EARTH

 

Introduction

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

  1. There are entirely continental, entirely oceanic and mixed plates. Totally 6 major and 20 minor plates.
  2. Plates interact due to earth’s rotation. These interactions are of types:
  3. Divergent plate boundary: below oceans the divergence of plates causes mid ocean ridges. And below continents it causes rift valley and nascent sea.
  4.  Convergent plate boundary: ocean and ocean plate convergence causes island arcs like in the Indian ocean and Philippines archipelago; ocean continent plate convergence causes volcanic mountains like in Andes, rocky and atlas mountains and continent plate convergence causes fold mountains like Himalaya, alps and Ural.
  5.  Transverse boundary: when the two plates slip past each other it doesn’t have any mountain formation but causes seismic activity. E.g.: San Andréa’s fault in USA.

is made up of Nickel and Ferrous/iron. The inner part of the core is solid.

Rocks in the earth’s crust:

  1. Igneous rocks:
  2. Formed by solidification of magma or lava.
  3. Don’t have strata or layers.
  4. Don’t have fossils.
  5.  They are non crystalline if volcano burst out and cools at the surface [e.g.: basalt] or crystalline if volcano doesn’t burst and cools inside surface [ e.g.: granite]

They are further divided into types based on mineral composition:

 

    1. Acid igneous rock: less dense and lighter
    1. Basic igneous rock: denser and darker

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

  1. Volcanic rocks: the molten rocks pour out of the surface as lava. This cools faster at the surface and crystals formed are small. E.g. basalt

Most igneous rocks are hard and resistant and so are mostly used for road making, gravestones and monuments.

  1. Sedimentary rocks
  1. Formed when sediments compact under pressure lose fluid and become solid called lithification. They are non crystalline.
  2.  Have fossils and strata. E.g.: shale, clay, limestone, silica

Types:

  • Mechanically formed sedimentary rocks: These rocks are made from accumulation of material from other rocks that is cemented together.
  • Organically formed sedimentary rock: these are formed from remains of marine life having shells like oysters or snails. As the fleshy part perishes the calcareous part fuses together. Carbonaceous origin sedimentary rock is formed when vegetative matter is compressed into carbon forms due to weight of overlying matter. Peat, lignite and coal are formed in this manner.
  • Chemically formed sedimentary rocks: these are formed from chemical precipitation from solutions. E.g. rock salt, gypsum

 

Rock cycle:

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].

Formation of continents:

Theory 1: Continental drift theory

  1. Alfred Wegener propounded it
  2. It was based on matching coastlines, similarity in age of rocks, fossil evidence i.e. similar plant and animal fossil found on both sides of the continents and glacial evidence in tropical lands.
  3. His theory was that the super continent – Pangaea broke up into continents 200 million years ago by drifting over the oceans.

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:

  1. A convention cell which is like a conveyor belt moves under the crust and is responsible for drifting of the continents.
  2. The sea floor has a lot of diversity: Mid ocean ridges, trenches, continental slopes, ocean floor. Some questions emerged as finding proved that oceanic crust was 200 million years old, age of crust increases away from mid ocean ridge and sea mounts deformed at trenches.
  3. The sea floor spreads due to the convention current and magma comes out this forms new ocean crust and expansion of ocean crust.

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

  1. Some questions that remained unanswered were the formation of Fold Mountains, reasons for earthquakes and reasons for Volcanism on land.
  2. The theory states that the crust isn’t continuous but broken into plates. The lithosphere floats over the Athemosphere.
  1. There are entirely continental, entirely oceanic and mixed plates. Totally 6 major and 20 minor plates.
  2. Plates interact due to earth’s rotation. These interactions are of types:
  3. Divergent plate boundary: below oceans the divergence of plates causes mid ocean ridges. And below continents it causes rift valley and nascent sea.
  4.  Convergent plate boundary: ocean and ocean plate convergence causes island arcs like in the Indian ocean and Philippines archipelago; ocean continent plate convergence causes volcanic mountains like in Andes, rocky and atlas mountains and continent plate convergence causes fold mountains like Himalaya, alps and Ural.
  5.  Transverse boundary: when the two plates slip past each other it doesn’t have any mountain formation but causes seismic activity. E.g.: San Andréa’s fault in USA.

Chapter Review


Score more than 80% marks and move ahead else stay back and read again!

Q1:Upper part of the Earth’s crust is called
1.SiMa
2.NiFe
3.SiAl
4.none

Q2:Which has the largest radius
1.crust
2.mantle
3.core
4.none

Q3:Which rock type has fossils
1.igneous
2.metamorphic
3.sedimentary
4.all

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
4.all

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