Earth Structure (Cambridge International IGCSE Environmental Management)

How is the earth structured?

• Understanding the Earth’s internal structure is essential to understanding tectonic hazards
• The planet is made of several layers, each with different compositions, temperatures, and physical states
• The Earth is composed of three main layers: the crust, mantle and core

Crust

• The thin, outermost layer of the Earth

• Made of solid rock

• Broken into tectonic plates that move slowly across the planet

• The thickness varies and it is made up of two types of crust: oceanic and continental

  • Oceanic crust is thin and dense
  • Continental crust is thicker and less dense

Mantle

• The mantle is the thickest layer, reaching a depth of approximately 2900 km
• Mostly semi-molten rock (magma) in the upper mantle and solid in the lower mantle
• Heat from the core drives convection currents within the mantle
• These convection currents are the engine of plate movement, shaping earthquakes and volcanic activity

Core

• Divided into:

  • Outer core: liquid iron and nickel
  • Inner core: solid iron and nickel due to immense pressure

• Temperatures exceed 5000°C

• Heat from the core powers mantle convection

Earth's structure

Earth’s continents

• Understanding continents helps visualise plate boundaries, earthquake zones and volcano belts

• The continents include:

  • Africa
  • Antarctica
  • Asia
  • Europe
  • North America
  • Oceania (including Australia and surrounding islands)
  • South America

Seven continents of the world

Distribution of earthquakes & volcanoes

• Earthquakes and volcanoes do not occur randomly

• Their distribution is mainly controlled by the structure and behaviour of tectonic plates

• The surface of the Earth is divided into tectonic plates that have moved throughout geological time

  • The Earth's crust is broken into 15 major tectonic plates and several minor ones
  • Plates move slowly (mm–cm per year) because of convection currents in the mantle
  • The place where plates meet is called a plate boundary or plate margin

• Tectonic plates move apart, slide against each other, or collide

Tectonic plates

Earthquakes

• The majority of earthquakes (about 95%) occur close to or at a plate boundary
• Plates are constantly moving due to mantle convection
• Where plates interact—colliding, separating or sliding—stress builds up
• This stress is released suddenly as earthquakes

Map showing global earthquake distribution

Volcanoes

• Like earthquakes, most active volcanoes occur at or near plate boundaries
• Many (about 75%) occur around the 'Ring of Fire' surrounding the Pacific Ocean

Global distribution of active volcanoes

Patterns to remember:

• Pacific Ring of Fire:

  • A tectonically active region surrounding the Pacific Ocean
  • It contains ~75% of the world’s volcanoes and frequent major earthquakes

• Mid-Atlantic Ridge:

  • A divergent boundary where new oceanic crust forms, creating volcanic activity and seafloor spreading

• San Andreas Fault:

  • A transform boundary in California known for frequent earthquakes

Hotspots

• Volcanic activity can occur far from plate boundaries

• A hot spot is an area where plumes of very hot mantle rise towards the surface

  • The magma rises to the surface through cracks in the crust
  • Occurs away from the plate boundaries (hotspots usually occur away from plate boundaries, in the middle of tectonic plates)
  • As plates move over a hotspot, a chain of volcanoes forms, e.g. Hawaii

Hot spot and island formation

• Hotspot volcanoes tend to produce runny, basaltic lava and create broad shield volcanoes