Plate Tectonics

The Theory of Plate Tectonics

The Earth's crust is divided into large sections called tectonic plates that move slowly (a few centimetres per year) across the mantle. This movement is driven by convection currents in the mantle — hot material rises, spreads sideways and cools, dragging the plates with it.

There are around 15 major plates (and several smaller ones). The main ones include the Eurasian, North American, South American, African, Indo-Australian, Pacific and Antarctic plates.

Types of Plate Boundary

1. Destructive Margin (Convergent)

• Two plates move towards each other
• Oceanic-continental: The denser oceanic plate is forced beneath the continental plate (subduction). Creates deep ocean trenches (e.g., Mariana Trench), fold mountains (Andes), and explosive volcanoes (e.g., Mt St Helens). Shallow to deep earthquakes.
• Oceanic-oceanic: One plate subducts. Creates island arcs (Japan, Philippines) and trenches.
• Continental-continental: No subduction — rocks buckle and fold upward. Creates fold mountain ranges (Himalayas) but no volcanoes.

2. Constructive Margin (Divergent)

• Two plates move apart
• Magma rises from the mantle to fill the gap → new oceanic crust is created
• Creates: mid-ocean ridges (Mid-Atlantic Ridge), rift valleys (East African Rift) and shield volcanoes (Iceland — on the Mid-Atlantic Ridge)
• Earthquakes are shallow and less powerful

3. Conservative (Transform) Margin

• Two plates slide past each other horizontally
• No magma = no volcanoes
• Creates: powerful shallow earthquakes (e.g., San Andreas Fault, California; the 1994 Northridge and 1989 Loma Prieta earthquakes)

Distribution of Volcanoes and Earthquakes

The Ring of Fire: A horseshoe-shaped zone around the Pacific Ocean where ~90% of the world's earthquakes and 75% of its volcanoes occur. Due to the Pacific Plate subducting under surrounding plates. Includes Japan, the Philippines, New Zealand, the west coasts of the Americas, and Alaska.

Mid-Atlantic Ridge: Constructive boundary — Iceland sits on it; some of the world's most active but non-explosive volcanoes (Eyjafjallajökull, 2010).

Hotspots: Not at plate boundaries — plumes of magma rise through the mantle from a fixed "hot spot." Example: Hawaii (Hawaiian Islands formed as the Pacific plate moves over a hotspot — older islands further northwest).

Key pattern: Earthquakes and volcanoes are not randomly distributed — they occur mainly at plate boundaries. The exam expects you to describe this distribution pattern with examples.