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Lecture 4

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Environmental Science
Nick Eyles

EESA06 Lecture 4 Page 1 EESA06 Lecture 4 th Topics for Lecture 4 and for Quiz 3 on Thrusday Feb 10 1) Introduction to mid-ocean ridges: Modern and Ancient “Fossil” MORs (Called Ophiolites) 2) Why are more MORs Fractured? The role of transform faults 3) Iceland: Walking over a Mid-Ocean Ridge 4) Magnetic Stripes Recorded in Ocean Floor Crust and The Age of Ocean Floors 5) Basalt Lava Flows: Fissure Eruptions 6) Volcanic Activity Under Glaciers: Jokulhlaups! 7) Geothermal Energy Oceans only widen(rift) to a certain width then they start to close - subduction - cooling away from ridge and thickens at sides - It wants to sink back down to the mantle - History of continent is dictated by ocean history Ophiolites – Ancient pieces of ocean crust - Combining the information can put a picture of history of MORs - Contain rocks that are very mineral-rich Mid Ocean Ridge - Middle of Atlantic, constantly volcanic eruptions - Are not continuous, are fractured Fractures – horizontal lines on MORs - enable ocean floor spreading on a curved earth surface Dykes – result of hot magma intruding into older rock, sheeted Dykes - surrounding rock will be extended - basic fuel behind plate tectonics Plumes – supply the large amounts of magma How Oceans Widen: Continuous intrusion of dykes - continuous cooling of margins of magma chamber and continuous intrusion of new magma at center - symmetrical distribution on each side Pillow Basalt – pillowed lava, rounded masses where basalt magma was cooled very quickly Gabbro – chemically equivalent of basalt but has larger crystals since it cools slowly, igneous rock Peridotite – dominant mineral is olivine (Peridot), the mantle is made out of peridotite Distill – Gabbro and Basalt is distilled from huge mantle plumes Ridge Push- Hot magma pushes up and dykes intrude Slab pull - Ocean floor sinks under its own weight Continental crust - is the product of melting ocean crust plus sea water 1 EESA06 Lecture 4 Page 2 Ma – Mega-annum (million) Ga – Giga-annum (billion) ka – kilo-annum (thousands of years) 750Ma to 350Ma - closing of an ocean – aiapotis - oceanic crust – some gets subducted, some gets pushed up onto land (Himalayas) - Ophiolites – ancient ocean crust that is now on land, fossil ocean crust preserved on land Eastern Canada: Chert (Flint) – sediments that were being slowly deposited on top of MORs, igneous processes, and accumulates Sheeted Dykes – continuous intrusion of basalt Pillow Lava – igneous rock, cooled very quickly Oceanic crust is made of: - Mafic and Ultra-Mafic rock– magnetism and iron, pre-dominant in olivine Contiential crust is made of: - Acidic Rocks – more silica Red – is young Green + Blue is old Youngest Rocks are all along the MOR Pacific ocean can see old rock but not along south American coast - Oceanic crust has already been subducted - Feeding Andian Volcano, has been recycled Maximum age of oceanic crust is: - Late Jurassic (150 mill. years ago) – Pangea broke up Continent underlay by two plumes, push two contintents apart, flood basalts develop into oceans fractures allow spreading on curved surface or when there is weakness in the crust 2 EESA06 Lecture 4 Page 3 Fractures – a crack or break - Black arrows moving in the same direction Transform Faults – movement of rock on either side of a fracture (can be small or b
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