Internal Structure of the Earth

Introduction

Geography in the Kenyan Senior School CBC (KICD) begins with understanding the planet we live on. The Earth is not solid throughout; it has distinct layers with different composition, temperature and thickness. This helps explain mountains, volcanoes, earthquakes and the Rift Valley.

Layers of the Earth

1. Crust - the outermost solid layer, 5 to 70 km thick.
   • Continental crust is thicker and made mostly of granite (SIAL: silica and aluminium).
   • Oceanic crust is thinner and made of basalt (SIMA: silica and magnesium).
2. Mantle - about 2,900 km thick, made of dense semi-molten rock. The upper mantle with the crust forms the lithosphere; the plastic layer below is the asthenosphere.
3. Outer core - about 2,200 km thick, liquid iron and nickel.
4. Inner core - about 1,200 km thick, solid iron and nickel under extreme pressure (the NIFE).

Temperature and pressure increase with depth. The core is about 5,000 degrees Celsius or more.

Evidence of Earth's structure

• Behaviour of seismic waves (P and S waves) during earthquakes
• Volcanic eruptions bringing magma to the surface
• Deep mines and boreholes showing rising temperature
• Meteorites with similar composition
• The Earth's magnetic field

Plate tectonics

The lithosphere is broken into plates that move slowly, driven by convection currents in the mantle.

• Divergent boundaries - plates move apart (Rift Valley, mid-ocean ridges).
• Convergent boundaries - plates collide, forming fold mountains or trenches.
• Transform boundaries - plates slide past each other.

The Great Rift Valley

Running from Jordan through East Africa to Mozambique, the East African Rift is a divergent zone. Tension caused the crust to crack. The central block dropped between parallel faults, forming the valley floor with escarpments such as the Kinangop and Elgeyo. Volcanoes such as Mt. Longonot and Menengai, and lakes such as Nakuru, Naivasha and Baringo, formed along it.

Importance of internal processes

• Create landforms: mountains, valleys and plateaus
• Produce minerals and geothermal energy (Olkaria)
• Cause hazards: earthquakes and volcanic eruptions

Worked example 1

The thickness of the mantle is about 2,900 km and the Earth's radius is 6,371 km. If the crust is 30 km, find the core's radius. Answer: 6,371 - 2,900 - 30 = 3,441 km.

Worked example 2

Explain why S waves do not pass through the outer core. Answer: S waves cannot travel through liquids, so their absence on the opposite side of the Earth shows the outer core is liquid.

Try it

• Draw and label a cross-section of the Earth.
• Name three features along the Rift Valley in Kenya.
• Differentiate between the lithosphere and the asthenosphere.
• Explain how geothermal power at Olkaria relates to internal Earth processes.