Structure of the Earth
Lithosphere- Solid outer layer of the earth. Broken up into tectonic plates.
- Continental crust: 900ºC, 2.7cm³ density, made of granite, 30-50km thick, solid
- Oceanic crust: 900ºC, 3.3cm³ density, made of basalt, 5-10km thick, solid
Mantle- Layer between the crust and the core.
- Asthenosphere: 900-1600ºC, 3.4-4.4cm³ density, made of peridotite, 175km thick, semi-molten liquid
- Lower mantle: 1600-4000ºC, 4.4-5.6cm³ density, made of peridodite, 2200km thick, solid
Core-Cente layer of the Earth.
- Outer core: 4000-5000ºC, 9.9-12.2cm³ density, made of iron and nickel, 2370km thick, liquid
- Inner core: 5000-7000ºC, 12.8-13.1cm³ density, made of iron and nickel, 1100km thick, liquid
Radioactive decay: Heat from the inside of the Earth is called geothermal. The heat is produced by radioactive decay in the core and mantle. Atoms of elements such as uranium and thorium decay, releasing particles from their nuclei and give off heat.
Convection currents: A substance is heated from below, rises, cools and sinks in a continuous cycle. The rising part of a convection current is called a plume.
Convection currents in the outer core: The outer core is liquid iron and moves in continuous convection currents. This creates our magnetic field which causes us to have magnetic poles and creates an invisible force field protecting the Earth from harmful radiation from space and the sun.
Convection currents in the asthenosphere: Plumes in the convection current of the upper mantle rise to the top, but then get stuck under the tectonic plates of the lithosphere. This forces the material to spread outwards, causing it to cool and sink. As the material spreads, it drags the tectonic plates above.
Hot spots: A plume occuring in the middle of a tectonic plate is a hot spot. This is where molten material heats and melts its way through the litosphere, creating a submarine volcano.
Destructive: An oceanic crust plate and a continental crust plate move towards each other, destroying the oceanic crust. Two convection currents are moving towards each other as they cool. When they meet, they sink. This drags the plates towards each other.
E.g. Andes Mountains in Peru and Chile are being created by the continental South American plate colliding with the oceanic Nazca plate.
Constructive: Two plates move apart, forming new ocean floor as magma erupts. A convection current plume of basaltic magma in the asthenosphere rises to the surface, cools and spreads outwards which pulls the plates above it apart. The magma rises out the fissures in the crust.
E.g. The Mid-Atlantic Ridge is constructed by Eurasian and North American plates pulling apart.
Conservative: Two plates are sliding past each other in either opposite directions or the same direction, but at different speeds. No plates are created or destroyed. Convection currents drag the overriding plates past each other.
E.g. The Pacific plate and North American plate are sliding past each other creating faults on the west coast of the USA.
Shelid (E.g. Mount Kilauea, Hawaii. Hot spot volcano)
- Lava type: Basaltic
- Lava temperature: High (1200°C approx.)
- Lava silica content: Low
- Lava viscosity: Fluid, runny lava
- Explosivity: Not explosive, (gentle eruptions) as gases escape easily.
Each eruption forms a new layer. They can erupt regularly and gently so they keep a wide side.
Composite (E.g. Mount Merapi, Indonesia. Destructive plate boundary)
- Lava type: Andesitic
- Lava temperature: Lower (900-1000°C approx.)
- Lava silica content: High
- Lava viscosity: Thick, sticky lava.
- Explosivity: Explosive as gases get trapped.
Each eruption forms a new layer. They don't erupt often because gases cannot easily escape, this means that it forms a cone-shaped volcano.
- Lava flow
- Lava bombs (made of hot rocks from volcano crater, flies through the air)
- Ash and gas column (material is called ejecta or tephra. Hot ash and gas shoot into sky)
- Ash (tiny fragments of rock float in air and settle on surfaces. Can be heavy-cause roofs to collapse)
- Lahar (ash and water that travels quickly down the volcano and drowns people)
- Pyroclastic flow (made of hot ash and gases that explode out of volcano. It's the build up of pressure when gases are trapped in magma causing the explosion.
- Landslide (made of dry soil and rock which slides downhill)
Sheild volcano- Ash and gas column, ash fall, lahar and lava flow
Composite volcano- Ash and gas column, ash fall, lahar, lava flow, pyroclastic flow, landslide, lava bombs
Predicting volcanoes and VEI
We can predict by:
- Aircraft measure gases given off by volcano. E.g. more sulphur dioxide indcates magma chamber is filling.
- Boreholes down into the volcano measure water temperature because if the magma is rising, it will heat the water.
- Hot springs are monitored for temp or activity changes.
- Tunnels contain seismometers which measure earthquakes.
- Tilitmeters detect when the volcano swells as it fills with magma.
Most volcanoes have planned evacuation routes which are signposted.
VEI- Volcanic Explosivity Index. It measures eruptions from 0-8. 8 being the most powerful.
Mount Merapi, Indonesia, 2010 (volcano case study)
- Composite volcano. Above where the Indo-Australian plate subducts beneath the Eurasian plate on a destructive plate boundary.
- Many eruptions between 25th Oct - 30th Nov 2010
- VEI: 4
- 353 died, 577 injured, 350000 evacuated
- Lava- some flowing into mountain rivers
- Pyroclastic flows- travelling over 11km from crater
- Airplane distruption due to ash clouds and people suffering respiratory difficulties
- Deaths, injury, loss of property, livestock injured.
- Villages destroyed, property buried from landslides
- Panic- loud explosions causing people to flee and block up roads.
- The focus of the earthquake is where it occurs.
- The epicentre is the point directly above the focus, at the surface
- The fault is where the different plates meet or where the crack in the plate is.
- Seismic waves are where the ground shaking travels through the rock.
The closer to the surface the focus of the earthquake is, the more powerfully the waves of the earthquake can be felt.
The magnitude of an earthquake is how powerful it is. It is measured of the Richter Scale (strength of the ground shaking) and the Modified Mercalli Scale (amount of damage done by the earthquake)
- The Richter scale is measured from 0-10. Each step up on the scale is 30 times more powerful than the previous stage. It gives people a basic understanding of the scale however doesn't measure the scale of damage caused by the earthquake.
- The Modified Mercalli scale is measure from 1-12 in Roman Numerals. It measures the damage done by the earthquake. It is good because it is easy to compare the damage done but doesn't give a scientific measurement.
- An underwater earthquake occurs where an oceanic plate is subducting below a continental plate. When the pressure is released, the oceanic plate flips up which causes the water above the fault line to move and create huge ripple-like waves outwards from the epicentre.
- In open water the tsunami travels very fast but as the wave reaches shallower water near land, the base of the wave slows down due to friction against the now shallower sea floor. The crest of the wave is not slowed down meaning that water piles up and gets higher as the wave reaches the shore.
- The wave doesn't break and wash back out to sea. Instead, it just keeps on coming into land as the water behind it keeps pushing.
LEDC- Indonesia: 26.12.04- 9.1 magnitude earthquake under the Indian Ocean. Waves up to 30 meters high. Over 130,000 deaths. They are still trying to recover.
MEDC- Japan: 11.03.11- 9.0 magnitude earthquake under the Pacific Ocean. Waves up to 40 meters high. Over 15,000 deaths. Area is mostly recovered now.
Impacts of earthquakes
The impacts of earthquakes can vary between countries as..
- Low GDP, so less money for emergency response. Often have to rely on foreign aid for help.
- More doctors per person. Medical response is better and can occur quicker.
- Construction laws are less strict in some countries- especially developing countries. Injury and death toll, as wel as cost of repair, will be higher.
- Limited access (in mountains or roads blocked). Makes it difficult to reach people trapped under rubble so a higher death toll. Also, services can't get through with supplies etc.
- Focus closer to the surface. The ground shaking will be felt more strongly so more damage.
- Stronger magnitude. The earthquake is more powerful.
- Secondary hazards. Additional injuries and deaths
Liquefaction- When the water-saturated sandy layers of earth act like liquids due to the pressure created by earthquakes. Force from an earthquake can cause the water to increase in pressure and sometimes break the friction in the grains and fill the spaces, causing liquefaction.
Earthquake resistant designs
Developed countries (e.g. Japan, USA, New Zealand):
- Damper in the roof acts like pendulum, reducing building sway. Stops building collapsing.
- Cross bracing means the building is more flexible. Stops floors collapsing.
- Shock absorbers strengthen the building. Works with cross-bracing to keep building standing.
- A strong, flexible steel frame. Prevents frame from cracking which could lead to collapse.
- Strong double glazing on tall buildings. This prevents windows shattering.
- Very deep foundatins to strengthen building (can work for shaking and liquefaction)
Developing countries (e.g. Pakistan, Indonesia and Haiti):
- Lightweight thatch roof. Helps to prevent collapse but if it does, it doesn't cause much damage.
- Walls made of mud and straw packed between wooden slats. Strengthens frame.
- Simple steel rod foundations. Prevents building from collapsing or toppling over people.
- Concrete rings ties the walls to the foundations. Works with the foundations
- Cross-braced wood or bamboo frame. Stops roof or other floors collpasing.
Earthquake case studies
Developing country, Haiti, 12th January 2010
- Conservative plate boundary. North American and Caribbean. Magnitude: 7. Focus depth: 8km
- Primary impacts- 316,000 people killed, 300,000 people injured, 1million made homeless, 60% of goverment buildings damaged, transport and communication links damaged badly, 4000 inmates escaped prison, hospitals and schools damaged.
- Secondary impacts- 1 in 5 lost their jobs, bodies piled on streets so lots of disease, difficulty getting to areas in need because of rubble on roads, people squashed together in camps etc.
Developed country, New Zealand, 22nd February 2010
- Conservative plate boundary. Pacific and Australian. Magnitude: 6.3. Depth of focus: 5km
- Primary impacts- 181 people killed, 2000 injured, 60% of buildings in city damaged, liquefaction, 80% of city without electricity.
- Secondary impacts- Businesses out of anction for long periods, schools damaged, difficulty getting emergency services due to liquefaction, mental effects of people, Christchurch lost benefits from holding world cup matches (tourism and income).