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Theory of Plate Tectonics | Plate Boundaries | Earthquakes
Volcanic Activity | Fold Mountains

Plate Tectonics, Earthquakes and Volcanoes

Plate tectonics examines major movements in the earth's crust and other processes which contribute to the structure of the earth's surface. For this study it is important to have knowledge of the theory of plate tectonics, of variations in activity at plate boundaries, and of the consequences of plate movement. Earthquakes and volcanic activity are the two consequences which most need study. You should be able to explain their physical formation and show knowledge of their effects and of possible preventative measures. Ireland's fold mountains are a consequence of volcanic activity in the remote past, but the country is now too far from fault lines to experience any further volcanic or earthquake activity.

Theory of Plate Tectonics

This theory has developed from the merging of two other theories - the theory of Continental Drift and the Spreading of the Sea Floor theory. The basis of plate tectonic theory is that the earth's crust is made up of sixteen plates that move around the globe, an action caused by convection currents in the mantle. At one time all continents were joined together (Pangaea), but drifting led to a break-up into different continents and to the creation of plates. This theory also states that cracks in ocean floors are filling with lava, forming a new crust. As evidence to back up plate tectonic theory, the discovery of identical fossils in separate continents and the matching shapes of South America and West Africa are often cited. Detailed knowledge of this theory is necessary in the study of the formation of such features as earthquakes and volcanoes.


Plate Boundaries

Plate boundaries are described as destructive, constructive or passive, depending on the tectonic forces involved. Information about each of these types of plate boundary must be learned. Destructive boundaries occur when plates meet. In Japan, for example, the heavier oceanic plate dips under the lighter continental plate and melts into the mantle. This is known as subduction and leads to the creation of volcanoes, mountain ranges and earthquakes. The separation of plates creates constructive plate boundaries, as in the Atlantic Ocean. Magma rises to form volcanoes, mid-oceanic ridges and earthquakes. Passive boundaries are where plates slide past each other and neither create nor destroy - for example, the San Andreas Fault, along the west coast of the United States.



You need to know about the causes of earthquakes and your knowledge of the theory of plate tectonics and of plate boundaries will help here. You must also be able to describe an earthquake, so you need to be able to talk about its focus and its epicentre and about shockwaves. The effects of an earthquake are normally negative - they include death, fire, landslides, tsunamis, and the destruction of property and routeways. An earthquake is measured with a seismograph and the measurement is graded on the Richter Scale. Finally, you need to know how the effects of an earthquake can be reduced - for example, by constructing specially reinforced buildings and by ensuring excellent recording of seismic activity so as to improve predictions. The Los Angeles earthquake of 1994 makes for a good case study.


Volcanic Activity

As with earthquakes, the causes of volcanoes can be understood by reference to the theory of plate tectonics and to plate boundaries. There are different types of volcanic mountains: they may be ash cone, composite, or dome and shield. The various results of volcanic activity with which students must be familiar include volcanic mountains, lava plateaux, mid-ocean ridges, volcanic island arcs, hot spots, geysers, and hot springs. And in referring to the life cycle of a volcanic mountain, one must understand what is meant by the terms 'active', 'dormant' and 'extinct'. The contribution of volcanic activity to human society can be both positive and negative. Positive effects include tourism, the generation of heat and electricity from hot springs and the creation of fertile soil. The negative effects include death, damage to property and mudslides.


Fold Mountains

To complete the study of volcanoes, examples of folding found in Ireland can be studied. Caledonian fold mountains are found in the North and Northwest of the country, and Munster is home to many examples of Armorican fold mountains. It is particularly useful to understand these effects of volcanic activity when asked to identify physical features on an Ordnance Survey map.

An excellent and attractive site on plate tectonics. In particular, check out the sections on 'Understanding plate motions' and 'Plate tectonics and people'.
A worthwhile site with interactive illustrations of plate movement, continental drift, volcanoes, etc.
A helpful overview of plate tectonics with a particularly useful list of 'rules' at the end.
Excellent site on volcanoes, with details of the eruption of Mount St. Helens in 1980 and the formation of composite volcanoes.
The causes of earthquakes are simply explained on this site.


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