Oceanic and continental crust
The crust is the surface of the Earth, it is a rock layer forming the upper part of the Lithosphere. The Litosphere is split into tectonic plates, which move slowly on the Asthenosphere
There are two types of crust:
Oceanic crust -Found under the oceans, thin but dense, made of basalt.
Continental crust- Forms land, thick but has a low density, made of granite.
Asthenosphere and mantle
The movement of the tectonic plates is evidence there is a 'lubricating layer' under the litosphere, the asthenosphere.
Geologists think the asthenosphere is partly molten rock and partly solid rock, because if it were liquid the tectonic plates would sink through it.
The asthenosphere is the top layer of the mantle. The mantle is the largest of the Earths layers by volume and is mostly solid rock.
Geoligists think that the core is metal mainly nickel and iron, evidence of this are meteorites, which are fragments of rock and metal that fall to Earth from space.
Meteorites may be fragments of the Litosphere, core and mantle of a shattered planet.
Earths heat engine
We know the inside of the Earth is hot because of:
- Molten lava spewing from active volcanoes.
- Hot springs and geysers.
Geothermal heat: Heat from inside the Earth. The heat is produced by the radioactive decay of elements such as uranium and thorium in the core and mantle.
The inner core is so deep and under so much pressure it stays solid. The outer core is liquid because it is under less pressure.
As heat rises from the core, it creates convection currents in the liquid core and mantle. These convection currents are strong enough to move the Earths tectonic plates on the Earths surface.
Plumes - The part of the convection currents where the heat moves towards the surface.
These are concentrated zones of heat, in a plume the mantle is less dense.
Plumes bring magma to the surface. If it breaks through a crust it erupts as lava in a volcano.
- Some plumes rise like long sheets of heat, forming constructive plate boundaries at the surface.
- Others rise in columns of heat, forming hotspots. They can be in the middle of a tectonic plate, like Hawaii and Yellowstone in the USA.
The magnetosphere - The huge magnetic field surrounding the Earth.
The magnetosphere protects the Earth from harmful radiation from space and the sun.
The Earths magnetic field is made by the outer core.
As liquid iron in the outer core flows, it works like an electric dynamo, producing the magnetic field.
The continents were all once joined together - Pangea.
We know this because identical rocks were found in Western Africa and Eastern South America for example.
Today the Earths lithosphere is split into 15 large tectonic plates and over 20 large ones.
The plates meet at plate boundaries:
- Constructive plate boundaries are formed when 2 plates move apart.
- Destructive plate boundaries are formed when 2 plates collide.
- Conservative plate boundaries are formed when 2 plates slide past each other.
Most Earthquakes and volcancanos are found on plate boundaries
Crust: old and new
New oceanic crust forms constantly at constructive plate boundaries:
- Convection currents bring magma up from the mantle.
- The magma is injected between separating plates.
- As the magma cools it forms new oceanic crust.
- The plates continues to move apart, allowing more magma to be injected.
Old crust is destroyed by subduction at destructive plate boundaries.
Continental crust was formed bilions of years ago and has not formed since, this is because it is less dense than oceanic crust, so cannot be subducted and destroyed.
Constructive plate boundaries
- Two tectonic plates moving apart.
- New oceanic crust is forming constantly at the gap created.
- The magma is injected between the two plates, as it cools it forms new oceanic crust.
- The magma is runny so shallow sided volcanoes are formed.
- Small earthquakes are caused by friction as the plates tear apart.
- Volcanoes that are not very explosive/ dangerous are formed. E.g. Iceland the Mid Atlantic Ridge.
Destructive plate boundary
- Where an oceanic plate meets a continental plate. They are moving towards each other.
- The denser oceanic crust is subducted under the less dense continental crust.
- As it sinks and melts it makes magma called andesite, sea water is dragged down with the oceanic plate.
- This makes the magma less dense so it rises through the continental crust.
- The water erupts as steam making the volcanoes very explosive.
- Sinking oceanic plate can stick to the continental plate. Pressure build up against the friction. When the plates snap apart alot of energy is released as an Earthquake.
- Very destructive earthquakes.
- Very explosive, destructive volcanoes which cool to be steep sided. E.g. Andes mountains, Peru, Chile.
Conservative plate boundary
Formed where two plates are sliding past each other.
- Destructive earthquakes.
- Small Earth tremors daily.
- No volcanoes. e.g. San Andreas Fault, California.
- A type of destructive boundary where two continental plates move toward each other.
- As they meet, they push upwards forming moantain ranges, e.g. The Himalayas.
- Destructive Earthquakes.
- Volcanoes are rare.
There is a clear relationship between the distribution of Earthquakes and volcanoes and the plate boundaries.
There are exceptions however for example the Hawaiin islands have formed in the middle of the pacific ocean.
This is explained as a hotspot theory, That there are fixed spots in the mantle where magma rises to the surface in the form of plumes.
Volcanic Explosivity index (VEI)
Measures the destructive power of a volcano on a scale of 1 to 8.
Volcanoes are measured using a seismograph or seisometer, which is an instrument used to detect and record earthquakes.
It does this using the richter scale or moment magnitude scale.
- An aircraft is used to measure the amount of gas the volcano gives off.
- Tiltmeters detect when the volcano swells up as it fills with magma.
- Seismometers measure Earth tremors which will increase as magma rises.
- Boreholes to measure water temperature as magma heats up.
- Made from andesitic lava (lower temperatures, high silica and lots of dissolved gases). So magma is less fluid and more likely to explode when it reaches the surface.
- They have steep sides with layers of ash and lava.
- The ash is formed out of the material destroyed in the explosive eruptions.
E.g. Mount Fuji
Made from basaltic lava (high temp very low in silica with low gas content) means it flows long distnces on the Earths surface.
Creates very large gently sloping shapes.
e.g. Mauna Loa
Montserrat (composite volcano)
Island in the Carribbean
- Monserrat lies on a destructive plate boundary.
- As the Eurasian and Caribbean plate merge the Oceanic plate is subducted under the continental plate.
- As it is forced down pressure increases which triggers Earthquakes and at the same time heat produced by friction melts the descending crust to form molten magma.
- 2/3 of the island was covered in ash.
- 50% of the island were evacuated to the North of the island to live in makeshift shelters.
- Plymouth- the capital became a ghost town.
- Floods as the valleys were blocked with ash.
- The airport and the port were blocked with ash.
- Farmland was destroyed.
- Forest fires caused by pyroclastic flows.
- Many schools and hospitals destroyed.
- Most of the Southern area was destroyed any remaining inhabitants have had to endure harsh living conditions in the North.
- Transport remains a problem for people travelling to the island as the port and airport remain closed.
- The tourist industry is still suffering with few visitors except for cruise ships looking at the volcano.
- Over half the population left the island and have not returned.
- 41 million was given in aid by the British Government.
Money was given to individuals to help them move to other countries.
Riots occured as locals complained that the British were not doing enough.
The MVO was set up to study the volcano.
Icelandic eruption (shield volcano)
- A constructive plate boundary where the Eurosian plate meets and the North American plates are moving apart.
- This causes magma to rise to fill the gap and magma is erupted into the Earths surface (Iceland itself was originally formed in the same way).
- Opening of a ground fissure (crack) of 150m
- Rivers saw a rise in their temperatures and water levels due to melting ice cap water = flooding.
- Contamination of water sources and rivers due to ash.
- Ash cloud reaches Europe within 2 days disrupting air flights.
- Eruption cloud reached 8 Km high.
- Long term impacts on farming due to the layer of ash that has fallen on some Icelandic farms and pastures has become wet and compact making it difficult to continue farming.
- Increase in short-term volcano tourists who wanted to see the eruptions.
- Global disruption of air flights leads to increased insurance claims and decline in profits for airline companies.
- A possible chain reaction, leading to eruptions from other volcanoes in the area.
- Possibly a shorter and colder summer in Europe.
- 800 farmers and their families evacuated from the local area.
- Flights to and from Iceland were postponed.
Why are people in developing countries at greater risk from tectonic hazards than people in developed countries?
- More live in risky conditions- no where else for them to live.
- Can't afford safe well built houses, they collapse easily.
- Don't have insurance.
- Governments dont have money to provide aid.
- Poor communications, no warning or evacuation.
Mountainous region between Pakistan and India
- A collision plate boundary involving the Eurasian and Indian plates.
- Measured 7.6 on the Richter scale.
- Pakistan suffered 73,000 deaths.
- Many of the dead were children as it was a Saturday morning- normal school day.
- Many more died as it was Ramadan so people were sleeping after getting up early for the pre-dawn meal.
- 2.8 million homeless.
- Lack of food, clean water and shelter meant 120,000 people are at risk of death.
- Secondary land slides expected.
- Winter snows were due to start making the relief effort difficult.
- International aid agencies used helicopters to fly in blankets, tents, basic provisions and medical supplies into the area.
- A conservative plate boundary involving the pacific plate slipping past the North American plate.
- Measured 6.9 on the richter scale.
- 63 people died
- 3,757 injured
- 12,000 homeless
- Increeased number of home insurance claims due to it beaing a MEDC
- Decline in tourism income as people don't feel the area is safe.
- Collapse of overpasses means that emergency services have difficulty accessing certain areas.
Planning and prediction
- Emerency plans
- Warning systems
- Training of people
- Evacuation plans
Improving building design:
Hollow concrete bricks designed to cause minimal damage if they fall in another earthquake.
Roof made from reinforced cement concrete (RCC).
Foundations made of stone largely from the remains of destroyed houses.
Reinforced steel cornor pillars providing strength and flexibility.
Short term: Includes mainly immediate aid to keep people alive e.g. tents, blankets and food.
Long term: Looks to improve the preparedness of the people and reduce to impact of future events.