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chapter_3_modified_lectures_slides.pptxOceanography-Chapter 3
Marine Provinces
The seafloor can be divided into three distinct regions:
Continental Margins
next to a continent
made of granite / continental crust
transition between the continent and the ocean
Deep sea Basins
beneath the open ocean (away from the continent)
built of basalt / oceanic crust
Mid-Ocean Ridges
Volcanic mountain range
built of basalt
Near the center of most ocean basins
Fig. 3.8, p. 80
Continental Margins come in two types:
Active margins (leading edge or Pacific Type) are located at plate boundaries
have earthquakes +/- volcanoes
lead the continent into the plate boundary like the front bumper of a car (leading edge).
mostly around the Pacific Ocean (Pacific Type)
Passive margins (trailing edge or Atlantic Type) are not at plate boundaries
no earthquakes or volcanoes
trail behind the continent like the back bumper of a car (trailing edge).
mostly around the Atlantic Ocean (Atlantic Type)
Fig. 3.9, p. 81
Parts of a Passive Continental Margin
Abyssal Plain
Figure 3.10 showing the internal structure of a Passive Continental Margin.
Parts of a typical passive continental margin
The continental shelf = shallow, flat area just offshore.
Average slope ≈0.1°
Up to 350 km (220 mi) wide
Ends at shelf break 135 m = 443 ft deep worldwide.
The continental shelf is believed to have formed by erosion and deposition while exposed above water during glacial low sea level stand.
The continental slope = a “steeply” sloped region extending seaward from the shelf break.
Average slope ≈ 4° (too steep for sediments to stick)
Extends to an average depth of 4 km or 2.4 miles (varies between 1 – 5 km).
The Continental Rise is found only at Passive Margins.
Thousands of feet of sediments deposited on oceanic crust.
Slope = < 1°
Average depth = 4 – 5 Km (2.4 – 3 miles)
Figure 3.10
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Active Continental Margins = Plate Boundaries.
Because of plate tectonics, active margins look different than passive margins.
We can define two types of Active Continental Margins:
Convergent Active Margins (e.g. west coast of South America, or coast of Oregon and Washington).
Transform Active Margins (e.g. southern and central California coast).
Convergent Active Continental Margins compared to Passive Margins
Continental Shelf=> These are narrower and more deformed than at passive margins
Shelf Break at 140 m or 460 feet
Continental Slope => steeper and longer than at passive margins, ending at 8 – 12 km
Deep Sea Trench instead of a continental rise => these extend up to 12 km (7 miles) deep. Note, your book includes these with the deep sea floor.
Active Continental Margins have:
Continental Crust
Oceanic Crust
Deep Sea Trenches mark ocean-ocean and ocean-continent convergent plate boundaries => subduction of oceanic crust.
Deep sea trenches trap and subduct sediment as it slides down the continental slope, so no continental rise forms on passive continental margins.
Figure 3.17
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Figure 3.16
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Transform Active Continental Margins do not have a deep sea trench!
They do have Continental Borderland -> a very bumpy continental shelf with lots of faults and islands.
Submarine Canyons are “narrow”, v-shaped features most often found on passive continental margins. A few small canyons exist on active continental margins. Submarine canyons are only narrow when compared to the vast size of the oceans. Our very own Monterey Submarine Canyon (a relatively small example) is every bit as spectacular as the Grand Canyon.
Most submarine canyons start just offshore of large rivers, and act as “pipes” carrying continental sediments to the deep sea floor.
We think the canyons originated as rivers when sea level was lower.
Submarine canyons continue to remain open and even get deeper due to the erosive action of the turbidity flows that
move sediments through them!
http://www.dkimages.com/discover/Home/Science/Earth-Sciences/Hydrologic-Sciences/Oceans-and-Seas/Ocean-Floor/Submarine-Canyons-and-Deep-sea-/Submarine-Canyons-and-De-3.html
Garrison, 2012, Essentials of Oceanopgraphy
Turbidity currents = underwater landslides Move fast down the steep slope, slow down near the flatter deep sea. Graded bedding – steep on the bottom of the layer, fining upward.
Wicander and Monroe, 2008, Essentials of Geology
Submarine Fan = a thick, fan-shaped deposit of turbidity deposits found on the seafloor at the mouth of a submarine canyon. 1000’s of meters thick
On passive margins these merge to form the continental rise.
But how can we continue to deposit thousands of meters of sediments into an ocean basin that is only 5 km deep without filling it up?
Remember isostasy?
Added sediments = added weight
Mantle displaced to balance, and the crust sinks.
Less weight here
More weight here
Less weight here
More weight here
Garrison, 2012, Essentials of Oceanography
Features of the Deep Sea Floor Abyssal Plains are flat sediment draped areas of the deep sea floor that are only found next to passive continental margins. Abyssal Plains also form as turbidity currents or underwater landslides bring sediments down the continental slope and rise.
Features of the Deep Sea Basin:
Abyssal hills, seamounts, guyots (gee-o) and islands
Volcanic in origin
The name depends on height and shape
Abyssal hills are only a few 100’s of meters tall
Seamounts are several thousand meters tall
Guyot are flat-topped seamounts
Islands are tall enough to stick out of the water.
Islands, seamounts and guyot form as the seafloor moves over a hotspot in the mantle. They are part of the hotspot chains – Hawaii and the Emperor Seamount Chain are examples.
Guyot form when islands are eroded flat by waves and wind while above the water, then sink deep below the surface.
But…
Why do they sink?????????????
Wicander and Monroe, 2008, Essentials of Geology
Isostasy strikes again….
When the volcano is active,
The seafloor is warm = less dense.
Less dense things float higher and
the seafloor is shallower
Once the volcano dies, the seafloor cools = more dense.
More dense things sink deeper, so the seafloor is deeper.
The cooler it is the deeper it sinks.
This takes thousands of years, plenty of time to erode the island before it sinks away.
Hot
Cool
Cold
Features of the Deep Sea Basin:
Mid-ocean Ridges!
Chains of volcanic mountains that run through all ocean basins like the seams on a baseball
Mid-ocean Ridges = Divergent Plate Boundaries
Chains of volcanic mountains that run through all ocean basins like the seams on a baseball
Up to 2000 km (1200 miles) wide
2-3 km (1.2 – 1.8 miles tall
May have a central rift valley < 2 km (1.2miles) deep
Interesting fact about Mid-Ocean Ridges:
They are dotted by hydrothermal vents = underwater hot springs
Seawater seeps through cracks in the seafloor,
is heated by underground magma chambers,
and makes its way back to the seafloor, dissolving minerals
Once erupted at the seafloor, the dissolved minerals reform and create weird and fantastic shapes called black smokers.
The mineral-rich waters support an ecosystem based on chemosynthesis or chemical energy (not the sun)
Ecosystems based on chemosynthesis or chemical energy (not photosynthesis)!
Methane “eating” bacteria live inside the clams and tube worms,
providing food for their hosts and obtaining nutrients in return – a symbiotic relationship.
www.botos.com / marine / tube_worm_colony.jpg
http://www3.ncc.edu/faculty/bio/fanellis/biosci119/marineorg.html
http://www.divediscover.sr.unh.edu/images/3747_006lg.jpg
Fracture Zones
Long, linear features that cut across the mid-ocean ridge at right angles
Extend for 1000’s of km across the ocean basin
These are transform plate boundaries
between the offset ridge segments
Away from the ridge, fracture zones are scars of old faulting.
Fig. 3.22, p. 92
