The Earth & Plates Notes | Junior Cycle Geography

Structure of the Earth and Convection Currents

The Earth has three layers: the crust, the mantle, and the core. The core is divided into an outer core and an inner core.

Crust: The thin outer layer of solid rock, varying from 10 km thick under oceans to 60 km under continents.
Mantle: The thick middle layer of very hot, semi-molten rock that flows slowly.
Outer core: A molten, liquid metal layer made of nickel and iron.
Inner core: A solid metal ball of nickel and iron, kept solid by immense pressure despite extreme heat.

Earth cutaway showing a thin crust surrounding the mantle, liquid outer core and solid inner core.

Convection currents circulating in the mantle move the overlying crust. Heat from the core warms magma in the lower mantle, making it rise. Near the crust, it spreads sideways and drags the plates along, then cools and sinks back down to be reheated.

Mantle circulation rises centrally, spreads beneath two separating plates, and sinks farther away; magma rises at the central ridge.

Types of Plate Boundaries

The crust is cracked into large slabs called tectonic plates. The place where two plates meet is called a plate boundary.

Boundary Type	Plate Movement	Key Features and Examples
Constructive (Divergent)	Plates separate and pull apart.	Magma rises to fill the gap, creating new crust, mid-ocean ridges, and volcanic islands (e.g. Mid-Atlantic Ridge in Iceland).
Destructive (Convergent)	Plates collide and push together.	Oceanic plate is denser, so it sinks and melts in subduction. The melted rock rises as volcanoes (e.g. Andes). Where two continental plates meet, neither sinks and rock buckles into fold mountains (e.g. Himalayas).
Conservative (Passive)	Plates slide past each other.	Pressure builds as the plates lock together; when they suddenly slip, an earthquake happens (e.g. San Andreas Fault). No volcanoes form here.

An oceanic plate moves beneath a continental plate, with magma rising towards a volcano on the overriding plate.

Two continental plates converge, compressing layered rock into fold mountains, with convection currents shown in the mantle beneath.

Earthquakes and Volcanoes: Where They Happen

Earthquakes and volcanoes occur in distinct belts along plate boundaries rather than being spread evenly.

Plates can lock together along cracks in the crust called faults. Pressure builds up, and when the plates suddenly slip, shockwaves (tremors) travel through the crust.
The focus is the underground point where the earthquake originates. The epicentre is the point on the surface directly above the focus.
An earthquake under the sea can push up a huge wave called a tsunami.
Magma is molten rock underground, whereas lava is molten rock that reaches the surface.
A volcano erupts through an opening called a vent; the hollow at the top is the crater; molten rock running down the side is a lava flow.
The Pacific Ring of Fire is a major horseshoe-shaped belt around the Pacific Plate where most volcanoes and earthquakes occur.
Fold mountains form long belts where plates collide: the Himalayas in Asia, the Alps in Europe, the Andes in South America, and the Rockies in North America. In Ireland, MacGillycuddy's Reeks in Co. Kerry are old fold mountains.

An underground focus lies on a sloping fault, directly beneath the surface epicentre, with shockwaves spreading from the focus.

Pacific-centred map highlighting the horseshoe-shaped tectonic belt around the ocean's margins.

Social and Economic Impacts and Disaster Responses

Tectonic activity presents dangerous hazards as well as valuable economic opportunities for people.

Volcanic ash breaks down over time into fertile soil, which gives farmers very good crops.
Underground volcanic heat provides clean geothermal energy to heat homes and generate electricity in Iceland.
Volcanic landforms and hot springs attract tourists, which creates jobs and brings money into the area.
Shockwaves can collapse buildings, break gas pipes, and destroy roads, bridges, and railways.
New Zealand is a high-income country, so strict building regulations mean buildings sway rather than collapse during tremors.
Haiti is a low-income country, so poorly built homes collapsed and weak emergency services could not reach survivors quickly.
Short-term response: emergency aid such as search-and-rescue teams, clean water, food, tents, and medical supplies, provided by governments and agencies like the Red Cross and Concern.
Long-term response: rebuilding homes to earthquake-proof standards, training emergency services, and setting up early-warning systems and evacuation drills.

Key terms

Convection current: The circular movement of semi-molten rock in the mantle driven by heat from the core, which moves tectonic plates.
Subduction: The process where a heavier, denser oceanic plate sinks beneath another plate into the mantle and melts.
Fold mountains: Mountains formed when colliding tectonic plates compress and buckle layers of rock upwards.
Focus: The exact underground point where an earthquake begins.
Epicentre: The point on the Earth's surface directly above the focus of an earthquake.
Richter scale: A scale that measures the magnitude, or strength, of an earthquake.
Pacific Ring of Fire: A horseshoe-shaped zone along the edges of the Pacific Ocean where most of the world's earthquakes and volcanoes occur.

Check yourself

What drives the movement of tectonic plates?
Convection currents circulating in the semi-molten mantle, powered by heat from the core.
At which type of plate boundary does subduction occur?
At a destructive (convergent) plate boundary where a denser oceanic plate sinks beneath another plate.
What does the Richter scale measure?
The magnitude, or strength, of an earthquake.
Name two types of short-term emergency aid needed after a severe earthquake.
Search-and-rescue teams, clean water, emergency food, medical supplies, or temporary tents.

The Earth & Plates