The Sea Floor

Chapter 2

Continental Drift

• Theory proposed by Alfred Wegner (German geophysicist),

• Stated that all the continents were joined together at one time in to a supercontinent called “Pangea.”

• Pangea began to break apart 180 million years ago.

Pangea

Theory of Plate Tectonics

• Theory that provides the explanation for continental drift.

• It is the process involved in the movement of large plates on the earth’s crust.

Mid Ocean Ridge

• Discovered from sonar images after WWII. • It is a continuous chain of volcanic mountains

that encircles the globe like a baseball’s seams.

• It is the largest geological feature on Earth!!!• The formation was made when

transformation faults were displaced by cracks in the Earth’s crust.

The Sea Floor

• The sea floor is geologically distinct from continental land masses.

• Geological processes sculpt the shoreline, determine water depth, control if the sea bottom is muddy, sandy, or rocky.

• Geological processes create new islands and undersea mountains for colonization and determine specific marine habitats.

Earth is a water planet.

• Oceans cover 71% of the globe.• Oceans regulate Earth’s climate and

atmosphere.• The Northern Hemisphere is 61% covered by

oceans.• The Southern Hemisphere is 80% covered by

oceans.

Ocean Classification

• Oceans are divided into four basins.1) Pacific Basin- deepest and largest2) Atlantic Basin- second largest3) Indian Basin- third largest4) Arctic Basin- smallest and shallowest

Connected to basins are seas.

• Examples of seas are: Mediterranean, Gulf of Mexico, South China, Red, Dead, Baltic, etc.

• All oceans are interconnected and compromise a single “world ocean” referred to as Panthalassa.

• The Southern Ocean is a continuous body of water that surrounds Antarctica.

Internal Earth Structure

The Core

• The innermost portion of Earth.• The inner core is made of solid iron.• The outer core is made of liquid iron that has

a swirling motion.• This motion is responsible for Earth’s

magnetic field.

The Mantle

• Found outside the core.• Most is solid and very hot, near the melting

point of most rocks.• Some flows very slowly like a liquid.

The Crust

• Outermost layer made of thin, rigid rock that floats on top of the mantle.1) Continental Crust- mostly made of granite and is

light in color. It is less dense than oceanic crust so it floats on the mantle. Approx. 3.8 million yr. old.

2) Oceanic Crust- mostly made of basalt and is denser and thinner than continental crust. It does not float as high on the mantle. Aprox. 180 million years old.

Mid Ocean Ridge

Classification

• In the Atlantic- Called Mid Atlantic Ridge• In the Pacific- Called East Pacific Rise

Trenches

• Deep depressions in the sea floor especially common in the Pacific.

Significance of Mid Ocean Ridge

• Lead to the discovery that from time to time the Earth’s magnetic field has reversed direction.

• This happens about every 700,000 years.• Reversal takes 5000 years.• Reversal occurs from the movements of

materials in the Earth’s core.

Magnetic Anomalies

• Geologists found striped patterns of magnetic bands in the sea floor.

• Show alternate periods of normal and reversed periods of earth’s magnetic field.

The Sea Floor

• Created by oceanic crust separated at the mid-ocean ridge allowing some mantle to rise through the rift.

• When the mantle rises through the rift it cools and solidifies to form new oceanic crust.

Sea Floor Spreading

• Process of making new oceanic crust.• What do you remember about the oceanic

crust?

Lithosphere

• Litho = “Rock”• “Rock sphere”• Composed of plates of crust and upper

mantle.• These plates can contain continental/oceanic

crusts or both.• The lithosphere floats on the denser, more

plastic athenosphere.

Plate Movement

Subduction

• The “downward” plate movement into the mantle at a subduction zone to form a “trench.”

• This movement causes earthquakes and when some molten material rises to the surface it may form volcanoes.

• Ex. When oceanic crust subducts under continental crust the Andes mountains of South America were formed.

Subduction

• When two oceanic crusts collide one will subduct below the other and form trenches that lead to the formation of volcanic island chains called “island arcs.”

• Ex: The Aleutian and Marianas Islands

• When two continental plates collide both tend to float and buckle and neither is subducted.

• This forms mountains ranges such as the Himalayas.

• What properties of the continental crust make this happen?

Shear Boundary

• When two plates slide past each other causing extreme friction.

• This stress can cause earthquakes.• Ex: A shear boundary formed the San Andreas

Fault in California.

Shear Boundary

Convection

• When heat from the mantle swirls the plates.• This used to be the main hypothesis of plate

tectonics.• Now it is believed to be an auxiliary form of

plate motion.

Convection

Geologic History

Pangea

• 200 million years ago Pangea was surrounded by Panthalassa.

• What was Panthalassa?• Panthalassa is thought to be the ancestor to

the Pacific ocean.• Tethys Sea- separated Eurasia from Africa (It is

the modern Mediterranean Sea.• Sirius Borealis- is the modern Arctic Ocean.

180 million years ago. . .

• A new rift formed between North America and combined the continents of South America and Africa.

• This rift formed the early Mid-Atlantic Ridge and the North Atlantic Ocean.

• Pangea broke into two continents:1) Laurasia- North America and Eurasia2) Gondwana- South America, Antarctica, India, and

Australia. A rift split at this time forming the Indian Ocean.

Marine Sediments

• Lithogenous- sediments derived from physical and chemical weathering of rocks.

• These are most common.• Ex: red clay on the open ocean floor.

• Biogenous- made of skeletons, shells of marine organisms such as diatoms, radiolarians, foraminiferans, and coccolithophorids.

• Composed of calcium carbonate- sometimes referred to as “calcareous ooze.”

Diatoms Foraminiferans

• Microfossils- microscopic preserved remains of marine organisms that give clues about the type of water the organism lived in such as whether it was cold or warm.

Microfossils

Radiometric Dating

• The use of radioactive isotopes to determine the exact age of a fossil.

• Also referred to as carbon dating.

Climate and Changes in Sea Level

• Earth’s climate fluctuated between warm interglacial periods and cold ice ages.

• During ice ages the sea level falls because water is trapped as glaciers.

Pleistocene Epoch

• Two million years ago- was the last major period of glaciation.

• Sea level has risen over the past 3,000 years due to greenhouse gases such as carbon dioxide and methane.

• These gases are said to cause global warming therefore melting glaciers and subsequently causing a rise in sea level.

Geology of the Sea Floor

The sea floor is divided into two main regions:

• Continental margin- are boundaries between continental and oceanic crust.

• They consist of shallow, gently sloping continental shelf, a steeper continental slope, and a gently sloping continental rise.

Continental shelf

• Shallowest part of the continental margin.• Makes up only 8% of the ocean’s surface area

but are the most biologically rich portion of the ocean.

• When the sea level rises- submarine canyons have formed here.

• The shelf ends at the “shelf break” where the slope gets steeper 120-400m.

Continental Shelf

Continental Slope

• Begins at the shelf break and descends to deep sea floor.

• Submarine canyons cause its depth to reach 3000-5000m.

• The continental slope channels sediments to the sea floor.

Continental Slope

Continental Rise

• When sediment from the submarine canyons accumulates and deep sea fans (deposits that are similar to a river delta) piles on the sea floor.

Continental Rise

Active and Passive Margins

Active Margins

• Continental margins with intense geological activity.

• Earthquakes and volcanoes are characteristic here.

• Have steep, rocky shorelines, narrow continental shelves, steep continental slopes, and lack a developed continental rise.

Active Margin

Passive Margin

• Inactive geologically.• Flat, coastal plains with wide continental

shelves and gradual slopes, leading to a thick continental rise.

Passive Margins

Deep Ocean Basins

• Most of the deep sea floor lies at a depth of 3000-5000m (10,000-16,500 ft).

• Abyssal plain- deep sea floor, rises at gentle slope toward mid-ocean ridge.

• Relatively flat but can contain submarine channels, abyssal hills, plateaus, rises and other features.

Abyssal Plain

Seamounts

• Comprised of volcanic islands and submarine volcanoes.

• Ex: Guyots- flat-topped seamounts that are common in the Pacific.

• These areas have a great diversity of marine life.

Trenches

• Areas where plates descend into mantle.• Are the deepest parts of the ocean.• The deepest is the Mariana Trench in the

Western Pacific at 11, 022m (36,153ft).

Mariana Trench

Mid-Ocean Ridge and Hydrothermal Vents

• The Mid-Ocean Ridge contains a gap called “the central rift valley.”

• Formed when fractures and crevices let seawater seep downward where it is then heated to a very high temperature by the mantle.

• This heated water is forced back through the crust via hydrothermal vents or deep sea hotsprings.

Mid-Ocean Ridge

Hydrothermal Vents

Deep Sea Hot Springs

• Water from hydrothermal vents can be warm, 10-20C (50-68F) or blistering 350C (660F)!

• This hot water dissolves minerals such as sulfides. These minerals cool and form deposits around the vents.

Black Smokers

• One type of deposit. They look like a chimney.• Made of solidified minerals.• The smoke the boils from them is a dense

cloud of mineral particles.

Black Smokers

• Deep Sea hot springs are of interest to geologists and biologists.

• They are an abundant source of marine life.• Many marine organisms are found around

these vents.

Tube worms around a vent.