GC Leong Test : The Earth's Crust - 1 - BPSC (Bihar) MCQ

• The outer layer is the earth's crust the lithosphere—which comprises two distinct parts. The upper part consists of granitic rocks and forms the continents.

• Its main mineral constituents are silica and alumina so it is collectively referred to as the sial.

• The lower part is a continuous zone of denser basaltic rocks forming the ocean floors, comprising mainly silica, iron and magnesium. It is therefore called sima.

• Starting at the centre, Earth is composed of four distinct layers.

• They are, from deepest to shallowest, the inner core, the outer core, the mantle and the crust.

• Except for the crust, no one has ever explored these layers in person.

• It is made mostly of iron and nickel, It is extremely dense, The inner core spins a bit faster than the rest of the planet. It’s also intensely hot: Temperatures sizzle at 5,400° Celsius.

• Igneous rocks are formed by the cooling and solidification of molten rock (magma) from beneath the earth's crust.

• They are normally crystalline in structure. They do not occur in strata (layers) nor do they contain fossils.

• Igneous rocks may be subdivided based on mineral composition.

• Sedimentary rocks are formed from sediment accumulated over long periods, usually underwater.

• They are distinguished from the other rock types in their characteristic layer formation and are termed stratified rocks.

• The rocks may be coarse or fine-grained, soft or hard.

• The materials that form sedimentary rocks may be brought by streams, glaciers, winds or even animals.

• They are non-crystalline and often contain fossils of animals, plants and other microorganisms. Sedimentary rocks are thus the most varied in their formation of all rocks.

Mechanically formed sedimentary rocks - sandstones.

Organically formed sedimentary rocks - limestones and chalk

Chemically formed sedimentary rocks -gypsum

• All rocks whether igneous or sedimentary may become metamorphic or changed rocks under great heat and pressure.

• Their original character and appearance may be greatly altered by such forces, particularly during intense earth movements.

• In this manner, clay may be metamorphosed into slate, limestone into marble, sandstone into quartzite, granite into gneiss, shale into schist and coal into graphite.

• Fold mountains: These mountains are by far the most widespread and also the most important.

• They are caused by large-scale earth movements when stresses are set up in the earth's crust.

• Such stresses may be due to the increased load of the overlying rocks, flow movements in the mantle, magmatic intrusions into the crust, or the expansion or contraction of some part of the earth

• In extreme cases, fractures may occur in the crust, so that the upper part of the recumbent fold slides forward over the lower part along a thrust plane, forming an overthrust fold.

• The overriding portion of the thrust fold is termed a nappe. When the crest of a fold is pushed too far, an overfold is called mountains of elevation.

• Block mountains: When the earth's crust bends folding occurs, but when it cracks, faulting takes place.

• Faulting may be caused by tension or compression, forces which lengthen or shorten the earth's crust, causing a section of it to subside or to rise above the surrounding level.