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Oceanic ridges
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of basalt (rich in iron and magnesium). It is thinner than continental crust. It is denser, having a mean density of about 3.0 grams per cubic centimeter as opposed to Continental which has a density of about 2.7 grams per cubic centimeter.
6 - 7 Km 35 Km
Oceanic ridge (or midoceanic ridge) is a continuous submarine mountain range extending approximately 60,000 km through all the world’s oceans. Individually, ocean ridges are the largest features in ocean basins.
Oceanic ridges are found in every ocean basin and appear to girdle the Earth. The ridges rise from depths near 3 km and are roughly symmetrical in cross section. They can be thousands of Kilometres wide. In places, the crests of the ridges are offset across transform faults within fracture zones (Transform faults are those along which lateral movement occurs.)
The arrows show the direction of movement across the transform faults.
Spreading occurs as basaltic magma from the mantle rises into vertical tensional fractures in the lithosphere. This zone is constructive, as volcanism adds basalt rock to the lithosphere.
Oceanic ridges can be classified as slow (spreading up to 50 mm per year), intermediate (up to 90 mm per year), fast (up to 160 mm per year). Slow-spreading ridges are characterized by a rift valley at the crest. It is typically 1.4 Km deep and 20–40 km wide. Faster-spreading ridges lack rift valleys.
Map showing the age of Earth’s oceanic crust
Rising magma assumes the polarity of Earth’s geomagnetic field before it solidifies into oceanic crust. At spreading centres, this crust is separated into parallel bands of rock by successive waves of emergent magma. When Earth’s geomagnetic field undergoes a reversal, the change in polarity is recorded in the magma, which contributes to the alternating pattern of magnetic striping on the seafloor.
Seafloor spreading and magnetic striping
ICELAND
TASK
2- What is the mid-Atlantic ridge?
1- Make your labelled drawing of an oceanic ridge
