Before we get into talking about tectonic hazards, we must first understand the composition of the Earth – that might sound cool, but it’s actually unbelievably hot!
So here goes: the dense core of the Earth is surrounded by a mantle. The mantle is surrounded by a thin outer layer called a crust. A thin layer of gases called the atmosphere surrounds the crust.
The crust is very thin, ranging from five – 70 km. The average thickness of the crust under the ocean is six km and the average thickness under continents is 35 km.
The Earth’s crust is made up of seven principal tectonic plates and numerous other smaller plates. The plates are sections of the crust that “float” on the mantle, which is made up of molten rock.
Where the plates meet the scale of the forces involved mean that they can form features such as volcanoes, fold mountains, deep-sea trenches and earthquakes – our tectonic hazards!
There are two main types of tectonic plate. Oceanic crust is often only about five km thick, but is very dense. Continental crust is considerably thicker, often being approximately 30 km deep, but is less dense.
The Earth’s tectonic plates all move very slowly on the mantle (and I mean really, really slowly), meeting along the four main types of plate boundary. The plates move due to convection currents in the mantle. These are hot currents of molten rock that slowly move within the mantle and cause the plates above them to move, usually by as little as one or two centimetres each year.
Destructive plate boundaries (also known as convergent or compressional boundaries) are found when two tectonic plates move towards each other (usually an oceanic plate and continental plate). The denser oceanic plate dives under the lighter continental one, creating a deep ocean trench. As the oceanic plate goes deeper into the mantle it melts in the subduction zone, due to friction and the increased temperature.
The molten rock is less dense than that which surrounds it, so it will rise towards the surface and cause volcanoes on the earth’s surface (bring on the tectonic hazard). The continental crust is crumpled by the collision of the two plates creating fold mountains. If the magma rises offshore it will form an island arc, like the West Indies and Japan. A good example of a destructive plate boundary is where the Nazca plate dives underneath the South American plate. This has caused volcanoes, earthquakes and the formation of the Andes Mountain Range.
Constructive plate boundaries (also known as divergent, passive or tensional boundaries) are found when two plates are moving away from each other. Although often not as violent as those on destructive plate boundaries, volcanoes and earthquakes do occur on constructive plate boundaries. They also cause mid-ocean ridges to form.
Molten rock (magma) rises from the mantle to fill the gap between the two plates. This forms a mid-ocean ridge. Volcanoes can also form here, along the edges of the plate boundary, due to the rising magma. These volcanoes are called shield volcanoes. A good example of a constructive plate boundary can be found where the North American plate is diverging from the Eurasian plate. This has caused the Mid-Atlantic ridge to form and has created Iceland among other volcanic islands through volcanic activity.
There are three main volcanic cones: acid lava cones, composite cones and basic lava cones. Acid cone volcanoes are steep sided due to the lava being thick and acidic, meaning that it doesn’t flow far before solidifying, e.g. Mount. Pelee.
Shield cone volcanoes are wide-based, with gentle slopes. Their lava is runny and thin, which means that it can travel a long way before cooling and solidifying. Often these eruptions are non-violent and can last for years, such as the one at Kilauea in Hawaii.
Composite Cone volcanoes are steep-sided plus have alternate layers of ash and lava. Often the lava cools in the pipe of the volcano to create a plug in the vent, meaning that a large explosion is needed to remove it. The best example is Mount St. Helens.
So hopefully that gives you an idea of not just the composition of the Earth, but also the hazards we face when our tectonic plates move!