Lecture Thirteen Flashcards
What are the three main types of plate boundaries?
Divergent plate boundaries: Plates move apart, creating new ocean floor. Convergent plate boundaries: One plate with oceanic crust at the front collides with another plate with continental curst at the front. The denser oceanic crust subducts beneath the lighter continental crust. Trasform plate boundaries: Two plates slide past each other.
What is a passive margin?
An oceanic-continent boundary that is not a plate boundary is a passive margin.
What are the characteristics of oceanic crust?
7- 10 km thick. Fe, Mg, rich silicate. Mostly basaltic. Density of ~g/cm^3. We call oceanic crust ophiolite. Created at mid ocean ridge due to melting of the asthenosphere. Recycled into the mantle because it is less buoyant than continental crust.
What are oceanic basins?
Oceans occupy 75% of the Earth surface. Oceanic crust occupies 65% of the Earth surface. Compositionally, oceanic crust is comparatively uniform - basalt (with respect to continents (granite)), however the oceans contain a diverse range of morphologic and topographic features.
How do oceanic basins form?
Make the ocean floor: Can be done in two ways - 1) Ocean crust forms at a mid ocean ridge (MOR) = ocean-ocean diverging plate boundary. Ocean crust is being pulled apart and magma rises up from mantle and cools to form oceanic crust. Note: Mantle is not molten, but if you take a solid mantle rock, and reduce the pressure on it, it melts = decompression melting (this happens at mid ocean ridges). 2) Ocean crust also forms when a continent is stretched apart by plate tectonics = continent-continent boundary. Continental crust is pulled apart (rifts) and magma rises up from mantle and cools to form oceanic crust = baby ocean.
What is continental rifting?
Forms a rift valley with a chain of volcanoes in its centre. When mature and wide enough, all central crust has become basaltic ocean crust = baby ocean basin.
What is a mid ocean ridge (MOR)?
Submarine mountain chain, with a central valley full of basaltic volcanism. Difficult to get to, as is usually under ~ 3-4km of ocean. Has composition of basalt, but varies with depth in crust. Can study parts of MOR that have become stranded in the land when continents collide - ophiolites. The height of the mountains at MOR’s are determined by how fast it spreads (fast = low, slow = high). Terms: Graben = rift valley. Dykes = thin sheets of cooled magma injected into other rocks. Gabbro = basalt that cooled slowly in magma changer = big crystals. Peridotite = matel. Sediments = from land and ocean sources. Pillow bassalt = magma erupted into water at MOR.
Why do MOR’s spread?
Gravity. MOR = hot = buoyant = high, so rocks slide away from it = ridge push. Cole and old ocean crust = dense = sinks = subduction = drags rest of the crust away from MOR = slab pull. Spreading makes new sea floor at MOR.
What are black and white smokers?
MOR magma chamber and sea water above it drives a hydothermal circulation system. Seawater is drawn down though the ocean crust, heated, and vented back out. It dissolves chemicals from the rocks, and precipitates them back out into the sea water as black or white ‘smoke.’ Highly acidic water. This water rapidly cools when it comes out at vents, and precipitate all of the dissolved material in mounds and chimneys.
What life is there in the deep ocean?
The heat and every (sulphur and carbon dioxide) supplied by the smoker vents drives life - extremophiles. Communities are generally quite small and localised
What are transform faults and fracture zones?
Transform faults = where crust slides horizontally at a fracture zone. Transform faults occur either at right angles to the plate boundary where they offset mid ocean ridges or parallel to plate boundary.
What is an abyssal plane?
Flat, featurless sediment-covered ocean floor. Average 4-6km deep. The further from the MOR you go, the thicker the sediment is. Sediment travels with the ocean floor until it reaches a trench.
What is a sea mount?
Volcanoes formed over a mantle plume = hot spot = an upwelling of unusually hot mantle. Mantle plumes are thought to come from the core-mantle boundary. They rise because they are hot and buoyant. Hot spots are long lived, and are stationary with respect to over riding tectonic plates. As hot springs are effectively stationary, plates moving over them generate a chain of volcanoes, sea mounts are evidence for plate movement and its direction.
What is a submarine plateau?
Broad regions where ocean is unusually shallow. Associated with hot sports and massive amounts of submarine lava = large igneous provinces (LIPs). Primarily basaltic.
What is continental shelf?
The edge of a continent which is below sea level. Generally less than 400m deep. Slopes downwards, via the continental slope and continental rise to the ocean floor. Rock type gradually changes from granitic (continent) to basaltic (ocean).
What are the ocean basin features?
