Oceanic crust

Life cycle of oceanic crust spreading ridges Ophiolites

1. Life cycle of oceanic crust spreading ridges Ophiolites Geo-Tectonics

2. Khawaja umair majeed roll no 38 section: Morning Semester: 7 th Dr. Raja Jhanzaib

3. Life Cycle of oceanic crust Oceanic crust is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it. The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges. As the mantle rises it cools and melts, as the pressure decreases and it crosses the  solidus. The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr −1  half-rate) produce thinner crust ( 4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at lesser depth, thereby producing less melt and thinner crust.

4. Spreading ridges A spreading ridge is the fracture zone along the ocean bottom where molten mantle material comes to the surface, thus creating new crust. This fracture can be seen beneath the ocean as a line of ridge that form as molten rock reachs the ocean bottom and solidifies. Mid oceanic ridges perfectly falls in this category.

5. Mid Oceanic ridge A long mountain range on the ocean floor, extending almost continuously through the North and South Atlantic Oceans, The Indian Ocean and the South Pacific Ocean. A deep rift valley is located at its center, from which magma flows and forms new oceanic crust. As the magma cools and hardens it becomes part of the mountain range.

6. DEEP SEA TRENCHES Deep-sea trench,  also called  oceanic trench  , any long, narrow, steep-sided depression in the ocean bottom in which occur the maximum oceanic depths, approximately 7,300 to more than 11,000 meters (24,000 to 36,000 feet). They typically form in locations where one tectonic plate subducts under another. The deepest known depression of this kind is the  Mariana Trench , which lies east of the  Mariana Islands  in the western North  Pacific Ocean ; it reaches 11,034 meters (36,200 feet) at its deepest point.

7. Ophiolites An Ophiolite is a section of the Earth's  oceanic crust  and the underlying upper  mantle  that has been uplifted and exposed above sea level and often emplaced onto  continental crustal  rocks.  Ophio  is Greek for snake , and  lite  means  stone  from the Greek  lithos   , after the often-green-color rocks (spilites  and  serpentines) that make up many ophiolites . The term ophiolite was originally used by Alexandre Brongniart for an assemblage of green rocks (serpentine subgroup, diabase) in the Alps; Gustav Steinmann  later modified its use to include serpentine, pillow lava, and chert ("Steinmann's trinity"), again based on occurrences in the Alps.

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