60% water. Big Bang: 14,000 millions years ago The Earth originated about 4,500 millions years ago its orbit allows water to exist in a liquid state!

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1 Ch2. The Sea Floor #1 Why geology of the oceans? Marine habitats are directly shaped by geological processes The form of the coastlines The depth of the water Type of bottom (muddy, sandy, rocky) #2 Geological times are very different to biological times #3 Unequal distribution of the Ocean Basins Pacific Ocean equals all the others Oceans work as a big interconnected system 60% water 80% water A) Earth s structure Big Bang: 14,000 millions years ago The Earth originated about 4,500 millions years ago its orbit allows water to exist in a liquid state! The Earth was probably molten because of the amount of heat generated This allowed materials to arrange in layers according to their density Density = Mass / Volume Earth s Internal structure Inner and Outer Core Mixture or alloys of iron Inner core is solid, but outer core is liquid Swirling motion in the outer core: earth s magnetic field Lower and Upper Mantle Near melting point of the rocks Mantle flows very slowly, almost like a liquid Crust Very thin : divided in Oceanic and Continental Crust 1

2 A.1) The Earth s Crust is very thin! Contour lines (isolines, iso=same) are in Km (USGS) 5 x 5 degree gridded crustal model CRUST 5.1 (Mooney et al., 1998) Probably less than 30 km (18 miles) on average! A.2) Continents and sea floor have different composition It is the nature of the rocks that determines the elevation of a particular area of the earth s crust, and therefore whether or not it is covered by water Both, oceanic and continental crust are less dense than the mantle, but they have differences in density between them Sea floor Basaltic rocks (basalt, a type of mineral, dark in color) More density Geologically young (200 millions) Continents Granite rocks (lighter in color ) Less density Geologically old (3,800 millions) These differences will matter when considering how the different Earth plates interact! 2

3 B) Geological History of the Earth Earth is not static and unchanging B.1) Sir Francis Bacon, 1620 Coasts of America and Africa fit together like pieces of a puzzle Evidence that continents had been once joined ---> slowly accumulated Coal deposits and other geological formations are in both sides Fossils collected in opposite sides of the Atlantic also matched B.2) Continental Drift (Alfred Weneger, 1912) The continents move! How is that possible? Sinus Borealis The process includes the entire earth surface But no mechanism, no experiment, no facts could prove that at that time! (1) (2) Tethys sea Only one super continent: Pangaea Only one ocean: Phantalassa A shallow sea, Tethys, separated Africa (1) from Eurasia (2), will eventually become the Mediterranean sea! Pangaea began to break up about million years ago North Atlantic Ocean (3) Laurasia: North America + Eurasia Gondwana: Africa, South America, India, Antarctica, Australia (3) 3

4 Indian ocean appears India migrates towards Asia Africa still attached to South America Antarctica still attached to Australia South Atlantic ocean appears India still migrating towards Asia Antarctica still attached to Australia The collision between India and Asia created the Himalayas Australia is the last continent to separate from Antarctica B.3) Plate Tectonics (1950 and up) Sonar allowed mapping the sea floor Mid-Ocean ridge system Submarine chain of mountains that encircle the world Occasionally the mountains rise so high that give origin to islands e.g. Iceland, Galapagos and the Azores islands Trenches Especially common in the Pacific Ocean 4

5 Plates limits are determined by the location of ridges and trenches Plates = Crust (oceanic & continental) + upper mantle = Lithosphere (60 mi) Caribbean plate 2 Cocos plate 3 Juan de Fuca plate 4 Philippine plate 5 Arabian plate What are the Mid-ocean ridge system and the trenches important? Plates move ones against the others Important geological activity at the ridges and trenches Oceanic Crust disappears (is destroyed) at the trenches New seafloor material is originated at the ridges B.4) Seafloor spreading Mid-ridge system Young rocks are located near the ridge Almost no sediment near the ridge Sediment gets thicker and older moving away from the ridge Cracks or rifts Molten material rises from the mantle Seafloor at the ridge system: bands of material with normal and inverse magnetization Bands are symmetric at each side of the midocean ridge 5

6 B.5) Plates interactions 1: Activity at the Trenches Trenches originate by the collision of two plates, one at least has to be oceanic Continental-oceanic plate Is always the oceanic plate (denser) the one that descends into the mantle Subduction zone Rising magma as a result of melting mantle Earthquakes Occur as the plate sinks and breaks up Oceanic-oceanic plate One of the plates dips beneath the other to form the trench. * The plate melts * Some material returns to the surface * Again, this associated with earthquakes and volcanoes 6

7 Volcanoes Concentrate around the world at the trenches B.6) Plates interactions 2: continental plates Continental-continental plate Two plates of low density, none is subducted Both plates weld and rise The Himalayas! Continental-continental plate NO collision The plates slide past each other San Andreas fault! 7

8 C) The Record in the Sediments 2 main types of sediments in the sea Lithogenous (Geological) Physical and chemical breakdown (weathering) of rocks (mostly continents) Diatoms (algae) Biogenous (Biological) Skeletons and shells of marine microorganisms Coccolithophorids (algae) Radiometric dating methods Methods based on the known, regular decay of certain radioactive elements (isotopes) into other isotopes or "daughter products." By measuring the amount of "parent" and "daughter" products in a rock sample, its approximate age may be calculated Foraminifera (protozoa) Radiolarians (protozoa) D) Geological Provinces in the Oceans Edge between the continental shelf and the continental slope Limit between continental and oceanic crust Thick layer of sediment piled up on the sea floor Shallowest part of the continental margin Ranges from 0.6 to 450 mi 8% of the Ocean s surface accounts for the richest diversity of organisms of the ocean and the best fishing Is the real oceanic crust. Most of the deep-sea floor lies at a depth of 10,000 to 16,500 ft 8

9 Active vs. Passive Continental Margins Intense geological activity Presence of trenches NO continental rise Steep slopes Narrow shelf Minimal geological activity Flat coastal plains Continental rise Gentle slopes Wide shelf 9