Page 207 - NS-2 Textbook

Earth's Oceanographic History

Our study of oceanography will begm with a discussion oxygen. Because the hydrogen was lightel; much of it es-
of the origin of Earth and its seas. Where did it all caped into space, ,,,,hile the heavier oxygen atoms vvere
begin-how and why? A basic idea of how our planet retained in the atmosphere by gravity. Gradually, poison-
Earth began is essential in our. study of the life-giving ous ammonia and methane gases in the atmosphere were
seas. More about the scientific theory of the formation of dissipated by the S,m as both it and Earth cooled. Slowly,
the universe and the solar system is given in the astron- the atmosphere cooled enough to cause the water ,;apor in
omy unit (unit 4) of this text. the air to condense and return to the surface in the form of
rain. Falling on Earth's hot surface, some water hissed into
steam, joined with new water vapor brought to the surface
FORMATION OF THE OCEANS
by volcanoes, and rose to be condensed and fall again and
Modern science has given scientists a good idea of how again as rain and, later, as snow.
Earth began. This study is a part of astronomy called C08- This continuous precipitation (rain and snow) prob-
lIIologlj, the science concerned with the nature of the uni- ably went on for thousands, maybe millions, of years. Fi-
verse and its origin. Scientists who study cosmology are nally, about 4 billion years ago, Earth had cooled to about
called cosmologists. its present size and temperature. Lighter granite (granitic
Cosmologists believe that what is now our solar sys- rocks) had risen to higher elevations on the surface, and
tem (the Sun, the planets, and their moons) began about the heavier basalt (basaltic rocks) sank, creating high and
4.5 billion years ago as a large cloud of gas and dust. low areas. Eventually most of the low spots in the crust
Gradually, gravity and centripetal forces caused this filled ·with rain,vater. These gigantic ,vater pools eventu-
cloud to spin and take the shape of a huge disk, with the ally formed the world ocean-not in the same geo-
infant Sun in its center. From time to time, eddies, s'\vids} graphic shape we see the oceans today but, nevertheless,
and collisions occurred in this disk, causing a number of covering about 70 percent of Earth's surface.
smaller clusters of materials to separate and whirl in or- TIle cycle of evaporation and condensation continues
bits arolmd the large cluster forming the S,m. One of today, though now only a small percentage of the vapor
these swirling masses became the planet Earth. ascending into the ahnosphere COTIleS fronl volcanoes and
After millions of years of increasing pressure and tem- other cracks in the Earth. Most of the water vapor today
perature, metallic crystals of iron and nickel melted and conles fron1 the ocean surface and trapped ground\vater,
sank toward the core, or center, of the Earth. Because of the which is heated and recycled by the S,m. Over millions of
intense heat created within the Earth by compression, years, the oceans have overrun some coastal edges of the
molten rock (magma) called lava often broke through the early continents as the result of ,vind and ,vater erosion,
surface, either in large cracks in the Earth's crust or in ac- earthquakes, and landslides in those areas. At the same
tive volcanoes that expelled gases and solid materials. The time, the buildup of polar icecaps has kept an almost con-
hydrogen molecules, other gases, and water vapor that es- stant an10lult of water in the seas. Ours is a continually
caped from the Earth gradually rose. The Sun's rays acted changmg geologic world. However, these changes hap-
on the released gases and soon distributed them arOlUld pen too slowly to be seen in the lifetimes of humans, ex-
the new planet to form an atmosphere. Meanwhile, the cept in instances of violent natural change, such as vol-
Earth continued to contract into a lllore solid mass, devel- ·canie eruption or massive earthquake.
oping what is now the planet's crust.
The intense heat created by the compression of the THE EARTH'S CRUST: CONTINENTAL DRIFT
Earth continued to cause thousands of volcanoes to bring
lava and water vapor to the surface. Radiation from the Earth is made up of several "shells," some,vhat like a
Sun also continued to form Earth's atmosphere by breaking golf ball. Earth's core consists of two parts: a solid inner
up water molecules into separate atoms of hydrogen and core of nickel and iron with a diameter of about 860

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