Plate Tectonics (Constructive, Destructive, Conservative, Collision)

Plate Tectonics Overview

The Earth's outer layer, the lithosphere, is broken into large pieces called tectonic plates. These plates float on the semi-molten layer beneath them called the asthenosphere. The plates constantly move due to heat-driven currents in the mantle called mantle convection currents. Hot magma rises, cools, and sinks in a circular motion, dragging plates along.

There are three main types of plate boundaries where plates interact:

• Constructive boundaries – plates move apart
• Destructive boundaries – plates move towards each other
• Conservative boundaries – plates slide past each other
• Collision boundaries – two continental plates collide

These interactions cause most of the Earth's seismic and volcanic activity.

Constructive Plate Boundaries

At constructive boundaries, two tectonic plates move away from each other. This gap allows magma from the mantle to rise up and solidify, creating new oceanic crust. This process is called sea-floor spreading.

Constructive boundaries are commonly found under oceans and form mid-ocean ridges, such as the Mid-Atlantic Ridge. As magma cools, it builds underwater mountains and volcanoes.

Because new crust is formed here, these boundaries are also called divergent boundaries. Earthquakes can occur but tend to be less severe than at destructive boundaries.

For instance, the Mid-Atlantic Ridge is a constructive boundary where the Eurasian and North American plates are moving apart at a rate of a few centimetres per year.

Example: If the plates at the Mid-Atlantic Ridge move apart at $2\,\mathrm{cm/year}$, in 1 million years, the gap created would be:

$$2\,\mathrm{cm/year} \times 1,000,000\,\mathrm{years} = 20,000,000\,\mathrm{cm} = 200\,\mathrm{km}$$

Destructive Plate Boundaries

Destructive boundaries occur when an oceanic plate meets a continental plate and moves towards it. The denser oceanic plate is forced underneath the lighter continental plate in a process called subduction.

This creates a deep ocean trench at the subduction zone. As the oceanic plate sinks, it melts due to the high temperatures and pressure, forming magma. This magma can rise to the surface, causing volcanic eruptions.

Destructive boundaries are also associated with powerful earthquakes caused by the plates locking and then suddenly slipping.

An example is the boundary between the Nazca Plate and the South American Plate, which has formed the Peru-Chile Trench and the Andes Mountains.

Example: The oceanic plate subducts at a rate of $5\,\mathrm{cm/year}$. How far has it subducted after 2 million years?

$$5\,\mathrm{cm/year} \times 2,000,000\,\mathrm{years} = 10,000,000\,\mathrm{cm} = 100\,\mathrm{km}$$

Conservative Plate Boundaries

At conservative boundaries, two plates slide past each other horizontally. Unlike constructive or destructive boundaries, no crust is created or destroyed.

The plates can become stuck due to friction, building up pressure over time. When this pressure is released suddenly, it causes earthquakes.

A famous example is the San Andreas Fault in California, where the Pacific Plate slides past the North American Plate.

Earthquakes here can be very powerful but there is no volcanic activity because no magma is produced.

Collision Plate Boundaries

Collision boundaries occur when two continental plates move towards each other and collide. Because both plates have similar densities, neither sinks beneath the other, so no subduction occurs.

Instead, the collision causes the crust to crumple and fold, forming large mountain ranges called fold mountains. The Himalayas are the best-known example, formed by the collision of the Indian and Eurasian plates.

Earthquakes are common in these areas due to the intense pressure and folding of the crust.

Example: The Indian Plate moves northwards at about $5\,\mathrm{cm/year}$ towards the Eurasian Plate. Over 10 million years, the plates will have moved:

$$5\,\mathrm{cm/year} \times 10,000,000\,\mathrm{years} = 50,000,000\,\mathrm{cm} = 500\,\mathrm{km}$$