Plate tectonics is a theory that attempts to explain the structure of the Earth’s crust, including the distribution of earthquakes, volcanoes, fold mountains and rift valleys. It is mainly based on Alfred Wegner’s theory of continental drift and Harry Hesse’s theory of sea-floor spreading.

Layers of the Earth

The Earth consists of 4 main layers: crust, mantle, outer core and inner core. The layer between the crust and mantle is known as the lithosphere.

The crust

File:Earth's Inner Layers denoting the LAB.png
Internal structure of the Earth (By NealeyS, CC BY-SA 3.0)

The crust is the outermost layer of the Earth, between 0 and 70 km thick. It floats on the semi-molten upper mantle and consists of multiple oceanic and continental plates.

The mantle

The mantle amounts for 84% of the Earth’s volume and 67% of its mass. It is approximately 2,900 km thick and can be divided into two layers: the asthenosphere (upper mantle) and the lower mantle.

The outer core

The outer core is up to 5100 km thick and consists of slow-moving metals that create the Earth’s magnetic field.

The inner core

The inner core is a solid mass of metal that is up to 6378 km below the surface and reaches temperatures of over 5000°C.

Theory of plate tectonics and mechanism of plate movement

File:Oceanic spreading.png
Convection currents are indicated in red, black arrows show plate movement

The crust is divided into several lithospheric plates, or rigid slabs of rock, that float on the asthenosphere.

But this not a constant process. Magma from the mantle rises to create mid-oceanic ridges and then moves apart, where other parts of the crust are destroyed by subduction (they re-enter the mantle) to reduce the pressure from the newly formed crust.

The circulation of magma and movement of plates is caused by convection currents that arise from temperature differences due to  radioactivity and the residual heat of the Earth.

Alfred Wegner on Continental Drift (the forerunner of plate tectonics theory)

(not explicitly required by the syllabus)

Alfred Wegner suggested in 1912 that “around 300 million years ago all continents were joined together by a single supercontinent called Pangea, surrounded by an ocean called Panthallassa. These continents have since drifted apart from one another.”

He suggested various pieces of evidence of continental drifting.

  • The Jig-Saw Puzzle: The borders of continents seem to compliment each other, eg. South America and Africa.
  • Fossil remains: Spores of ferns have been founds on all southern continents, but they can not have crossed a wide ocean. Same is true for fossil evidence of many land species.
  • Rock sequences and geological structures were found to be similar on multiple continents.
  • Oil and coal has been found on Antarctica, meaning that the continent was once located in a different climatic zone, with active plant growth.

Distribution of tectonic plates

The map shows the major tectonic plates and their direction of movement.

Types of plate boundaries

A plate boundary is the border between two or more tectonic plates. There are different types of plate boundary, depending on the direction in which the plates are moving and whether the plates are continental or oceanic. The main types of plate boundary are: constructive, destructive, collisional and transform.

Constructive (divergent) plate boundary

A constructive boundary is formed when two plates are moving apart, creating gaps in the Earth’s crust and thereby allowing magma to surface and form new crust. More than often, magma bursts through the crust, giving rise to volcanoes as the lava cools.

Many constructive plate boundaries are mid-oceanic ridges, eg. the fault line between the Eurasian and North American plates.

In the rare case that continental plates diverge, land is pulled apart and forms a rift valley. The most famous rift valley is East African rift system, where the African plate is splitting into the Somalian plate and the Nubian Plate. These plates are moving away from each other, so Africa will be divided into two land masses. Between them, a new oceanic curst will form as magma from the mantle will surface to fill any possible gap between the two plates.

Destructive (convergent) plate boundary 

A destructive plate boundary is formed when an oceanic plate and a continental plate (or occasionally: oceanic and oceanic plates) collide. The oceanic plate is subducted under the continental plate (or oceanic plate), as it is more dense. File:Active Margin.svg

This often leads to the formation of volcanoes at destructive plate boundaries, as friction and heating cause the oceanic plate to melt and pressure builds up beneath the crust. This pressure is eventually released when magma rises through lines of weakness in the crust. Lava then solidifies to create a volcano.

When violent eruptions occur over an extended period of time, islands may form between oceanic plates, eventually giving rise to island arcs.

Also, subduction causes the seafloor and outermost crust (the lithosphere) to bend and form a steep, V-shaped depression.

Collisional plate boundary

A collisional plate boundary is formed when two continental plates collide (eg. Eurasian and Indian plate).

Fold mountains form along collisional boundaries as continental plates move towards each other and exert pressure on each other. No subduction occurs as rocks are of the same density. Instead, the crust becomes crumpled into a series of folds and the layers of crust are uplifted.

For example, the Himalaya’s were formed at the margin of the Eurasian and Indian plates, which are converging due to mantle convection currents. As neither plate was more dense, no subduction occurred and pressure built up. This forced the plates to fold upwards and form an area of fold mountains known as the Himalaya’s. (Prior to their collision, the two land masses were separated by the Tethys sea.)

Conservative plate boundary

Transform faults are formed by plates gliding past each other in opposite directions or at opposite speeds. Transform faults are very prone to earthquakes.

Example: San Andreas fault (Pacific plate and North American plate)

Earthquakes

Earthquakes are sudden, violent vibrations of the Earth’s crust produced by the shock waves (from displacement along a fault).

The point in the Earth where an earthquake originates is called the focus. The epicentre is the point on the Earth’s surface vertically above an earthquake. The epicentre is usually the location where the greatest damage associated with an earthquake occurs.

Why people choose to live in earthquake-prone areas

People may live near fault lines where earthquakes occur, as severe earthquakes are very infrequent. Earthquake monitoring systems and earthquake resistant infrastructure may convey a false sense of security. Also, earthquake-prone areas may have good economic opportunities, eg. California. Additionally, local communities may have bonded to the soil due to family tradition, culture or religion. Furthermore, people may be forced to live near earthquakes, due to population pressure and poverty (they can not afford to live elsewhere).

Impacts of earthquakes

Road torn apart after Hebgen Lake earthquake
  • Deaths/injuries
  • Collapse of buildings and homelessness
  • Flash flooding
  • Electricity supply disrupted
  • Gas pipeline fractured/fires break out;
  • Roads blocked/destroyed

Factors affecting the intensity of earthquakes

  • The depth and magnitude (energy released)
  • The duration and time of the earthquake
  • The local geology (rock structure)
  • The location of the epicentre (in a rural or urban area)
  • The building standards (eg. earthquake prone housing or low quality materials)
  • The awareness and education of residents regarding evacuation and emergency services
  • Secondary hazards

Earthquake monitoring

Earthquake Richter Scale.jpg

Earthquakes are being monitored to predict further seisomological activity (movement in the ground caused by earthquake vibrations) and reduce the impacts of tectonic hazards.

A seismograph and seismometer are used to measure the magnitude of an earthquake. This is displayed in the form of the Richer Scale, a logarithmic scale that records the energy released by movement in the ground. The Richter Scale goes from 0 to infinity, however, most earthquakes are between 0 and 9.

The following website shows all current earthquakes with a magnitude of more than 2.5: Earthquakes US Government Map

Another important scale is the Mercalli scale, which is used to depict the impacts of an earthquake. The Mercalli scale classifies earthquakes from 1 to 12, depending on the damage caused. An earthquake intensity of 1 would be measurable only using instruments, an intensity of 6 would cause trees to sway and create slight damage. An intensity of 12 would result in total destruction.

LEDC’s vs. MEDC’s

Earthquakes tend to have much more devastating impacts in LEDC’s than in MEDC’s, as LEDC’s often have poor quality housing which easily collapses, especially if building regulations are not enforced. Also, medical infrastructure is of a comparatively low standard, so people can not be treated for their injuries. Furthermore, LEDC’s often lack emergency systems and may not be educated regarding precautions. Additionally, LEDC’s may not have sufficient money for rebuilding and may have to rely on international aid, which could be delayed by poor transport and communications such as destroyed airports. Common problems in LEDC’s such as lack of food, poor water, and sanitation may be worsened by an earthquake.

Earthquake proof buildings

People have invested into buildings that are more resistant to earthquakes to minimise the damage by these hazards, especially in areas where they occur on a regular basis.

Some common techniques include:

  • using hollow concrete blocks
  • using shatterproof glass
  • making foundations of stone
  • using springs
  • constructing a reinforced concrete roof and reinforced steel corner pillars

Volcanoes

Volcanoes may be active, dormant or extinct.

  • Active volcanoes are expected to erupt soon or are erupting already. eg. Kilauea on Hawaii which has been erupting since 1983. Currently, around 1500 volcanoes are considered active.
  • A dormant volcano is not currently active or erupting, but geologists think that it will erupt in future. eg. Yellowstone caldera
  • An extinct volcano is a volcano that is considered dead, meaning it should not erupt again, usually because it has no lava supply. eg. Kilimanjaro  in Tanzania

Stratovolcano or shield volcano

There are two main types of volcano. Stratovolcanoes have steep sides and viscuous lava and pyroclastic materials. They are explosive with dormant phases and form along destructive margins involving an oceanic plate. Composite volcanoes (stratovolcanoes) have a steep slope.  Shield volcanoes have a more gentle profile and are found along constructive boundaries and at hotspots, where they erupt continuously, but not violently. They release non-viscous lava.

black-and-white, nature, mountain
Stratovolcano in Guatemala
Erta Ale shield volcano in Ethiopia

 

 

 

 

 

 

Characteristics of volcanoes

All volcanoes share a few common features. A volcano receives a supply of magma from the magma chamber (beneath the volcano). During an eruption, magma rises from the magma chamber through the main vent, where it is ejected into the atmosphere. The top of a volcano is known as a crater, because of the depression in the ground from an eruption.

Why people (choose to) live near volcanoes

People may choose to live near volcanoes, as volcanic ash often provides fertile soils for farming (cultivation of crops). Furthermore, volcanic soils are often rich in resources, such as sulfur, and thus allow for employment in mineral intensive industries. Volcanoes are also a popular tourist destination, and therefore provide job opportunities is tourist-related industries. Besides, volcanic areas are often rich in geothermal energy, and thereby meet the energy needs of nearby settlements. Additionally, local communities may have bonded to the soil due to family tradition or they may rely on eruption prediction techniques, such as seismology. In these cases, benefits of living near a volcano would seem to outweigh the risks.

However, people may be forced to live in close proximity to volcanoes, as they can not afford to move elsewhere or the country/area they live in is severely overpopulated.

Impacts of volcanoesFile:MSH04 crater eruption image 1213PDT 10-01-04.jpg

  • Deaths/injuries due to burning from the pyroclastic flow and suffocation from toxic fumes
  • Disruption of transport facilities (eg. roads) and infrastructure due to falling debris and lava bombs
  • Crops destroyed by ash
  • Ash cloud reduce visibility and block air traffic
  • Ice caps may melt and trigger flash floods due to intense heat
  • etc.

 

Further information:

Case Studies on Eyjafjallajökull (volcano in Iceland) and Haiti earthquake 2010