Describe the internal structure of the Earth (crust, mantle, core) and their major characteristics (location, size/thickness, density and gross composition)

The crust is the thin, solid outer skin of the Earth. It is the coolest and least dense layer.

Key characteristics

• Thickness: About 8 km to 50 km (continental crust is thicker than oceanic crust)
• Density: Average 2.7 g/cm³
• Composition: Made of two sub-layers:
  • SIAL – the continental crust, rich in silica (Si) and aluminium (Al). It forms the continents.
  • SIMA – the oceanic crust, rich in silica (Si) and magnesium (Ma). It forms the ocean floor.
• These two layers are separated by the Conrad discontinuity.

The mantle lies beneath the crust and above the core. It is the thickest layer, making up about 84% of the Earth's volume.

Key characteristics

• Thickness: Extends from about 51 km to 2,900 km deep
• Density: 3.0–3.4 g/cm³ (increases with depth)
• Composition: Dense igneous rocks rich in iron, magnesium, silicon, and oxygen
• Structure: Divided into two parts:
  • Upper mantle (lithosphere) – solid rock, together with the crust forms the rigid lithosphere
  • Lower mantle – contains the asthenosphere (partially molten), where convection currents move
• Separated from the crust by the Mohorovicic discontinuity (also called Moho)

Why the mantle matters

Convection currents in the mantle cause plate movements, leading to earthquakes, volcanic eruptions, and the formation of mountains. In Tanzania, the East African Rift Valley is a result of these internal forces.

The core is the innermost layer of the Earth. It is extremely hot and dense, consisting mainly of iron and nickel.

Key characteristics

• Average density: About 10.5 g/cm³
• Composition: Iron and nickel (sometimes called the "nickel-iron" layer)

Outer core

• Thickness: About 2,300 km
• State: Liquid (molten)
• Temperature: Around 3,700°C at the boundary with the mantle
• Importance: The movement of molten iron and nickel creates electric currents that generate Earth's magnetic field, which protects us from harmful solar radiation

Inner core

• Diameter: About 2,600–2,700 km
• State: Solid (despite temperatures of about 5,500°C)
• Reason for being solid: The immense pressure at the centre of the Earth forces the iron and nickel to remain solid even at such high temperatures
• Density: About 16–17 g/cm³

The core is separated from the mantle by the Gutenberg discontinuity.

Understanding the Earth's internal structure helps Tanzanian geologists locate valuable minerals. For example, the presence of the East African Rift Valley indicates volcanic activity that brings iron ore, gold (like in Geita), and other minerals closer to the surface. This knowledge supports mining companies in Tanzania in exploring resources that contribute to the country's economy through exports and local employment.