Plate Tectonics Flashcards
Structure of the earth
Crust: Relatively thin layer of solid rocks around the outside of the earth
Mantle: A layer of melted rock 2900km beneath the crust. Very hot and main rock type is peridotite
Outer core: A layer of molten rock 2900-5000 km below the crust. Average temps of 3000 c and an iron/nickel composition
Inner core: Centre of the earth
Major plates
The Pacific, Eurasian, African, American, Indo Australian, Nazca plate, Antarctic plate
Plates
Plates are sections of very thin, they float like rafts on the semi-molten material that makes up the Earth’s mantle. They move on top of the mantle by a series of heat/convectional currents. Movement is small at less that 1cm per year, but can result in eruptions/ earthquakes
Destructive plate
Happens when Oceanic and continental plates move together. Oceanic plate is denser and is forced down into the mantel (below continental). It melts and is released as magma (volcano). Continental plate is forced uo and earthquakes occur due to the movement of the plates e.g Monsterrat
Constructive plate
New crust forms in undersea valleys in mid-ocean. Oceanic plates move apart causing magma to rise up as a volcanic eruption and one cooled, new land is formed. New crust gradually pushes the older crust sideways and away from the ridge. A mid-ocean ridge. Minor earthquakes occur e.g Mid Atlantic ridge
Conservative plate
Two plates move past one another. Pressure builds up as the plates move causing a massive earthquake. No crust is created or destroyed and no volcanic eruptions occur e.g Haiti earthquake
Collision plate
Continental plates converge (move together). They are pushed up to form a fold mountain (e.g Himalayas). Eruption occurs and sometimes violent earthquake (Nepal 2015)
Divergent plate boundary
Plates move apart; earthquakes and magma are common
Convergent plate boundary
Oceanic crust is often forced down into mantle where it begins to melt. Magma rises into and through the other plate, solidifying into granite. So continental crust is created and oceanic crust is destroyed
Transform plate boundary
Two plates slide past each other (e.g San Andreas fault zone)
Volcanoes form
Volcanoes are formed when magma from within the Earth’s upper mantle works its way up to the surface. AT the surface, it erupts to form lava flows and ash deposits. Over time, it will get bigger and bigger
Volcanic hotspots
Hotspots are areas in the lithosphere (crust/ upper mantel) that are underlain by hot magma. This heat causes partial melting of the lithosphere, eventually leading to volcanic activity. Hawaiian Islands are a classic example of a volcanic grouping formed over one hot spot
Active volcano
If a volcano has erupted recently
Dormant volcano
If it has erupted in the past 2000 years
Extinct volcano
If it hasn’t erupted for many thousands/ millions of years
Shield volcano
These volcanoes have runny lave (non vicious) because of this they don’t have an explosive eruption. Lava spreads quickly across the landscape. Each eruption adds a new layer or rock built on the previous. Eventually, a wide dome of rock is built. Looks like a curved Shiel and slopes are very gentle. Hawaiian islands are a chain of shield volcanoes
Composite volcano
Most common type of volcano. Classic shape. They are formed by hardened layers of lava and ash from successive eruptions. The lava is vicious (therefore thicker than Shiel volcano). This lava cools and hardens before spreading very far though and therefore they are steep sided. The eruptions tend to be very violent, capable of producing deadly pyroclastic flows. Chances peak, Monsterrat is an example of one
Features of volcano
Magma chamber (bottom), cone (shape), vent (vent of lava going upwards), crater (where lava comes out), lava (liquid coming out), ash (cloud above)
Case study: volcanic example in an LEDC
Monsterrat
Date: July 1995
Eruption lasted: 5 years
Chances peak volcano had been dormant for 20 years. Emergency plans were to evacuate people to nearby islands or Britain. Killed 23 people. Destroyed a lot of homes and farms. Lots of jobs lost. pyroclastic flow covered large areas of island. Located on a destructive plate boundary (North American plate moves towards Caribbean plate), North American plate is denser so it is forced into mantle. Molten rock is lighter than surrounding rock so it rises through the cracks towards the surface. Ends up producing andesitic lava (strato volcano) and lava is so thick it created a dome on the side of chances peak. Dome suffered serious collapse in 1997 and led to ash fall and pyroclastic flows engulfing a lot of the south of the island.
Effects:
landscapes changed, animals like cows are now feral as they were set free, mountain chicken frogs’ habitats destroyed, gases released, coral reefs covered in ash, business decline, economy damaged, Plymouth destroyed, less housing
Prediction
- lasers to detect swelling of volcano
- chemical sensors to measure increase in sulphur levels
- seismometers to detect the large number of earthquakes that occur due to the magma rising up
- ultrasound can monitor low frequency waves within the magma
- gas sampling
prevention
- have an evacuation plan
- nobody allowed to live in risky zones
Case study: Why live near a volcano?
Monsterrat
- fertile land for farming-> jobs
- nice landscapes
- volcanic activity always monitored so it’s not a really negative factor
- tourims
- geothermal energy can be harnessed
- minerals are contained in lava, e.g diamonds can be mined to make money
Earthquakes
An earthquake is the shaking and vibration of the crust due to movement of the Earth’s plates
The focus
The point inside the Earth’s crust where the energy is released
