Plate Tectonics

To answer the questions "why do volcanoes erupt?", "where do they erupt?", and "why in this specific way?" it's very instructive to look at the processes behind volcanoes. Promise, I'll make it short and interesting! :-)

Inside Earth


© Volcano World

When geologists began to study earthquake waves in detail they found evidence the earth is made up of different layers with different properties.

Some background (you might skip this paragraph safely): At layer boundaries shockwaves are reflected and deflected which results in late and unexpected arrivals of wave fronts. Depending on the material of the layer, the wave velocity and dampening is different.

Today, the following layers have been identified:

The upper mantle is not fluid but moves very much in the way a glacier or cold honey does. It provides a kind "soft cushion" for the crust which floats on in. Why that is important we see in the next section.


Moving Plates


© Volcano World

Did you ever notice the coastlines of South America and Africa match quite well? This lead Alfred Wegner to his theory of Plate Tectonics which postulates the earth's crust is made up of plates which move in relation to each other.

Though it looks obvious, Wegner had a hard time proving his theory to other Geologists. To support his idea Wegner looked for mineral formations and fossils in areas which according to his theory would once have been adjacent.

He indeed found such evidence. But he encountered two difficulties: How can the continents plough through the oceanic plates? What is the engine behind the plate movement? These issues remained open questions at this time.

If plates move in relation to each other obviously gaps are created (where they move in opposite directions), bruising occurs (where they slide past each other), and collisions happen (where they converge). What exactly happens on the plate boundaries? The next section looks into this.


Sea Floor Spreading


© Volcano World, modified by C. Treber

It was not after Wegner died on an expedition through Greenland answers to his open questions were found. Through world wide surveys of the sea floor it became known ridges run through the middle of all oceans. At these ridges often active volcanism could be found. This looked very much like plates ripping apart with new material emerging in the middle.

But due to the slowlyness of plate movement this is a pretty static picture. Help to support this assumption came from the geomagnetic maps of the sea floor. Positive and negative magnetic anomalies where found which form a stripe pattern which runs parallel to the ridges and which is symmetrical to it. Now, how did that help?

The explanation: It is known the polarity of earth's magnetic field reversed a couple of times. During an eruption the current polarity gets "recorded" in lava because in molten rock magnetic particles are able to align in the direction of the geomagnetic field. When the lava cools down and solidifies the orientation is frozen. The stripe pattern is created by new matrial being extruded from the central ridge (hence the parallelity) and pushed to both sides (hence the symmetry).

This provided evidence of diverging plates and the process became known as "sea floor spreading". The volcanism associated with sea floor spreading is called "rift volcanism".


Subduction


© Volcano World, modified by C. Treber

Since the surface area of Earth remains constant (since Earth does neither shrink or grow) and new oceanic plate is being produced old plate material has to disappear. But where and how? The seafloor surveys revealed deep ocean trenches, often directly adjacent to continents. Close to the trenches often rather explosive volcanism can be found. Is this where the plates disappear into the mantle?

More evidence was needed. Earthquake recordings showed frequent events in the area of deep sea trenches. The depth of these events increases with the distance from the trench, a circumstance which was difficult to account for at first.

The explanation: The dense oceanic plate (made from basalt) collides with the lighter continental plate (made from minerals rich in silica) and is forced under it. The slab bends and decends into the half-solid mantle where it finally gets melted. Friction causes earthquakes in the rigid oceanic plate and rock melts and rises to the surface. Since the slab descends at an angle the location of the earthquakes becomes deeper with the distance from the trench.

This provided evidence of converging plates and the process became known as "subduction". The volcanism associated with subduction is called "subduction volcanism".


Hot Spots


© Volcano World, modified by C. Treber

Some volcanoes are neither near spreading centers or deep sea trenches but sit right in the middle of a plate. What kind of anomaly could be responsible for this activity? Some kind of blow torch melting through the crust?

Evidence came in the form of analysing the age of rocks in volcanic island chains. It was found the islands become continually older in the direction of the plate movement. Again the seafloor surveys provided interesting information, showing some island chains continue for many thousand kilometers under the sea until they disappear in deep sea trenches.

The explanation: A comparatively thin stream of hot lava rises vertically from the mantle and melts its way through the crust. The resulting volcanism creates an island. Because this mantle plume as it is called is stationary and the plate with the volcano on it moves a chain of islands is created over the time.

This provided evidence of hot spots. The volcanism associated with hot spots is called (you guessed it!) "hot spot volcanism".


Summary

This section summarizes the different types of plate boundaries and the resulting types of volcanism.

Just move the mouse over your section of interest.


© Volcano World

Sea Floor Spreading

Convection currents in the upper mantle push vertically against the oceanic plate and spread outwards laterally. The relatively thin plate rips and the pieces are transported away to both sides. At the rift zone basaltic magma is being extruded and forms a mid ocean ridge.

Example: Atlantic Ridge (with Iceland being the only part above sea level), Pacific Ridge.

In some cases a continental place rips and is torn apart. Instead of "Continental Spreading" you will likely encounter the term "Rift Volcanism".

Example: East African Rift, maybe Rhine Graben.

Subduction

When an oceanic plate encounters a continental plate the specifically heavier oceanic plate is subducted under the continental plate. Through friction on the plate boundaries heat is generated which melts the rock. The hot magma rises through cracks created in the process and forms volcanic chains, usually of viscous lava.

Example: Indonesia, Japan.

When continental plates collide neither plate gets subducted. As to be expected in a head on collision something gets crumpled and piled up; in this case mountains. Volcanologically this process is pretty uninteresting :-).

Example: Himalaya, European Alpes.

Hot Spots

In some places a narrow current of magma (a so called mantle plume) melts through the middle of a plate. Because the plume usually remains in one place a volcanic chain is created through the movement of the plate. Islands in the chain are youngest directly over the hot spot and become increasingly older in the direction of the plate movement.

Example: Hawai'ian Islands, Canary Islands (?).

In rare occasions hot spots occur in continental plates as well.

Example: Yellowstone.

Some examples in short:

Contact/ Movement Diverging Converging Sliding Hot Spot
Ocean/ Ocean Atlantik Ridge Tonga Trench ? Hawai'ian Islands
Ocean/ Continent ? Andes Mountains San Andreas Fault N/A
Continent/ Continent East African Rift Himalaya ? Yellowstone


1999 Anita Ford & Christian Treber Subduction Hot Spot Sea Floor Spreading Afar Kerguelen (?) Reunion Crozet (?) Hawai'i Yellowstone Tahiti (?) Easter Island Galapagos Iceland Azores Canary Islands Cape Verde Islands Ascension (?) St. Helena (?) Tristan da Cunha (?)