chapter 2 - the sea floor

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CHAPTER 2 THE SEA FLOOR

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about 4.6 billion years old Geology: the science that deals with the

dynamics and physical history of the earth, the rocks of which it is composed, and the physical, chemical, and biological changes that the earth has undergone or is undergoing

The earth is…

Oceans cover 72% of the globe and 80% of that is still considered undiscovered◦ 2/3 of the earth’s land mass is in the Northern

Hemisphere, which is only 61% ocean◦ 80% of the Southern Hemisphere is ocean

Oceans

1. Pacific - Largest and deepest◦ almost as large as the

other 3 combined 2. Atlantic - Second

largest◦ similar to Indian in avg.

depth 3. Indian - Third largest

◦ similar to Atlantic in avg. depth

4. Arctic - Smallest and shallowest

Ocean Basins

The Earth and our solar system is thought to have originated about 4.6 billion years ago from clouds or clouds of dust.

  Big Bang: A great cosmic explosion that is

estimated to have occurred 13.7 billion years ago in which dust particles collided with each other merging into larger particles that again collided with each other eventually building up the earth and other planets

Earth Structure

Density: the mass of a given volume of a substance; Mass/Volume

When the Earth was formed it was most likely molten◦ This allowed the

interior of the Earth to form based on density Fig 2.3

Earth Structure

Core: Innermost layer of the Earth that is composed mainly of iron◦ Pressure at the core is more than a million times

the pressure on the Earth’s surface◦ Made up of a solid inner core and a liquid outer

core◦ It is thought that the swirling motions of the liquid

material in the iron-rich outer core produce the earth’s magnetic field

Internal Earth Structure

Mantle: The layer outside the earth’s core◦ most of it is

thought to be solid, but very hot - near the point of melting rock

◦ much of the mantle slowly flows almost like liquid

Internal Earth Structure

Lithosphere: The fairly rigid layer of the earth’s surface composed of the crust and upper-most part of the mantle.◦ About 100 km (60 mi) thick◦ Means “rock sphere”◦ Broken up into a number of plates called

lithospheric plates Lithospheric Plates: A part of the lithosphere

that can contain continental crust, oceanic crust, or both

Internal Earth Structure

Asthenosphere: The denser, more plastic layer of the upper mantle on which the lithosphere floats

The distinction between the lithosphere and the asthenosphere is based on how easily the rock flows

Internal Earth Structure

Sir Francis Bacon ◦ noted that the coasts of the continents on

opposite sides of the Atlantic fit together like pieces of a giant puzzle

◦ Evidence in coal deposits, geological formations, and fossil findings

Early Evidence of Continental Drift

Mid-ocean Ridge: A continuous chain of submarine volcanic mountains that encircles the globe like the seams on a baseball◦ The largest geological feature on earth◦ Displaced at locations by cracks known as faults◦ Occasionally these mountains rise so high that

they break the surface to form islands such as Iceland and the Azores

◦ Earthquakes tend to cluster near the sites of ridges

Mid-Ocean Ridge

Mid-Atlantic Ridge: a chain of submarine volcanic mountains that runs down the center of the Atlantic Ocean, closely following the curves of the opposing coastlines. ◦ The ridge forms an inverted Y in the Indian Ocean

and runs up the eastern side of the Pacific◦ The main section of ridge in the Eastern Pacific is

called the East Pacific Rise.

Mid-Atlantic Ridge

Surveys of the sea floor also revealed a system of trenches

Trench: A deep depression in the sea floor◦ especially common in the Pacific◦ Usually many volcanoes nearby◦ Lithosphere is destroyed

Trenches

A trench is formed when two plates collide and one of the plates sips below the other and sinks back down into the mantle

Subduction: The downward movement of the plate into the mantle

Trenches are also known as Subduction Zones

Trenches

The nature of the sea floor is related to the mid-ocean ridge

Sea floor rock right near the ridge is very young and the rock gets progressively older moving away from the ridge

Significance of the Mid-Ocean Ridge

From time to time the earth’s magnetic field reverses direction◦ Reversals happen every 700,000 years on average◦ thought to be related to movements of material in

the earth’s outer core Many rocks contain tiny magnetic particles

Magnetic Anomalies: Magnetic bands or “stripes” in the sea floor running parallel to the mid-ocean ridge

Magnetic Anomalies

Magnetic Anomalies

Rifts: Cracks in the oceanic crust separating at the mid-ocean ridges

releases some of the pressure form the underlying mantle

allows hot mantle material to melt and rise up through the rift

magma pushes up the oceanic crust around the rift to form the mid-ocean ridge (Fig. 2.9)

Rifts

Sea-floor Spreading: The entire process by which the sea floor moves away from the mid-ocean ridges to create new sea floor

Also known as spreading centers explains crust and sediment build up, and

magnetic anomalies

Sea Floor Spreading

Continental margins: are the boundaries between continental crust and oceanic crust

The shallowest part of the continental margin is the continental shelf

consists of the shelf to shelf break, continental slope, continental rise to abyssal plains

Continental Margins

Continental shelf make up about 8% of the ocean’s surface

area biologically richest part of the ocean

◦ most life and best fishing◦composed of continental crust

The continental shelf ends at the shelf break where the slope abruptly gets steeper

Continental Margins

Two types of margins Active Margins: zones of intense geological

activity, including earthquakes and volcanoes◦ ex. South America’s western coast

Passive Margins: zones of little geological activity with flat coastal plains, wide shelves and gradual continental slopes◦ ex. South America’s eastern coast

Continental Margins

Ocean sediment holds many clues to the earth’s past

Most marine sediments are of two basic types:

lithogenous sediment: which is derived from weathering and is usually deposited near coasts

biogenous sediment: which consists of the shells and skeletons of marine animals

Record in the Sediment

Likely leading to an increase in tropical areas and increased rates of desertification, changes in agricultural yields, modification of trade routes, glacier retreats, mass species extinctions and increases in the ranges of disease vectors

Climate Change