My OperaObduction
Tuesday, January 30, 2007 7:21:04 PM
When continental and oceanic plates collide, the oceanic plate is normally forced downwards beneath the continent because oceanic crust is denser than continental crust. This is called subduction.
However, very rarely an oceanic crust can be forced upwards over a continental plate. This is called obduction. In 2005 I saw obducted oceanic crust in Oman. The Semail Ophiolite. See Photo. (In the foreground desert sand with a few green trees to show the scale).
About 90 million years ago, in late Cretaceous, and before the al-Hajar mountains were formed, an ocean existed to the north of Oman (The Tethyan Ocean). The Arabian Plate (together with the Indian Plate) moved towards the Asian Plate forming a subduction zone in the ocean. The Arabian Plate, including North Oman was pushed under the ocean crust which was part of the Asian Plate. The obducted ocean crust formed the Semail Ophiolite.
On my field sketch from then I have marked the "Moho", short for the Mohorovicic Discontinuity, which is the boundary between the crust and the mantle. Above the Moho you see gabbro, a dark, coarse-grained, intrusive igneous rock with the same chemical composition as basalt, but formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass. The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basalt magmatism at mid-ocean ridges. Harzburgite is a type of peridotite, and peridotite is the dominant rock of the upper part of the Earth's mantle.
In my next blog I shall tell more about oceanic crust and ophiolites.
However, very rarely an oceanic crust can be forced upwards over a continental plate. This is called obduction. In 2005 I saw obducted oceanic crust in Oman. The Semail Ophiolite. See Photo. (In the foreground desert sand with a few green trees to show the scale).
About 90 million years ago, in late Cretaceous, and before the al-Hajar mountains were formed, an ocean existed to the north of Oman (The Tethyan Ocean). The Arabian Plate (together with the Indian Plate) moved towards the Asian Plate forming a subduction zone in the ocean. The Arabian Plate, including North Oman was pushed under the ocean crust which was part of the Asian Plate. The obducted ocean crust formed the Semail Ophiolite.
On my field sketch from then I have marked the "Moho", short for the Mohorovicic Discontinuity, which is the boundary between the crust and the mantle. Above the Moho you see gabbro, a dark, coarse-grained, intrusive igneous rock with the same chemical composition as basalt, but formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass. The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basalt magmatism at mid-ocean ridges. Harzburgite is a type of peridotite, and peridotite is the dominant rock of the upper part of the Earth's mantle. In my next blog I shall tell more about oceanic crust and ophiolites.
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