Structure of the earth
Lithosphere- the uppermost layer of the earth (top of the mantle and the crust)
Asthenosphere- Part of the earth's mantle, semi molten, beneath the tectonic plates. Upper layer of the mantle just below the lithosphere that is involved in plate tectonic movement.
Lithosphere is solid- less dense, Mantle is molten (asthenosphere) and solid (lower mantle) - more dense as depth increases. The outer core is liquid iron and nickel (largest section), and the inner core is solid iron and nickel due to immense pressure (most dense).
TYPES OF CRUST
Oceanic- younger rock (new rock is formed at constructive boundaries,) More dense (basalt rock,) as it is thinner 6-8km but is still dense.
Continental- Older, thicker in depth (30-50km.) Therefore it is less dense. Mostly granite rock
Why do plates move?
Heat energy is produced in the inner core of the earth due to radioactive decay of elements. As heat rises from the core, convection currents are created. These convection currents are strong enough to move plates on the earth's surface.
The convection current moving towards the crust is called a plume, which is a concentrated zone of heat. The plumes can push through the middle of tectonic plates as hot spots, form constructive plate boundaries, or it can erupt as lava from a volcano.
The convection currents cool after they rise, and sink down in a circular motion.
Evidence for Plate Tectonics
The continents fit together like a jigsaw
Mineral deposits match up over continents for where the land matched due to pangea
Fossils (especially dinosaurs) match up over continents.
Glacial evidence (ice caps)
Seafloor spreading- Hess
Mid-Atlantic ridge discovered in 1953
Plates moved by convection currents
Floor gets older as you move away from the ridge
Mid-ocean ridges are warmer than surrounding ocean floors
Types of Plate Margin
- Mid-Atlantic ridge
- Eurasian and N American plate pulling apart
- Small earthquakes
Convection currents from the mantle bring magma towards the surface, where the plates are pulling apart, magma is forced up between the plates. When it cools, new plate is formed. Lava erupts forming volcanoes.
- Example The Himalayas
- Eurasian and Indian plate
- Earth quakes and landslides
They are a type of destructive plate boundary. They occur when two continental plate boundaries are pushing into each other, creating mountains. Earthquakes happen on faults (cracks in the crust)
Types of Plate Margin
- Andes mountains in Peru and Chile
- Nazca plate is subducted under the S American plate
Where the dense oceanic plate meets the less dense continental plate, the oceanic plate is subducted, and forced back down into the mantle where it melts. Sometimes where the plate is melting, there are volcanoes where the magma can easily rise and erupt. Earthquakes can also happen, as when the plate is subducted, is can get caught, pressure builds up and it is suddenly released causing a quake.
- San Andreas Fault
- N American and Pacific plate sliding past each other.
When plates slide past eachother, friction between them builds up causing earthquakes. These are quite rare but can be very destructive.
Types of Volcano
- Shield Volcano
- Gently sloping
- Mauna Loa, Hawaii
- Runny Lava (Low silica content)
- Gentle eruptions
- Large circumference at base
- Hotspot or constructive margin
- Basaltic lava
- Low gas content
- Steep sides
- Viscous lava (high silica content)
- Small base circumference
- high gas content
- Mt St Helens, Waashington
- Subduction margin
Mount Sakurajima, Japan
- Location- sheltered bay of Kagoshima, destructive plate boundary. Island of Kyushu
- Key Facts and statistics- 40% of the land is fertile volcanic soil, and they grow tea and rice.
- The bay is important for fishing and trade
- It can erupt 200 times a year, and has been erupting since the 1950s
- Destructive plate boundary where the pacific plate is subducted under the Eurasian plate
- The pacific crust melts under the Eurasian, and magma rises through a volcano
- When it erupts, ash is deposited which makes the land fertile for farming.
- Hot springs and lava flows are a popular tourist attraction
- Japan is an MEDC, so can afford to spend money on monitoring protection.
- It has technologies such as boreholes, aircraft, and lahars to detect and divert the eruptions.
Mount Nyiragongo 2002
- Location- East border of the DR Congo, ner the city of Goma, north of lake Kivu
- Key facts and statistics- The DR of Congo is an LEDC
- The population of Goma is 1 million
- The volcano is a stratovolcano
- The lava here has a low silica content meaning it is runny and can flow quickly aided by steep sides
- Magma rises from the mantle where the two plates (African) that are pulling apart.
- Constructive plate boundary
- The lava cut off routes into Goma and divided the city in two, complicating aid effort
- 147 died and 14 villages were destroyed
- Each family received rations
- The UN loaded lorries with blankets and plastic sheets and fed 70,000 for a week.
- TV appeals
Living with volcanoes
Aircraft- measure the amount of gas the volcano gives off
Tiltmeters detect when the volcano swells up
Boreholes measure water temperature as magma heats up
Concrete lahar channels divert dangerous mudflows
Seisometers- monitor earthquakes, which increase as magma rises
- Kobe Earthquake 1995
- 6 am in the morning, magnitude 7.2
- 5,000 died
- The oceanic plated in subducted under the lighter continental plate. Earthquakes are common along this boundary
- Shallow depth of the focus, epicentre was close to a high populated area.
- There were devastating effect son the soft land that had been re-gained from the sea.
- Hanshin highway destroyed, over 100km of the bullet train track destroyed
- Older office blocks collapsed
- Fires caused by broken gas pipes
- New jobs created in the construction industry
- Now every year Japan has drills
- people keep emergency kits at home, e.g torch, food, radio.
- Emergency services practise rescue drills
- Sichuan Earthquake 2008
- 7.9 magnitude
- 70,000 died
- 5 million homeless
- Sichuan lies on a collision zone (himalayas), both continental crust. Earthquakes are common
- Primary effects
- 5 million buildings collapsed
- landslides blocked rivers
- Secondary effects
- 200 aftershocks
- broken phone lines
- soldiers were sent to help dig for survivors
- UK gave $2 million
- helicopters used to reach isolated areas
Long-term earthquake planning
- Yokohama Tower-
- Flexible structure which absorbs earthquake waves
- Mass dampers act as counter weights. as the building shakes one way, the damper balances it by swinging the other.
- Transamerica pyramid-
- Large base that sways and does not break
- Concrete/steel block that sits 52 ft underground
- (base isolation)
- Windows made from shatterproof glass, bookshelves screwed to walls
- cross braced wooden frame, steel rod foundations, lightweight thatched roof