Lecture 05: Ocean Basins. Hypsometric Curve. Consider Ocean Basins: What is the elevation of Chambana?

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Eric Bryan
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~ -3.7 km (well below sea level) Mean land elevation ~840m (~at sea level) Is this

1 Lecture 05: Ocean Basins 1 Hypsometric Curve What is the elevation of Chambana? Shows distribution of surface relative to sea level ~67% below sea level Mean ocean depth ~ -3.7 km (well below sea level) Mean land elevation ~840m (~at sea level) Is this significant? 2 Consider Ocean Basins: Do you have a feel for the shape and bathymetry of the oceans? 3 1

2 Ocean Basins: Shape of Ocean Basins Irregular? Distribution? Depth? This implies the process that forms oceans is complicated. 4 Ocean Basins: Are they isolated/ restricted Where do they connect? 5 Ocean basins: Are They Connected? Look at the shallow shelf (<130 m, in light blue) So, how well do oceans communicate? 6 2

Bathymetry: What do you Expect? Where are the oceans deepest? A. In the center of the basins? B. At the margins of the basins? 7 Bathymetry: What do you Expect?

3 Bathymetry: What do you Expect? Where are the oceans deepest? A. In the center of the basins? B. At the margins of the basins? 7 Bathymetry: What do you Expect? What sort of seafloor topography do you expect? A. Muted (smooth, no extreme relief)? B. Rugged (like the continents)? 8 Bathymetry: Deeps? Shallow? Ridges/Rises, Plateaus, Oddities So, are the processes that form the seafloor complex and worth studying? 9 3

10 Compare continent to ocean topography Oceanic mountains are taller and

Oceanic canyons are deeper and steeper (average). Why? 11 Active vs.

4 Profile of an Ocean Basin If you could walk along the seafloor, what would the topography be like? 10 Compare continent to ocean topography Oceanic mountains are taller and broader! Oceanic plains are wider and flatter! Oceanic canyons are deeper and steeper (average). Why? 11 Active vs. Passive Margins Leading vs. trailing edges of continents. For example, look at South America, and note difference in Shelf width on E and W side of SA 12 4

Passive Margins = trailing edge produced by continental rifting (we will talk about this idea later) Active

61-67 The Shelf is shallow < 140 m Flat, Gentle slope (m = 0.

at Antarctic and Greenland where the shelf break is at ~3-400 m. Why? The Slope is relatively steep (slope = 0.

009) 9 m per km (9 mm/m) Sediment derived from continents cover the rise Turbidites = sediment from turbidity

According the the UN Convention on the Law of the Sea (1982), The continental shelf is defined as the natural

harvest mineral and non-living material in the subsoil of its continental shelf, to the exclusion of others.

5 Passive Margins = trailing edge produced by continental rifting (we will talk about this idea later) Active Margins = Leading edge of a continent 13 The Shelf-Slope-Rise Read pp The Shelf is shallow < 140 m Flat, Gentle slope (m = 0.002) ~ 9 feet per mile or 2mm/m Shelf break = change in slope Marks end of shelf At 140 m depth globally except at Antarctic and Greenland where the shelf break is at ~3-400 m. Why? The Slope is relatively steep (slope = 0.07) 70 m per km (7 cm/m or 370 ft/mi) The Rise has gentle slope (0.009) 9 m per km (9 mm/m) Sediment derived from continents cover the rise Turbidites = sediment from turbidity currents. Sediment covering oceanic Lithosphere! 14 The Shelf is submerged continental lithosphere! According the the UN Convention on the Law of the Sea (1982), The continental shelf is defined as the natural prolongation of the land territory to the continental margin s outer edge Coastal states have the right to harvest mineral and non-living material in the subsoil of its continental shelf, to the exclusion of others. Coastal states also have exclusive control over living resources "attached" to the continental shelf, but not to creatures living in the water column... See the links page for web links to the Law of the Sea The Shelf is exposed occasionally during ice-ages Why/how? So, what happens then? 15 5

The shelf is exposed during ice ages: Rivers flow out onto exposed shelf and cut canyons through the

flooded when sea level rises again: e.g. Hudson R., Chesapeake Bay, Delaware Bay etc.

slope and rise, and out onto abyssal plain.

bacteria and decomposition of organic mater Under pressure and cold T, methane ice is stable

6 The shelf is exposed during ice ages: Rivers flow out onto exposed shelf and cut canyons through the shelf into the slope and deliver sediment to shelf, slope and rise (river channels, flood planes and deltas all move out onto shelf). Sediment derived from the continents and deposited on the shelf is called Neritic sediment Canyons flooded when sea level rises again: e.g. Hudson R., Chesapeake Bay, Delaware Bay etc. 16 Turbidites and Turbidity Currents Neritic sediment builds on shelf Avalanche from shelf across the slope and rise, and out onto abyssal plain. Often triggered by Earthquake or by melting methane ice Methane ice is produced by methanogenic bacteria and decomposition of organic mater Under pressure and cold T, methane ice is stable Turbidity currents supply sediment to deep-sea fans Turbidity current 1 Turbidity current 2 model 17 Deep-sea fans deliver vast quantities of sediment to the seafloor Indus Fan deposited from the Indus river, and the Ganges Fan both draining the Himalaya 18 6

Leading edge of continents Shelf-slope-rise is

the Rise Very narrow or non-existent Covered

wedge of oceanic sediments scrapped from the

19 Trenches Notice how trenches rim the Pacific

derived from continents Notice how deep they are

surface FLAT: < 5 m relief Sediment covered Mud

sediment on the seafloor Thick sediment: >1 km

7 Leading edge of continents Shelf-slope-rise is compressed compared to passive margin Especially the Rise Very narrow or non-existent Covered with Turbidite deposits And the accretionary wedge of oceanic sediments scrapped from the subducting oceanic slab Look at Active Margins: 19 Trenches Notice how trenches rim the Pacific Ocean basin They are a barrier to sediment derived from continents Notice how deep they are 20 Deep: m Vast: cover ~30% of earth surface FLAT: < 5 m relief Sediment covered Mud hills form as currents move and deposit fine sediment on the seafloor Thick sediment: >1 km Extreme pressure 1 atm/10 m atm >7000 lb/in 2 Abyssal Plane 21 7

Abyssal sediment Sediment far from continents is pelagic in origin - it settles from the water

1-2 km thick sediment buries the crustal topography, resulting in very flat abyssal plane.

marine Turbidites Wind blown Some from pelagic organisms Away from margins, shells of pelagic

23 Why are Pacific abyssal planes not covered with thick sediment? A.

8 Abyssal sediment Sediment far from continents is pelagic in origin - it settles from the water column and sinks like a snow fall onto the sea floor. 1-2 km thick sediment buries the crustal topography, resulting in very flat abyssal plane. 22 Sediment on the Abyssal Plane Near passive margins: Dominantly from continents and shallow marine Turbidites Wind blown Some from pelagic organisms Away from margins, shells of pelagic organisms and wind blown sediment become dominant. 23 Why are Pacific abyssal planes not covered with thick sediment? A. Pacific is vast, and planes are far from continents B. Equatorial Pacific has low productivity (few pelagic organisms) C. Trenches surround the Pacific, trapping sediment from continents D. All of the above 24 8

9 Review Questions Will seawater easily flow between the Oceans? Where do they communicate with each other? Describe the seafloor of an ocean - List some features you expect to find. What is the average depth of the oceans? Where is the deepest place in the oceans? Are ocean basins more complicated than you previously thought? What are some complications that you didn t know about previously? Why is there very little erosion of the seafloor? Are the continental shelves always below sea level? What is a turbidite, what may cause one, and why are turbidites important? Compare the sediments of the abyssal plain with those on the continental shelf. How are the sediments covering the Pacific s abyssal plains different from those covering the Atlantic s abyssal plains and why? 25 9

Foundations of Earth Science, 6e Lutgens, Tarbuck, & Tasa

Foundations of Earth Science, 6e Lutgens, Tarbuck, & Tasa Oceans: The Last Frontier Foundations, 6e - Chapter 9 Stan Hatfield Southwestern Illinois College The vast world ocean Earth is often referred