The upper layer of the Earth’s crust is called lithosphere. It has 2 parts – upper part is granitic rocks and forms the continents. Its main constituent is Silica and Aluminum and is also called SiAl. It has a density of 2.7. The lower part is denser and is called SiMa [Silica, iron, magnesium]. As the SiAl is lighter than SiMa the continents are said to be floating on denser SiMa. The thickness of the crust is about 30 miles.
Below the crust is the Mantle which is 1800 miles thick. It is made up of dense layer very rich in olivine. The interior of mantle is the core which has radius of 2100 miles. It is called NiFe layer as it
is made up of Nickel and Ferrous/iron. The inner part of the core is solid.
They are further divided into types based on mineral composition:
In terms of origin there are two main types of igneous rocks:
1.Plutonic rocks: these are formed after the magma has cooled beneath the Earth’s surface. Because of slow cooling the crystals formed are large and easily recognized. E.g. granite
Most igneous rocks are hard and resistant and so are mostly used for road making, gravestones and monuments.
Igneous rocks under lithification become sedimentary / [metamorphic under heat] and further on heat or pressure become metamorphic. This again on melting becomes magma which turns to igneous rocks.
Seismic wave studies: the waves are S waves [can travel through solids and gases] and P waves [can travel through all mediums but speed is fastest in solids and least in gases].
Theory 1: Continental drift theory
4. However his theory doesn’t explain how the gravitational forces could have affected the movement of continents. He couldn’t explain the driving force for the drift. Also his model of crust doesn’t agree with modern version. The speed of drift suggested by him too was implausibly high.
Theory 2: Convention current theory / sea floor spreading:
4. It could also explain trenches in the ocean. The diverging oceanic crust was heavier than the continental crust. It was subdued under and it melted inside the mantle.
Mariana trench: – Pacific Ocean near Philippines.
Diamantine trench: – Indian Ocean near Australia.
Theory 3: Plate tectonic theory
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Q1:Upper part of the Earth’s crust is called
Q2:Which has the largest radius
Q3:Which rock type has fossils
Q4:Physical weathering is due to
1.Repeated cycles of high and low temperatures
2.Repeated wetting and drying
3.Biotic factors like plant roots
Q5: Which theory on continent formation was based on matching coastlines, similarity in age of rocks, fossil evidence found on both sides of the continents and glacial evidence in tropical lands.
1.Modern element theory
2.Convention current theory
3.Plate tectonic theory
4.Continental drift theory