L2 - An introduction to tectonic hazards

• the earth is made of 4 distinct layers, the crust, the mantle, the outer core and the inner core
• the crust is known as the lithosphere and the upper portion of the mantle, the asthenosphere
• Crust: This is the outer layer of the earth. It is split into 'plates' of various sizes. These plates can be made up of either 'Continental Crust' or 'Oceanic Crust'. This layer of the earth is the thinnest, only 5-10 kms thick at the Oceanic Crust but can be 20-200 kms thick at the Continental Crust. The Oceanic Crust may be thinner, but it is more dense, meaning that it can sink below the Continental Crust and can be destroyed and renewed. Oceanic Crust tends to be young rock, less than 200 million years old whereas Continental Crust is older, up to 3.8 billion years old.
• Mantle: This section is solid but still able to flow but very slowly. This makes up most of the earth. It is very hot - around 5000oC. The upper portion of the mantle is a weak layer that can deform like plastic. These high temperatures create 'Convection Currents', where the heat rises within the magma, cools down slightly as it reaches the surface (crust) and then sinks towards the core. This 'cycle' creates movement, whichmoves the plates on the crust
• Core: This is the densest part of the earth. The inner core is solid, made up of iron and nickel and around 6000oC. The outer core is semi-molten and slightly less hot.
• At plate margins where the crust is moving away from each other, ocean ridges form high above the ocean floor. Beneath the ridges, the mantle melts, the magma rises and cools down to form new material. As the lithosphere cools, it becomes denser and starts to slide away from the ridge, which causes plates to move away from each other. This is called 'Ridge Push'.
• At destructive margins, the denser oceanic plate sinks back into the mantle under the influence of gravity, which pulls the rest of the plate behind it. This is called 'Slab Pull'.
• Volcanoes and Earthquakes are not randomly distributed
• Generally, they are found at plate margins/boundaries for example the Pacific and North American plate
• Volcanoes occur at Constructive and Destructive plate/margins boundaries for example the Nazca plate and South American plate
• Earthquakes occur at all 4 types of boundary - Destructive/Collision plate margins/boundaries tend to give the strongest ones for example the Nazca and south American plate
• Volcanoes and Earthquakes for this reason tend to occur in narrow 'belts
• Volcanoes and Earthquakes are found on both land and sea.
• There is a large band of Volcanoes and Earthquakes circling the Pacific Ocean, known as the 'Ring of Fire'.
• There are some exceptions to these patterns. Volcanoes can appear in the middle of a plate due to 'hotspots'.
• Where the crust is particularly thin, magma can rise and break through the plate. This can lead to a chain of volcanoes in the middle of the sea known as island arcs for example Kilvea
• Earthquakes can also occur in the middle of a plate for example India due to the reactivation of old fault lines.
• GIS can be really useful in studying plate tectonics – it can show us where plates are moving, it can show the area a volcano is likely to explode and therefore where to place an exclusion zone
• fold mountains are distributed along plate margins where continental crust is being uplifted, this includes continental crust at destructive margins such as on the west coast of south america
• fold mountains also occur at collision margins between two continental crusts, such as the himalayas that span several countries, including china and india
• ocean trenches are located at subduction zones where oceanic crust descends below continental crust at destructive plate margins, this includes the area around the pacific ocean known as the ‘pacific ring of fire’ such as off the west coast of south america
• fold mountains are simply mountains that have formed through the collisions of plates
• young fold mountains include the himalayas, rockies, andes and alps
• ocean trenches form some of the deepest parts of the ocean (such as the marianas trench) where subduction and slab pull are pulling the oceanic crust below the continental crust
• both fold mountains and ocean trenches result from plates moving towards each other
• if fold mountains and ocean trenches occur in the same place, they are associated with subduction and destructive plate margins / boundaries
• if fold mountains occur by themselves, they are in areas where collision is taking place