Transcript Powerpoint Presentation Physical Geology, 10/e

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?
Fracture Zones
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
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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
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Mid-Oceanic Ridge
Seamounts
Fracture Zones
Submarine canyons
Aseismic ridges
Continental Shelves &
Continental Slopes
• Continental shelf:
shallow, submarine
platform, 0.1º 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: at base of continental slope, wedge of
sediment from continental slope to deep-sea floor, slopes
0.5º.
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 10-15º
– Benioff seismic zone
– Volcanoes landward
– Low heat flow
– Negative gravity anomaly
Mid-Oceanic Ridges
• Mid-oceanic ridge:
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Undersea mountain range
Basalt
80,000 km long
1500-2500 km wide
2-3 km above ocean floor
• Rift Valley:
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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
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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:
Submarine ridges not
associated with
earthquakes.
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/cm3) 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
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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…