Question

The oceanic crust mainly consists of___?
a)basaltic rocks
b)granitic rocks
c)gabbroic rocks
d)ultramafic rocks

Answer 1

Hint: Earth’s crust and upper mantle is divided into pieces called tectonic plates. Oceanic crust is the top layer of oceanic portion of a tectonic plate.

Complete answer: Oceanic crust is thinner than continental crust and is made of rocks rich in magnesium and iron. Let us look at the options given to us
Option A- Basaltic rocks is an extrusive igneous rock which is rich in magnesium and iron. It is formed when lava is cooled near the surface of earth. Oceanic crust which extends 5-10km beneath the ocean floor is primarily composed of basalt rocks. Oceanic crust is constantly formed in mid-oceanic ridges where tectonic plates move apart forcing lava to well up and cool down. Basalt is fine grained and dark coloured rock.
Option B- Granitic rock is an intrusive igneous rock. It is a coarse grained rock which is rich in quartz and feldspar.
Option C- Gabbroic rock is an intrusive igneous rock. It is mostly found in lower oceanic crust and has coarse structure.
Option D- Ultramafic rocks are igneous and meta igneous rock which are rich in mafic minerals and low in silica. The Earth’s mantle is made of ultramafic rocks.

Therefore the correct answer is Basaltic rocks, option (A)

Note: One must note that basalt forms the upper oceanic crust while gabbro is found in lower oceanic crust. Basalt and gabbro are both igneous rocks . However basalt is rapid cooling lava and gabbro is lava that cools slowly. Rapid cooling leads to fine grained structure of basalt while slow cooling gives gabbro a coarse texture.Flood basalt eruptions are basically volcanic eruptions that cover large areas of land or ocean. Deccan traps of India is a flood basalt province.