Chapter 2

The Sea Floor

Geologic Processes

• Sculpt shorelines• Determine water depth• Control whether bottom is muddy, sandy, or rocky• Create new islands and undersea mountains• Determine the nature of marine habitats

Ocean Basins• World Ocean: all oceans

– 71 % of the planet

• Pacific Ocean– Deepest and largest– 64 mil sq miles– Avg depth= 13,741 ft– Mariana Trench = 36,163ft

• Atlantic Ocean– 33 mil sq miles– Avg depth= 12,258 ft

• Indian Ocean– 28 mil sq miles– Avg depth= 12,704 ft

• Arctic Ocean– 3.7 mil sq miles– Avg depth =4,364ft

History of Earth

Big Bang• ~13.7 billion years ago• Dust particles collided to

form larger planets

Earth• ~4.5 billion years ago• Materials settled

according to density• Densest= center• Lightest = thin crust• Atmosphere and oceans

formed

Internal Structure

Core• Innermost layer• Mostly iron• Pressure is 1 million

times more than the surface

• Over 7,200 degrees F• Solid inner core• Liquid outer core• Produce magnetic field

Mantle• Outside Earth’s core• Solid, but hot• Flows slowly

Crust• Outermost• Floats on mantle

Continental and Oceanic Crusts

Oceanic• Sea floor• Basalt: mineral that

makes up sea floor– Dark in color

• Denser than continental– Floats lower in mantle

• Thinner• Oldest= less than 200

million years old

Continental • Land rocks• Granite: different than

basalt– Lighter in color

• 3.8 billion years old

Evidence of Continental DriftSir Francis Bacon (1620)• Coasts in Atlantic ocean

fit together like a puzzle• Coal deposits match• Fossils similarAlfred Wegner (1912)• Continental drift:

continents were once joined in a single “supercontinent” – Pangaea– 180 million years ago

• Couldn’t explain HOW

Theory of Plate Tectonics

• The main features of plate tectonics are:– The Earth’s surface is covered by a series of

crustal plates– The ocean floors are constantly moving;

spreading in the center and sinking at the edges and being regenerated.

– Convection currents beneath the plates assist movement

– Heat from the mantle drives these currents

Evidence of Plate TechtonicsMid-Ocean Ridge System• Continuous chain of submarine volcanic mountains that

encircles the globe– Largest geologic feature on earth

• Earthquakes clustered at ridge• Volcanoes near trenches• Rock near ridge is young• Further away is older and consists of more sediment

• Runs down the center of the Atlantic– Forms a Y in the Indian Ocean

• main section of ridge in Eastern Pacific

Faults

East Pacific Rise

• Faults: cracks that displace the ridge

• Transform faults: in earth’s crust

• High submarine mountains form islands

Mid-Atlantic Ridge

Trenches

• System of deep depressions in the sea floor– Common in the Pacific

Earth’s Magnetism

• Magnetic field reverses about every 700,000 years– 5,000 years to occur– Movement of material in

outer core

Magnetic Anomalies

Creation of Sea Floor

Sea-Floor Spreading

Changing Oceans

• Lithogenous sediments – derived from the break-down of rocks (weathering)

• Biogenous sediments – derived from the skeletons and shells of marine organisms

Marine Sediments

• Biogenous sediments (composed of microfossils) can reveal information from the past

• Ocean temperature at the time the microfossils were formed can be determined by looking at the ratios of magnesium to calcium or by different ratios of oxygen isotopes

What Can We Learn from Sediments?

• The margins of continents are boundaries between continental crust and oceanic crust

• They generally consist of:– Continental shelf (most landward)– Continental slope – Continental rise (most seaward)

Continental Margins

• Makes up about 8% of the ocean’s surface area• However, it is biologically the richest area of the ocean• The width varies from 1 km (0.6 mi) to 750 km (470 mi)• Shelf ends at shelf break which occurs at a depth of

120 – 400 m (300-1300 ft)

The Continental Shelf

• Can be thought of as the “edge” of a continent• Begins at the shelf break and continues to

deep sea floor• Much steeper than continental shelf and

continental rise

Continental Slope

• Formed by sediments that have been pushed down from continental shelf and slope

• It can be thought of as an underwater river delta (the river in this case is formed of sediments)

Continental Rise

• Passive margins:– Relatively inactive geologically– Characterized by flat, wide coastal plains,

wide continental shelves and gradually sloping continental slopes

– Example: East Coast of US

Types of Margins

Active Margins: Sites of more intense geologic activity

including earthquakes, volcanoes and trenches

Characterized by steep, rocky shorelines, narrow continental shelves and steep continental slopes

Example: West Coast of US

Types of Margins

Climate and Changes in Sea Level

Deep Ocean Basins

Mid-Ocean Ridge

Hydrothermal Vents