Geology Reference
In-Depth Information
Several important resources, such as common salt,
come from seawater, and hydrocarbons are found in
some seafl oor sediments.
than parts of the planet not covered by water. Nevertheless,
the continents and the ocean basins are very different.
Oceanic crust is thinner and denser than continental crust,
and it is made up of basalt and gabbro, whereas continental
crust has an overall composition similar to that of granite. In
addition, oceanic crust is produced continually at spreading
ridges and consumed at subduction zones, so none of it is
older than about 180 million years. Continental crust, on the
other hand, varies in age from recent to 3.96 billion years old.
One important reason to study the seafloor is that it
makes up the largest part of Earth's surface (
INTRODUCTION
According to two dialogues written in about 350 B . C . by
the Greek philosopher Plato, there was a huge continent
called Atlantis in the Atlantic Ocean west of the Pillars
of Hercules, or what we now call the Strait of Gibraltar
(
Figure 9.1) According to Plato's account, Atlantis con-
trolled a large area extending as far east as Egypt. Yet despite
its vast wealth, advanced technology, and large army and
navy, Atlantis was defeated in war by Athens. Following the
conquest of Atlantis,
Figure 9.2).
Despite the commonly held misconception that the seafl oor
is fl at and featureless, its topography is as varied as that of
the continents. Furthermore, many seafloor features, as
well as several aspects of the oceanic crust, provide impor-
tant evidence for plate tectonic theory (see Chapter 2). And
fi nally, natural resources are found on the marginal parts of
continents, in seawater, and on the seafl oor.
Our discussion of the seafl oor focuses on (1) the physical
attributes and composition of the oceanic crust, (2) the com-
position and distribution of seafl oor sediments, (3) seafl oor
topography, and (4) the origin and evolution of the conti-
nental margins. Oceanographers study these topics, too, but
they also study the chemistry and physics of seawater, as well
as oceanic circulation and marine biology.
there were violent earthquakes and fl oods and one
terrible day and night came when . . . Atlantis . . . dis-
appeared beneath the sea. And for this reason even
now the sea there has become unnavigable and
unsearchable, blocked as it is by the mud shallows
which the island produced as it sank.*
No “mud shallows” exist in the Atlantic, as Plato asserted. In
fact, no geologic evidence indicates that Atlantis ever existed,
so why has the legend persisted for so long?
One reason is that sensational stories of lost civilizations
are popular, but another is that until recently no one had
much knowledge of what lies beneath the oceans. Much of
the seafl oor is a hidden domain, so myths and legends were
widely accepted. The most basic observation we can make
about Earth is that it has vast water-covered areas and conti-
nents, which, at fi rst glance, might seem to be nothing more
EXPLORING THE OCEANS
An interconnected body of saltwater that we call oceans and
seas covers 71% of Earth's surface. Nevertheless, this world
ocean has areas distinct enough for us to recognize the Pacifi c,
Atlantic, Indian, and Arctic Oceans. The term ocean refers to
these large areas of saltwater, whereas
sea designates a smaller body of water,
usually a marginal part of an ocean
(Figure 9.2). We should point out that
whereas the oceans and their marginal
seas are largely underlain by oceanic
crust, the same is not true of the Dead
Sea, Salton Sea, and Caspian Sea; these are
actually saline lakes on the continents.
During most of historic time, peo-
ple knew little of the oceans, and until
recently they thought the seafloor was
a vast, featureless plain. In fact, through
most of this time, the seafloor, in one
sense, was more remote than the Moon's
surface because it could not be observed.
Early Exploration
The ancient Greeks had determined
Earth's size and shape rather accurately,
but western Europeans were not aware
of the vastness of the oceans until the
1400s and 1500s, when explorers sought
trade routes to the Indies. Even when Christopher Columbus set
sail on August 3, 1492, in an effort to fi nd a route to the Indies,
Figure 9.1 Atlantis According to Plato, Atlantis was a continent west of the Pillars of
Hercules, now called the Strait of Gibraltar. In this map from Anthanasium Kircher's Mundus
Subterraneus (1664), north is toward the bottom of the map. The Strait of Gibraltar is the
narrow area between Hispania (Spain) and Africa.
*From the Timaeus, quoted in E. W. Ramage, Ed., Atlantis: Fact or Fiction?
(Bloomington: Indiana University Press, 1978), p. 13.
 
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