The Ocean Floor

The Ocean Floor

•The World Ocean•Imaging the Ocean Floor•Continental Margins•The Deep-Ocean Floor•Oceanic Ridges•Seafloor Sediments

Terms

PSCI 131: The Ocean Floor

Oceanography: An Interdisciplinary Science

• Geology– Ocean floor, oceanic crust

• Biology– Marine life

• Chemistry– Composition of ocean water

• Physics– Waves, heat transfer, ocean circulation

The World Ocean

PSCI 131: The Ocean Floor

The World OceanPSCI 131: The Ocean Floor

Northern HemisphereSouthern Hemisphere

Earth’s surface is 71% oceanMajority is in Southern Hemisphere

PSCI 131: The Ocean Floor: The World Ocean

PSCI 131: The Ocean Floor: The World Ocean

Oceans vs. Continents• Continents

– Average elev.: about 2800 feet above sea level– Highest point: about 30,000 feet a.s.l.

• Oceans– Average depth: about 12,200 feet– Deepest point: about 36,000 feet

Imaging the Ocean Floor

PSCI 131: The Ocean Floor

PSCI 131: The Ocean Floor: Imaging the Ocean Floor

Ocean Floor Mapping

• HMS Challenger– British– 1872-1876– All oceans except Arctic– Used weighted ropes to find ocean depths

PSCI 131: The Ocean Floor: Imaging the Ocean Floor

Ocean Floor Mapping• HMS Challenger’s route

– British– 1872-1876– All oceans except Arctic– Used weighted ropes to find ocean depths

PSCI 131: The Ocean Floor: Imaging the Ocean Floor

Ocean Floor Mapping• Sonar

– Single beam– Multibeam

PSCI 131: The Ocean Floor: Imaging the Ocean Floor

Ocean Floor Mapping• Sonar

– Travel time of ping / 2 = depth

PSCI 131: The Ocean Floor: Imaging the Ocean Floor

Seismic Reflection Profiles

• Seismic waves penetrate mud, bounce off rock

PSCI 131: The Ocean Floor: Imaging the Ocean Floor

Seismic Reflection Profiles

PSCI 131: The Ocean Floor: Imaging the Ocean Floor

Ocean Floor Provinces• Revealed by ocean floor imaging techniques

• Continental margins– Passive and active

• Deep-ocean floor• Oceanic ridges

PSCI 131: The Ocean Floor: Imaging the Ocean Floor

Ocean Floor Provinces

Continental Margins

PSCI 131: The Ocean Floor

PSCI 131: The Ocean Floor: Continental Margins

Continental Margin Types• Passive

– Little geologic activity– Gentle slope– Flatter coastlines

• Active– Frequent geologic activity– Steeper slope– More rugged coastlines

PSCI 131: The Ocean Floor: Continental Margins

Passive Margins

PSCI 131: The Ocean Floor: Continental Margins

Passive Margins

PSCI 131: The Ocean Floor: Continental Margins

Passive Margin Formation

Crustal stretching & thinning

Initial, narrow ocean basin forms

Mature basin with passive margins

PSCI 131: The Ocean Floor: Continental Margins

Passive Margins: Submarine Canyons

Undersea “landslides” move down continental slopes and cut into shelves to form submarine canyons.

PSCI 131: The Ocean Floor: Continental Margins

Passive Margins: The Hudson submarine canyon

Modern Hudson River mouth

Hudson River mouth during last ice age

PSCI 131: The Ocean Floor: Continental Margins

Active Margins (aka subduction zones)

PSCI 131: The Ocean Floor: Continental Margins

Active Margins (aka subduction zones)

The Deep-Ocean Basin

PSCI 131: The Ocean Floor

PSCI 131: The Ocean Floor: The Deep-Ocean Basin

Key Deep-Ocean Basin Features• Abyssal plains• Trenches• Seamounts

PSCI 131: The Ocean Floor: The Deep-Ocean Basin

Abyssal Plains

• Very flat• Deep sediment

PSCI 131: The Ocean Floor: The Deep-Ocean Basin

Abyssal Plains

Abyssal plains are dark blue

PSCI 131: The Ocean Floor: The Deep-Ocean Basin

Trenches

• Deepest places in the oceans• Subduction-related• Sediment traps

Puerto Rico Trench

Florida

PSCI 131: The Ocean Floor: The Deep-Ocean Basin

Seamounts• Undersea volcanoes• Form islands if peaks are above sea level

– Most are not

Oceanic Ridges

PSCI 131: The Ocean Floor

PSCI 131: The Ocean Floor: Oceanic Ridges

Oceanic Ridges• Elevated, linear features

PSCI 131: The Ocean Floor: Oceanic Ridges

Oceanic Ridges• Also called “spreading centers” or “divergent

plate boundaries”

• Two crustal plates are spreading apart

• New crust formed at center of ridge

PSCI 131: The Ocean Floor: Oceanic Ridges

Diagram of an oceanic ridge

Central rift valley w/ volcanoes

Fault blocks Rising molten rock from mantle

Plate motionPlate motion

PSCI 131: The Ocean Floor: Oceanic Ridges

Oceanic ridge formation

PSCI 131: The Ocean Floor: Oceanic Ridges

Earth’s Largest Topographic Feature• Over 70,000 miles long

Seafloor Sediments

PSCI 131: The Ocean Floor

PSCI 131: The Ocean Floor: Seafloor Sediments

Ocean Sediment Types• Terrigenous (~45% of ocean floor)

– “Terra” = earth– Derived from continents

• Biogenous (~55%)– Created by organisms

• Hydrogenous (<1 %)– Crystallize out of seawater

PSCI 131: The Ocean Floor: Seafloor Sediments

Terrigenous Sediments• Concentrated along continental margins

• Mineral and rock material

• From rivers, wind, glaciers

PSCI 131: The Ocean Floor: Seafloor Sediments

Biogenous Sediments• Concentrated away from continents

• Mainly dead plankton shells

Plankton shells at high magnification

White Cliffs of Dover, England – ancient biogenous seafloor sediment

PSCI 131: The Ocean Floor: Seafloor Sediments

Hydrogenous Sediments• Dispersed throughout ocean and along shorelines

• Chemical precipitation of minerals from seawater

• Common examples– Manganese nodules– Calcium carbonate

Manganese nodules in south Pacific, depth 15,000 feet

End of Chapter

PSCI 131: The Ocean Floor