Layers of Earth, Composition & Earthquakes

Generally speaking, Earth has 4 layers → the outer crust that we live on, the plastic-like mantle, the liquid outer core, and the solid inner core. Many geologists believe that as the Earth cooled the heavier, denser materials sank to the center and the lighter materials rose to the top. Because of this, the crust is made of the lightest materials (rock- basalts and granites) and the core consists of heavy metals (nickel and iron).

Crust

• Outermost solid part of earth surface, brittle in nature
• The crust is only about 8 kilometres thick under the oceans (oceanic crust) and about 32 kilometers thick under the continents (continental crust).
• The crust of the Earth is broken into many pieces called plates. The plates “float” on the soft, plastic mantle which is located below the crust.

• These plates usually move along smoothly but sometimes they stick and build up pressure.
• The pressure builds and the rock bends until it snaps. When this occurs an Earthquake is the result!
• The crust is composed of two basic rock types granite and basalt.
• The continental crust is composed mostly of granite. The oceanic crust consists of a volcanic lava rock called basalt.

Mantle

• Lies beyond crust till 2900 km, composed mainly of denser rocks rich in Olivine
• Consists of two layers viz. Asthenosphere & Lower layer
• Asthenosphere → Upper Mantle portion till 400 Km (Weak & Source of Magma)
• Lower Layer → 400 – 2900 km, Solid in appearance

Core

• Core – Mantle boundary lies at 2900 km.
• Earth’s Core is thought to be composed mainly of an iron and nickel alloy
• The core is divided into two different zones.
• The outer core is a liquid because the temperatures there are adequate to melt the iron-nickel alloy → 2900 – 5100 km.
• However, the inner core is a solid even though its temperature is higher than the outer core → 5100 – 6378 km.
• Here, tremendous pressure, produced by the weight of the overlying rocks is strong enough to crowd the atoms tightly together and prevents the liquid state

Lithosphere

• Crust + Asthenosphere → Made up of ( SiAl + SIMa) → Consists of two different parts

• The upper part consists of Granite rocks & forms the continents
• Its main constituents are Silica & Alumina, collectively referred as SiAl
• ~100 Km with average density 2.7

• The lower part is continuous zone of denser basaltic rocks, forming the ocean floors
• Mainly comprises of Silica, Iron & Magnesium, collectively referred as SIMa
• ~100-400 km with average density 3

• Since SiAl is lighter than SIMa, continents can be said to be floating on a sea of denser SIMa
• Pressure, Temperature & Density all increases with decrease in height

Composition of the interior of the earth

Earthquakes

• Tremors produced by vibratory waves through rocks of earth mainly along a fault 

• Place of origin → Focus / Hypocenter
• Point vertically above focus on earth surface → Epicenter

• Shock waves travel in all directions from focus & shaking is maximum at Epicenter on earth surface

• Earthquake is measured in following scales

• Intensity / Severity of earthquake → Measured by MMS (Modified Mercalli Scale)
• Magnitude of earthquake (Energy released at epicenter) → Measured by Richter scale
• Variation in direction of waves → Measured by Seismograph

• Earthquake waves → Most important source of earth’s interior

• P & S waves → Body waves
• L → Surface waves

• • P waves (Longitudinal Waves) → Primary waves, travel in all mediums solid, liquid & gases with speed of approx. 8 km/s
  • S waves (Transverse Waves) → Secondary waves, travel in solid & gas medium but not in liquid with speed of approx. 4 km/s
  • L waves → Travel only in solid medium at the surface of earth with speed of approx. 3 km/s

• Earthquake generally occurs when rocks underground breaks along a fault
• This cause a sudden release of energy known as seismic waves → mainly occur in Lithosphere
• Distribution of Earthquakes coincides very closely with that of volcanoes with almost 70 % of the earthquakes occurring in circum Pacific belt
• Another 20 % of earthquakes take place in the Mediterranean-Himalaya region

Types of Earthquakes

• Shallow focus earthquake – Depth ( 0 – 60 Km)
• Medium focus earthquake – Depth (70 – 300 km)
• Deep Focus earthquake – Depth (300 – 700 Km)
• Below 700 km – Not very dangerous