Structure of the Earth (Definition of Terms, Characteristics)

The arrangement of the earth’s crust from the surface to the interior is;

• Sial
• Sima
• Mantle
• Core

The internal (or inner) structure of the Earth is made up of three concentric layers which are:

i. Crust (Lithosphere) – made up of the sial and sima

ii. Mantle (Mesosphere) and

iii. Core (Barysphere)

1. The Crust or Lithosphere:

The Crust is a thin layer of rocks and minerals, which lies above the Earth’s Mantle. This layer is composed of two types of crust; Oceanic and Continental.

The Oceanic crust is denser and thinner and mainly composed of basalt.  The Continental crust is less dense, thicker, and composed mainly of granite. It sustains life and forms the soil. It is divided into upper and lower parts, these are;

– The upper part of the earth’s crust forms the continents, it consists mostly of granite rocks and is made of Sial (silica and aluminium). It has an average densityDensity is the measurement of how tightly a material is packed together i.e. how closely the particles are packed in the material. The tighter the material is packed the more its... More of 2.7 g/cm³.

– The lower part of the earth’s crust forms the ocean floor and consists of basalt rocks. It is made up of Sima (Silica, iron and magnesium). It has an average density of 3.0 g/cm³ (The Sima is heavier than the Sial).

The Sima and Sial are 5 km to 6 km below the ocean and 48 km below the surface continents.

• Moho: This is the boundary layer between the earth’s crust and mantle whose depth varies from about 5 km beneath the ocean floor to about 40 km beneath the continents. It was named Mohorovicic’s Discontinuity after the Croatian seismologist, Andrija Mohorovicic’ who discovered it.

• Asthenosphere: The asthenosphere is the denser, weaker layer beneath the lithospheric mantle. It lies between 100 km and 410 km beneath the Earth’s surface. The temperature and pressure of the asthenosphere are so high that rocks soften and partly melt, becoming semi-molten.

2. The Mantle or Mesosphere:

The Mantle is the layer inside the Earth below the crust and above the core. The Earth’s Mantle is the largest and most massive layer. It is a 290 km thick, dense layer of hot, solid material. Olivine (iron and magnesium) is the main mineral found here. It has an average density of 3.3 g/cm³. It is a middle layer which surrounds the core.

3. The Core or Barysphere:

The Earth’s Core is its innermost layer found beneath the Mantle. It is divided into a liquid outer core, which begins at a depth of 2,900 km and a solid inner core, at about 4,983 km. The core from about 2,900 km below Earth’s surface has a radius of about 3,476 kilometres. The Earth’s Outer Core consists mostly of two minerals iron (Fe) and nickel (Ni) which are collectively known as NiFe. It is also made up of molten rock. This layer of the Earth is subjected to constant high temperatures and pressure. The temperature here is known to go as high as 2000°C.