causes of natural disasters Flashcards
1
Q
explain the formation of tropical storms
A
- Tropical storms require ocean temperatures of approximately 26. ̊C and over, to a depth of 60m to form.
- This encourages evaporation of water from the sea surface.
- The rising air that is heavy with moisture created by this evaporation rises rapidly.
- Due to the upwards movement of air, the air pressure decreases, creating low air pressure.
- Air rushes in from high pressure areas to replace the air that is escaping upwards. It is warmed and starts to rise as well, creating more fuel for the storm.
- The rising warm moist air is seeded by dust and sand particles high up in the atmosphere.
- Off the west coast of Africa storms are seeded by sand blown, SW towards the Atlantic Ocean, by a tropical continental air mass. This usually happens during sandstorms in the Sahara Desert.
- Those particles when carried high up into the atmosphere, where temperatures are cooler, provide a cool surface for the warm moisture rich air to condense around.
- The rapid condensation forms large supercell thunder clouds.
- Due to the spinning of the Earth on its axis those storms start to spin and merge together.
- When wind speeds reach 120kph or 70mph an eye forms and the tropical storm officially becomes a hurricane
2
Q
explain how volcanoes form at convergent/destructive plate boundaries
A
- Where the convection currents move towards each other at the edge of two plates, the two plates will also move towards each other.
- If two plates of different densities move towards each other i.e. an oceanic plate and a continental plate, the heavier plate will sink down underneath the lighter one and melt into the mantle. Those types of plate boundaries are called destructive plate boundaries, because the edge of the oceanic plate is being destroyed.
- The two plates rub against each other. This creates a huge amount of friction. The friction releases heat.
- As the oceanic plate sinks down and melts into the mantle, the pressure in the mantle decreases.
- Gas bubbles generated by the friction, the heat and the changing pressure as well as by plankton on the oceanic plate releasing methane as the plankton are heated. Magma rises to the surface creating an explosive volcanic eruption.
- During those explosive eruptions, the lava is pulverised to form a huge cloud of superheated ash, gas and dust – a pyroclastic flow. Those can travel at over 70 mph and can reach temperatures of over 1000°C. At other times very viscous lava is ejected. Due to its thick, sticky nature, it is very slow moving.
- The volcanoes formed at destructive plate boundaries are called composite or stratovolcanoes, because their sides are made up of alternate layers of cooled lava and cooled ash.
3
Q
explain the formation of volcanoes at divergent/constructive boundaries
A
- Where there is ridge push and the convection currents move away from each other at the edge of two plates, the two plates will move apart.
- Pressure in the mantle reduces as mantle rock moves upwards at the plate boundary. This reduces the melting point of the magma
- The molten magma rises up from the mantle, forming lava when it reaches the Earth’s surface
- This lava cools to form new crust. This is why those types of plates are also called constructive plates, they are building/constructing new crust.
- The lava can also form volcanoes, volcanic islands, mid-ocean trenches (deep valleys along the sea-floor) and ridges
4
Q
explain how volcanoes form at hot spots/hot plumes
A
- Hot plumes are stationary up-wellings of unusually hot molten rock in the mantle.
- Convection causes the hot molten rock to rise up to the base of the crust. Reduced pressure encourages melting of areas of the crust at the hot plume.
- The point at where this occurs on the crust is known as a hot spot.
- Volcanoes form on the ocean bed or the land surface, at the point on the crust where the hot spot is located.
- As a section of the plate moves away from the hot spot, volcanic activity at that point on the plate ceases.
