rams into a continental plate.
The ocean plate, consisting largely of heavy basalt, plunges down into the
mantle where it eventually melts. At the same time, pieces of the ocean
plate that are shaved off by the continental plate may pile up to form a
range of mountains. The Andes of South America, for instance, came about
in this way and are still growing today. Molten rock from the downward-moving
edge of the ocean plate may also force its way back to the surface through
Everywhere on the Earth's surface where two plates meet, there is violent
activity. Sometimes, instead of colliding head-on, two plates slide past
one another. This happens along the famous San Andreas Fault in California.
Here the giant Pacific Plate rubs against the North American Plate. When
the plates suddenly slip, after having been stuck in one place for a number
of years, a large earthquake rips open the Earth's surface.
On October 17, 1989, movement along the San Andreas Fault near San Francisco
caused widespread damage. A mile-and-a-half elevated stretch of the Nimitz
Freeway collapsed, killing about 50 people who were driving on it at the
time of the quake.
In recent years, views of the Earth from space and the use of computers
to process large amounts of data quickly have greatly improved our knowledge
of plate tectonics. Earthquakes, too, can now be predicted with greater
accuracy. But their exact timing and location can still take scientists
|Vital Statistics of the Earth
about 4.6 billion years
about 6 billion trillion tons
Diameter from pole to pole
Diameter at the equator
Circumference around the poles
Circumference around the equator
Surface area of the land
57,292,000 square miles
Largest single land mass (Eurasia)
20,742,000 square miles
Surface area of the oceans
139,728,000 square miles
Total surface area
Volume of the oceans
308,630,000 cubic miles
Highest point (Mount Everest)
Lowest point (Mariana Trench, Pacific Ocean)
36,228 feet below sea level
Earth's Rocky Shell
Before beginning our journey down into the Earth, we should know more about
the crust. First, it is by no means the same depth at every point. The crust
beneath the ocean is very thin – less than four miles from top to
|The Landsat-1 satellite took this picture
of an area of southern California that includes Los Angeles, the Mojave
Desert, and the San Andreas Fault
Under the continents, though, the crust is at least 22 miles deep. Beneath
large mountain ranges, it may go down 37 miles.
There is also a big difference between the make-up of oceanic and continental
crust. Most of the crust under the ocean is composed of basalt, which contains
silicon, iron, and magnesium. The much thicker crust of continents, on the
other hand, consists mainly of GRANITE. This is a lighter rock rich in silicon
and aluminum. Oceanic crust usually has only a few separate layers, but
continental crust can have a complex structure of many layers.
The crust may seem unimportant compared with the incredibly deep mantle
and core that lie below it. Still, it is the best explored region within
the Earth and also, perhaps, the most varied and interesting. The ways in
which humans, machines and nature itself have penetrated this hard, outer
rocky shell of our planet will help guide future explorers.