The Seafloor

I. Introduction• The sea floor is geologically distinct from the continents. It is in

a perpetual cycle of birth and destruction that shapes the oceans and controls much of the geology and geological history of the continents.

• Geological processes that occur beneath the waters of the sea affect not only marine life, but dry land as well.

• The processes that mold ocean basins occur slowly, over hundreds of millions of years. Habitats, or places where organisms live, are directly shaped by geological processes. The form of coastlines, the depth of the water, whether the bottom is muddy, sandy, or rocky and many other features are determined by its geology.

The Big Bang • The earth and the rest of the solar system are thought to have

originated about 4.5 billion years ago from a cloud or clouds of dust. This dust was the debris remaining from a huge cosmic explosion called the big bang, which astrophysicists estimate occurred about 15 billion years ago.

• The dust particles collided with each other , merging into larger particles. The larger particles collided in turn, joining into pebble-sized rocks that collided to form larger rocks and so on; eventually building up the earth and other planets

• The young earth was molten and the densest material tended to flow toward the center of the planet, while lighter materials floated toward the surface. The light surface material cooled to make a thin crust. Eventually, the atmosphere and oceans began to form.

The internal structure of the earth reflects the planet’s early beginnings.

• For centuries people viewed the earth as static and unchanging. Evidence of change was all around, however, from earthquakes and volcanic eruptions to the slow erosion of river valleys. Gradually, people began to see that the face of the earth did indeed change and it is in constant transformation.

Continental Drift• As early as 1620 the English philosopher, Sir

Francis Bacon noted that the coasts of the continents on opposite sides of the Atlantic fit together like pieces of a puzzle.

• Moving Continents: Alfred Wegner proposed thatthere was only onesupercontinent named Pangea.

• Alfred Wegener was a German geophysicist and in 1912 he combined the evidence available at the time and proposed the first detailed hypothesis of continental drift. His evidence included:

• 1. Coal deposits, glacial deposits & other geologic formations matched up on opposite sides of the Atlantic.

• 2. Fossils collected on opposing coasts are also similar.

Plate Tectonics• Wegener’s hypothesis was not widely accepted because he

could not supply a plausible mechanism to account for the motion of the continents. But by the late 1950’s and 60’s, scientists concluded that the continents did drift, as part of a process that involved the entire surface of our planet. This process is called plate tectonics.

• The plate tectonic theory states that the earth’s outer shell (lithosphere) is composed of a number of rigid plates that move slowly and change size.

Geological Provinces of the Ocean

• The structure of the ocean floor is dominated by the workings of plate tectonics. Because this is a global process, the major features of the sea floor are quite similar from place to place. The sea floor is divided into two main regions:

• 1. Continental margins• 2. Deep sea floor

A. Continental Margins• Generally consist of a

shallow gently sloping continental shelf, steeper continental slope, and another gently sloping region, the continental rise.