Earth's Internal Structure

Earth is composed of distinct layers, each with different compositions, temperatures, and physical properties. Understanding these layers is essential to understanding plate tectonics.

Crust

Thin, solid outer layer. Oceanic crust (5–10 km, denser basalt) and continental crust (30–70 km, less dense granite).

Mantle

Thickest layer (~2,900 km). The upper mantle contains the semi-molten asthenosphere, where convection currents drive plate movement.

Outer Core

Liquid iron and nickel (~2,200 km thick). Generates Earth's magnetic field through convection of molten metal.

Inner Core

Solid iron and nickel (~1,200 km radius). Despite extreme temperatures (~5,500°C), it remains solid due to immense pressure.

Tectonic Plates and Their Movement

Earth's outer shell (the lithosphere) is broken into large slabs called tectonic plates. These plates float on the semi-molten asthenosphere below and move very slowly — typically 2–10 cm per year.

The movement is driven by convection currents in the mantle: hot material rises from near the core, spreads laterally, cools, and sinks back down. This circular motion drags the plates along.

Major Plates: Pacific, North American, South American, African, Eurasian, Indo-Australian, and Antarctic. The Australian plate moves northward at approximately 7 cm per year — one of the fastest-moving plates on Earth.

Types of Plate Boundaries

Convergent Boundary (Collision)

Plates move towards each other.

Ocean-continent: Denser oceanic plate subducts beneath the continental plate, forming deep ocean trenches and volcanic arcs.

Continent-continent: Neither subducts; instead they crumple upward, forming mountain ranges (e.g., Himalayas).

Hazards: Earthquakes, volcanic eruptions, tsunamis.

Divergent Boundary (Spreading)

Plates move apart from each other.

Magma rises from the mantle to fill the gap, creating new oceanic crust (seafloor spreading).

Forms mid-ocean ridges (e.g., Mid-Atlantic Ridge) and rift valleys on land (e.g., East African Rift).

Hazards: Mild earthquakes, volcanic activity.

Transform Boundary (Sliding)

Plates slide horizontally past each other.

No crust is created or destroyed.

Friction causes plates to lock, then suddenly release, producing powerful earthquakes.

Example: San Andreas Fault (California), Alpine Fault (New Zealand).

Evidence for Plate Tectonics

Continental Drift

Alfred Wegener (1912) proposed that continents were once joined as Pangaea. Evidence: coastlines of South America and Africa fit together like jigsaw pieces.

Fossil Evidence

Identical fossils (e.g., Glossopteris fern, Mesosaurus) found on continents now separated by oceans, suggesting they were once connected.

Seafloor Spreading

Magnetic stripes in ocean floor rock show symmetrical patterns either side of mid-ocean ridges, proving new crust forms and spreads outward.

Matching Rock & Climate Evidence

Glacial scratches found in tropical regions (e.g., India, Australia) and coal deposits in Antarctica indicate these landmasses were once at different latitudes.

Key Vocabulary

Term	Definition
Lithosphere	The rigid outer layer of Earth (crust + upper mantle), broken into tectonic plates.
Asthenosphere	The semi-molten layer of the upper mantle on which tectonic plates float and move.
Convection currents	Circular movements of hot material in the mantle that drive plate movement.
Subduction	When a denser plate sinks beneath a less dense plate at a convergent boundary.
Pangaea	The supercontinent that existed approximately 300 million years ago before breaking apart.
Seismology	The scientific study of earthquakes and seismic waves.

Worked Examples

1

Identifying a plate boundary type from a description.

Description: Two plates are moving apart. New rock is forming at the surface from rising magma. A ridge of underwater mountains is present.

Step 1: “Moving apart” indicates plates are separating → divergent boundary.

Step 2: New rock forming from rising magma confirms seafloor spreading.

Step 3: Underwater mountain ridge = mid-ocean ridge.

Answer: This is a divergent plate boundary, such as the Mid-Atlantic Ridge.

2

Explaining why earthquakes occur at transform boundaries.

Step 1: At transform boundaries, two plates slide horizontally past each other in opposite directions.

Step 2: The plates do not slide smoothly — friction causes them to lock together.

Step 3: Stress (potential energy) builds up over time along the fault line.

Step 4: When the stress exceeds the frictional force, the plates suddenly slip, releasing energy as seismic waves.

Answer: Earthquakes at transform boundaries result from the sudden release of built-up stress when locked plates overcome friction and slip past each other.

3

Using fossil evidence to support continental drift.

Evidence: Fossils of the freshwater reptile Mesosaurus are found in both South America and Africa, but nowhere else.

Step 1: Mesosaurus lived in freshwater — it could not have swum across the Atlantic Ocean.

Step 2: The simplest explanation is that South America and Africa were once joined.

Step 3: This supports Wegener's theory that the continents were part of a single landmass (Pangaea) that later broke apart.

Answer: Matching Mesosaurus fossils on two separate continents provide strong evidence that these landmasses were once connected, supporting the theory of continental drift.

Knowledge Check

Select the correct answer for each question. Click “Check Answer” to see feedback.

Question 1

Which layer of Earth is liquid?

Inner core Outer core Crust Lithosphere

Question 2

What type of plate boundary forms a mid-ocean ridge?

Convergent Transform Divergent Subduction

Question 3

What drives the movement of tectonic plates?

Earth's rotation Gravitational pull of the moon Wind patterns on the surface Convection currents in the mantle

Question 4

At a convergent boundary between an oceanic and a continental plate, which plate is subducted?

The oceanic plate (it is denser) The continental plate (it is heavier) Both plates are subducted equally Neither — they form mountains instead

Question 5

Which of the following is NOT evidence for continental drift?

Matching coastlines of Africa and South America Tides caused by the moon Identical fossils on separate continents Glacial deposits in tropical regions

Plate Tectonics