The Sea Floor

Physical Geology, Chapter 18

Origin of the Ocean Methods of Studying the Sea Floor Continental Shelves & Continental Slopes Submarine Canyons Passive Continental Margins Active Continental Margins Mid-Oceanic Ridges

form?

Seamounts, Guyots & Aseismic Ridges Reefs Sediments of the Sea Floor Oceanic Crust & Ophiolites The Age of the Sea Floor The Sea Floor & Plate Tectonics

Origin of the Ocean

• Early formation ~ 4 b.y.

• Degassing of Earth’s interior • Comet impacts

Methods of Studying the Sea Floor

• Rock samples – Rock Dredge – Corer – Sea Floor Drilling • Submersibles • Echo Sounder • Multibeam Sonar • Sidescan Sonar • Seismic Reflection Profiler • Magnetic Surveys • Gravity Surveys • Seismic Refraction Surveys

Features of the Sea Floor

• Passive continental margin (Fig. 18.5) – Continental shelf – Continental slope – Continental rise • Abyssal Plain • Active continental margin (Fig. 18.5) – Continental shelf – Continental slope – Trench • Mid-Oceanic Ridge • Seamounts • Fracture Zones • Submarine canyons • Aseismic ridges

Continental Shelves & Continental Slopes

• •

Continental shelf

platform, 0.1

: shallow, submarine º seaward dip

Continental slope

: 4-5º steep slope from a depth of 100-200 m at edge of continental shelf

Submarine Canyons

Submarine canyons

: V-shaped valleys that run across continental shelves & slopes

Abyssal fans

: fan-shaped deposits of sediment at base of submarine canyons

Turbidity currents

: masses of sediment-laden water pulled downhill by gravity

• •

Passive Continental Margins

Passive continental margin

: – Continental shelf, continental slope, and continental rise. – Extends to abyssal plain at 5 km depth.

Continental rise

0.5

º.

: at base of continental slope, wedge of sediment from continental slope to deep-sea floor, slopes

Passive Continental Margins

Continental Rise: Types of deposition • •

Turbidity currents

– flowing down slope

Contour currents

– flowing along slope

Passive Continental Margins

Abyssal Plains •

Abyssal plains

: – Very flat regions at base of continental rise. – Composed of horizontal sediment layers probably deposited by turbidity currents. – Flattest features on the Earth.

Active Continental Margins

•

Active continental margin

: – Earthquakes, young mountain belt, and volcanoes. – Consists of continental shelf & slope, and oceanic trench. – Lacks continental rise and abyssal plain. – Associated with convergent plate boundaries.

Active Continental Margins

Oceanic Trenches •

Oceanic trench

: – Narrow, deep trough parallel to edge of continent or island arc – Continental slope steepens to 10-15 º – Benioff seismic zone – Volcanoes landward – Low heat flow – Negative gravity anomaly

Mid-Oceanic Ridges

•

Mid-oceanic ridge

: – Undersea mountain range – Basalt – 80,000 km long – 1500-2500 km wide – 2-3 km above ocean floor •

Rift Valley

: – Crust extension – Along ridge crest – 1-2 km deep – Several km wide – Present in Atlantic & Indian Ocean, absent in Pacific Ocean

Mid-Oceanic Ridges

Geologic & Biologic Activity at the Ridges • Geologic Activity at the Ridges – Shallow focus (0-20 km) earthquakes – High heat flow decreasing away from the ridge – Basaltic eruptions – Hot springs (

Black smokers

) • Biologic Activity at the Ridges – Mussels – Crabs – Starfish – Giant white clams – Giant tube worms – Thermophilic bacteria

Europe

India

North America

•

Fracture zones

: – Major lines of weakness of the Earth’s crust – Cross MOR at right angles – Rift valley is offset – May extend onto continents

South America

•

Seamounts

: – Conical undersea mountains that rise 1000 m above the sea floor – Some are islands – 10,000 in W. Pacific

Seamounts, Guyots, & Aseismic Ridges

• •

Guyots

: Flattopped seamounts

Aseismic ridges

associated with earthquakes.

: Submarine ridges not

•

Reefs

Reefs

: – Wave resistant ridges of coral, algae, & other calcareous organisms – Warm, shallow, sunlit, clean water – Reef types • • •

Fringing reefs

: Flat table-like, attached directly to shore

Barrier reefs

: parallel to shore, detached by lagoons

Atolls

: Circular reefs rimming lagoons, surrounded by deep water

Sediments of the Sea Floor

• Basaltic oceanic crust • •

Terrigenous sediment

: – Land-derived sediment.

– Turbidity & contour currents

Pelagic sediment

: – Fine-grained clay & skeletons of microscopic organisms.

– Absent on ridge crests.

Oceanic Crust & Ophiolites

• Oceanic crust is thinner (7 km) and denser (3.3 g/cm 3 ) than continental crust.

• •

Layer 1

: Marine sediment (variable thickness & composition).

Layer 2

: 1.5 km, pillow basalts overlaying basalt dikes (closely spaced, parallel, vertical).

• •

Layer 3

: 5 km sill-like gabbros.

Ophiolite

: Slivers of oceanic crust emplaced on land represented by distinctive rock sequences

Oceanic Crust & Ophiolites

• Pillow basalt from a northern California ophiolite

The Age of the Sea Floor & The Sea Floor and Plate Tectonics • The age of the sea floor – Younger than 200 m.y.

• The sea floor & plate tectonics – Origin of most sea floor features related to plate tectonics (Chapter 19)

End of Chapter 18…