Earth’s Formation and Layers

STEPS IN THE FORMATION OF THE EARTH

Step 1: accretion of cm sized particles. countless numbers of these small particles collide at low relatively velocity and coalesce into objects which are a few km in diameter

Step 2: Physical Collision on km scale. once these km size objects are formed, they start to smack into one another and stick thus creating fewer new objects of size 10-100 km

Step 3: Gravitational accretion on 10-100 km scale. Once an object gets to a radius of 10-100 km, its cross section is larger than its physical cross section because it exerts a gravitational influence on surrounding smaller bodies and, in essence, sweeps them up. This process will create a planet with significant gaps between each swept up zone.

Step 4: Molten protoplanet from the heat of accretion . For each protoplanet that forms, there are thousands of left over planetisimal objects. These planetisimals are then accelerated by the relatively large mass of the formed planets and they start to move at high velocity throughout the inner solar system - many of them ultimately impact the surfaces of the cooling planets.

Core: Inner and Outer

• The study of earth’s history interior is most difficult because it is inaccessible. The information about Earth’s core is from seismic information and computer models.

• Inge Lehmann, a seismologist, discovered in 1936 that earth has solid inner core.

• Studies showed that the hot spherical solid has a radius of 1250 kilometers.

• It consist mainly iron-nickel alloy and is magnetic

• It has temperature of about 6000°c, amost as hot as the surface of the sun.

• The pressure in the inner core is so great that the alloy cannot melt. For this reason, the inner core is mostly solid.

• The outer core is made mostly of iron and nickel.

• The outer core is approximately 2300 km thick.• It is very hot; the temperature is between

4000°c to 5000°c . Because of the very high temperature, the outer core is liquid.

• The boundary between the outer and inner core is sometimes referred to as Lehmann discontinuity.

Inner Core and outer CoreInner Core

Iron and nickelMagneticSolid1250 km

Outer Core Iron nickelMagneticLiquid2300kmCause of Earth’s

magnetic fieldEarth’s molten metallic core gave rise to magnetic field. The magnetic field is attributed to the dynamo effect of circulating electric current.

Mantle: Lower and Upper

The mantle is the longest part of the Earth. It is the intermediate layer of Earth and is

subdivided into lower and upper mantle. It is made up of molten rocks called magma. The magma circulates in currents determined

by the cooling and sinking of heavier minerals, and the heating and rising of the lighter minerals.

The mantle has a total thickness of 2900km.

The lower mantle is hot and exhibits plasticity. The higher pressure in the layer causes of formation of minerals that are different from that of upper layer.

Gutenberg discontinuity is detected between Earth’s lower mantle and the outer core observed by changes in the seismic waves. I

It is 2240 km thick.

The uppermost mantle and earth’s crust that form the lithosphere are relatively rigid.

The upper boundary that separates the upper mantle from earth’s crust is defined by the sudden increase in seismic velocity.

This is called Mohorovičić (or Moho) discontinuity, as noted by Andrija Mohorovičić

In 1909. Moho is 5km below the ocean floor and 20 to 90 km

wit an average of 35km, beneath typical continents.

The asthenosphere is the layer that lies after the lithosphere (100km- 250km) beneath km, Earth’s surface

The name was given by British geologists Joseph Barell in 1914.

The balance between temperature and pressure is so great that the rocks have a little strength (weak) and are easily deformed.

It is believed that asthenosphere plays a critical role in the movement of plates on the surface of earth according to the plate tectonic theory.

Lower and Upper Mantle Lower/ Mid- MantleAsthenosphereSoft/ magmaExhibits “plasticity”Convection currents2240 km

Upper Mantle LithosphereRigid/ solidMoho boundary660km

Crust• There are two types of crust that make up

the surface of the lithosphere. These are the oceanic crust and the continental crust.

Oceanic Crust and Continental Crust

Oceanic CrustDark- coloredRock sample:

basaltMore dense50km

Continental CrustLight- coloredRock sample- graniteLess denseCoarse- texturedThick layer40-70 km