Investigating different Types of Plate Boundaries and Hazards

© British Broadcasting Corporation (BBC)

© British Broadcasting Corporation (BBC)

In the last edition of GITN we looked in detail at different plate boundaries and earthquakes. Refresh your memory with this diagram from the last edition or go back and read it again.

Image: Tectonic plate boundaries - Jose F. Vigil. USGS © Public Domain

In this edition we are going to look at the other main type of hazard associated with plate tectonics which are volcanoes and finally in the last linked article we will look at what we as humans can do to plan, prepare and possibly prevent some of the hazards caused by earthquakes and volcanoes.

Remember the structure of the earth; that the rigid plates (crust and upper mantle) move on convection currents in the hotter and softer rocks of the mantle below them.

Divergent Boundary

When two plates move apart the boundary will commonly have ‘relatively’ weak earthquakes but will have a lot of volcanic activity. Refresh your memory with this diagram from the last edition.

Image: Continental-continental constructive plate boundary - domdomegg © Wikimedia Commons under Creative Commons Attribution-Share Alike 4.0 International

In the last edition we looked at the Mid-Atlantic Ridge and Iceland as a good example.

Image: Iceland Mid-Atlantic Ridge Fig16 - USGS © Public Domain

As well as the ‘weak’ earthquakes people living in Iceland have to cope with a lot of volcanic activity.

The type of magma that is created at Divergent boundaries tends to be very fluid (runny) - go back to the previous edition to read a detailed account of magma formation. When it reaches the surface and becomes lava it can flow easily over several kilometres. As a result it creates wide volcanoes with gentle slopes which look like a warriors shield from Anglo Saxon or Viking times. These volcanoes are therefore called shield volcanoes.

Image: Skjaldbreidur Herbst 2004 - Reykholt © Wikimedia Commons under Creative Commons Attribution-Share Alike 3.0 Unported

1. Ash Cloud
2. Lava fountain
3. CRATER
4. Lava lake
5. Fumaroles
6. LAVA FLOW
7. LAYERS OF ASH & LAVA
8. Stratum/layer
9. Sill
10. Magma conduit
11. MAGMA CHAMBER
12. Dike

Image: Hawaiian Eruption-numbers - Sémhur © Wikimedia Commons

We will look at some of these in more detail later.

Convergent Boundary

When plates are forced together there may be volcanic activity if one of the plates is made up of oceanic crust. Oceanic crust can be forced downwards into the mantle where parts of it will melt.

Image: Continental-continental destructive plate boundary - domdomegg © Wikimedia Commons under Creative Commons Attribution-Share Alike 4.0 International

We looked in detail at how magma is made at convergent boundaries in the last edition; what is important in this edition is to understand that the magma found here is very thick and viscous.

Viscous magma cannot flow easily so will make steep sided cone shaped volcanoes when it reaches the surface and becomes lava.

Image: Vesuvius from plane - Pastorius © Wikimedia Commons under Creative Commons Attribution-Share Alike 3.0 Unported

Another feature of thick viscous magma/lava is that dissolved gasses within it cannot escape easily which is a very important feature.

Image: 04Sep2007 Etna from SE Crater - Jason Bott, Christopher Berger, Pete Garza © Wikimedia Commons under Creative Commons Attribution-Share Alike 3.0 Unported

The gasses dissolved in magma/lava are similar to the dissolved gasses in a bottle of fizzy pop. When a bottle of pop is opened you can see the gas escaping (coming out of solution) as small bubbles. This is what happens with the runny basalt (see last edition) magma/lava found at divergent boundaries.

When thick, viscous magma from a compressional boundary reaches the surface the pressure drops and the gas comes out of solution but cannot escape and builds up until it the pressure is so great that it causes an explosion. This explosion blasts overlying rock and magma in to the air as volcanic ash and volcanic bombs.

Image: Sarychev Peak eruption on 12 June 2009, oblique satellite view - ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center / NASA © Public Domain

This is why volcanoes at these margins are made up of thick layers of ash and layers of lava. These are often called composite cone volcanoes though their scientific name is a stratovolcano.

1. Ash plume
2. Magma conduit
3. Volcanic ash fall
4. Layers of lava and ash
5. Stratum
6. Magma chamber

Image: Plinian Eruption-numbers - Sémhur © Wikimedia Commons

These are the two main types of volcanic eruption that geography students are expected to know but there are many different types depending on location and magma differences use the slide show beneath to learn more about different types of volcano.

Volcanoes are not as a rule found where two plates made up of continental crust collide or at conservative boundaries. Volcanoes do however sometimes occur away from the edges of plates where a hotspot occurs in the Earth’s mantle. Scientists are not sure what causes these hot spots but we do know that as the overlying plate moves over the hotspot a volcano is formed.

The most well know hotspot is where the Hawaiian Islands have been formed as the pacific plate moves over a hotspot.

Image: Hotspot(geology)-1 - Los688 © Public Domain

By looking beneath the surface of the Pacific Ocean we can see a long line of old volcanoes called sea mounts rising up from the ocean floor. These are the old volcanic islands that used to be above the hotspot. Overtime a combination of erosion and subsidence of the sea floor as it cools away from the hotspot causes the island to sink below the surface. In time a new island volcano will grow above the hotspot and the current Hawaiian Islands will sink below the ocean surface.

Magma formed at a hotspot tends to be fluid basalt similar to magma formed at divergent boundaries so shield volcanoes are also formed here.

Image: Hawaii hotspot - National Geophysical Data Center / USGS © Public Domain

Image: Hawaiian seamount chain - Interiot~commonswiki © Public Domain