Because they float on the semi-molten upper mantle, and its direction of movement is determined by convection currents.
2) Describe the Earth’s Crust
It is divided into 14 major plates, which float on the semi-molten upper mantle. It has an average thickness of 30km below continents and 10km below oceans. It is divided into oceanic and continental crust.
3) Describe the Earth’s Mantle
It is mainly made of dense silicon-rich minerals. It is semi-molten, apart from a thin solid layer under the crust. It extends to a depth of 2900km. High temperatures up to 5000°C generate convection currents, which act as conveyor belts, dragging plates along.
4) Describe the Earth’s Outer Core
It’s a liquid layer mostly made of iron.
5) Describe the Earth’s Inner Core
A solid layer made of iron, where temperatures can reach up to 5,500°C.
6) What is Oceanic Crust Like?
It’s denser than Continental, newer (most less than 200 million years old), it can sink and it can be renewed and destroyed.
6) What is Oceanic Crust Like?
It’s denser than Continental, newer (most less than 200 million years old), it can sink and it can be renewed and destroyed.
Plate Margins
1) What happens at Destructive Plate Margins?
Convection currents in the mantle cause two plates to move together. If an oceanic plate and a continental plate move together, The denser oceanic crust sinks under the lighter continental crust in subduction. The oceanic crust being forced down causes earthquakes. The oceanic crust is destroyed by friction and heat from the mantle and it melts to form magma. The hot liquid rock can rise to the surface and result in violent volcanic eruptions.
2) What is a Collision Boundary?
A type of destructive plate boundary where two continental plates meet. They cannot subduct, so collide instead.
3) What happens at a Constructive Plate Boundary?
Convection currents drag plates apart. Cracks and fractures form between where there is no solid crust. Magma forces its way through the cracks and makes its way to the surface to form volcanoes. Volcanic islands and mid-ocean ridges are created by lava.
4) What happens at a Conservative Plate Boundary?
Plates slide past each other in similar, but not the same directions. As one plate is moving faster than the other, and in a slightly different direction, they tend to get stuck. Eventually, the build-up of pressure causes them to be released.This sudden release of pressure causes an earthquake. An example is the San Andreas fault. In a conservative plate boundary crust is neither being destroyed nor made.
Fold Mountains
1) How old are Young Fold Mountains?
They have been formed in the last 65 million years.
2) Give Details about Older Fold Mountains?
They are less high due to erosion, e.g. the Scottish highlands.
3) Describe the Distribution of Young Fold Mountains
They are mainly found in two linear belts: one down the west coast of America and the other from the Mediterranean through south Asia to Indonesia. Examples are the Himalayas, the Rockies, the Andes and the Alps.
4) How are Fold Mountains formed at Collision boundaries?
Where an area of sea separates two continental plates, sediments settle on the floor in depressions called geosynclines. These sediments gradually become compressed into sedimentary rock. When the two plates move together the layers of sediment become crumpled and folded. Eventually the layers of sediment appear above sea level as a range of fold mountains. Where the rocks are folded upwards, they are called anticlines. Where the rocks are folded downwards, they bulge into the mantle as mountain roots and are called synclines.
5) How are Fold Mountains formed at Destructive boundaries?
Where sections of oceanic crust and continental crust are driven together, denser oceanic crust is subducted beneath lighter continental crust. As the oceanic crust is subducted, sedimentary rock layers deposited on the ocean floor are deposited, piling up onto the continental. Sediment is compressed and folded in the process. Eventually, the entire ocean floor is subducted and the two sections of continental crust collide.
Andes Case Study
1) Where are the Andes and what Size are they?
The Andes are a range of fold mountains formed about 65 million years ago. It is the longest range of fold mountains in the world, (7,000km) and extends the length of South America through 7 countries! They are about 300 km in width and have an average height of 4,000 km.
2) How are the Andes used for Farming?
In Bolivia, many subsistence farmers grow a variety of crops on the steep slopes, including potatoes, which are their main source of food. Terraces are used to create flat land on the slopes. The flat areas retain water in areas that receive little, and limit the downward movement of soil in areas that receive little. Most crops are grown in the lower valleys, including cash crops such as soybeans, rice and cotton.
4) How are the Andes used for Mining?
They contain a range of important minerals and Peru ranks in the top ten for gold, silver and tin, and more than half of its exports are from mining. The Yanacocha gold mine is the largest gold mine in the world, and is an open pit where gold-bearing rock is loosened by daily dynamite blasts. It is then sprayed with cyanide and gold is extracted from the solution. This has led to contamination of water supplies. The nearby town of Cajamarca has grown by 210,000 inhabitants, bringing an alternate source of jobs, but this has brought a lack of services and an increased crime rate.
4) How are the Andes used for Hydroelectric Power
The steep slopes and narrow valleys can be easily dammed and the relief encourages the rapid fall of water to generate electricity. The melting snow in the spring increases the supply of water (but variation is a disadvantage). The El Platinal project, which started construction in 2009 will dam the Cañete river and is predicted to be the second largest in Peru.
4) How are the Andes used for Tourism?
There are natural attractions like mountain peaks, volcanoes, glaciers and lakes. Tourist attractions show how people settled in the inhospitable area, and the remains of their settlements, like Machu Picchu. The 45 km Inca Trail does both, but only allows 200 trekkers per day and is closed in February to prevent erosion.
Ocean Trenches
1) How are Ocean Trenches Formed?
They form at some of the deeper parts of the ocean, and are formed due to subduction at destructive plate boundaries (fold mountains are also formed)
Earthquakes
1) What is an Earthquake?
A violent shaking of the Earth’s crust caused by the sudden relief of enormous stresses. This leads to the crust ‘snapping’.
2) Where is the Focus of an Earthquake?
The focus of an earthquake is where the ‘snapping’ begins and can occur many kilometres below the Earth’s surface. But, deep focus earthquakes cause less damage and are felt less than smaller ones.
3) Where is the Epicentre of an Earthquake?
The epicentre is the point directly above the focus, where the greatest damage occurs. Radiating out from this point are shock waves, primary longitudinal waves and then stronger transverse waves and surface waves of both types.
4) What is the Richter Scale?
This is a logarithmic scale used to measure the magnitude of an earthquake. The scale ranges from 1-10 but earthquakes below 3.5 can only be detected by seismographs. Each number in the scale is 10x the previous value.
5) How can Geology affect the Damage an Earthquake does?
The effect of an earthquake is much stronger on weak sands and clays than on solid rock. E.g. in 1985, part of Mexico City had been built on silt and peat that had accumulated when the site was previously a shallow lake. The shock waves brought water to the surface, turning the silt into mud, and 10,000 buildings collapsed.
6) How can the Location of the Focus affect the Damage an Earthquake does?
Generally, the greatest damage occurs if the focus is close to the Earth’s surface. The distance to the surface is important as the strength decays with distance.
7) How can the Population Density of a Place affect the Damage an Earthquake does?
A densely populated area will experience more death and injury than a less densely populated area (e.g. Bam in Iran). This will also be linked to the time of day (people asleep/rush-hour traffic) and whether it is a weekday or weekend.
8) How can Building Design affect the Damage an Earthquake does?
The Bam earthquake in 2003 had 30,000 deaths as many of the buildings were made out of mud and clay, so collapsed on their occupants. Building design in earthquake-prone regions of MEDCs are very high-tech, including features such as reinforced foundations deep in bedrock and reinforced lift shafts with tensioned cables. Automatic shutters come down to prevent passers by from being showered with window glass.
9) How can the Wealth of a Country affect the Damage an Earthquake does?
MEDCs obviously have the financial resources and technology to be able to limit earthquake damage. There is more money to be spent on earthquake-proofing buildings, and spending more money on research, prediction and prevention.
10) Describe the Modified Mercalli Scale
It was created in 1981 and uses Roman numerals from I-XII. It doesn’t have a mathematical basis, but is a subjective ranking based on observed effects ranging from imperceptible shaking to catastrophic destruction. Structural engineers and architects will use this to asses the damage to buildings and improving future design.
11) Why do Earthquakes occur at Destructive Plate Boundaries?
The pressure resulting from the sinking of the subducting plate and its subsequent melting can trigger strong earthquakes as the pressure is periodically released.
12) Why do Earthquakes occur at Constructive Plate Boundaries?
The friction caused by plates moving apart causes earthquakes, but these are less intense than at destructive or conservative plate boundaries.
13) Why do Earthquakes occur at Conservative Plate Boundaries?
When plates slide past each other, the plates tend to stick for long periods of time. This causes stresses and pressure to build. The release of pressure occurs in a sudden, quick release of the plates, and the result is an earthquake.
Sichuan Earthquake
1) When was the Sichuan Earthquake and Why did it Occur?
On the 12th May 2008 at 2:28pm the pressure resulting from the Indian Plate colliding with the Eurasian plate was released and the Longmeshan fault line that runs beneath Sichuan. This led to an earthquake measuring 7.9 on the Richter scale, with tremors lasting for 120 seconds.
2) Give the effects of the Earthquake (9)
A death toll of 69,000. Between 5-11 million people became homeless. In rural areas near the epicentre, such as Beichuan county, 80% of buildings collapsed.The Wenchuan country at the centre was cut off by landslides.In Dujiangyan, 900 students were killed when Juyuan middle school collapsed. In Shifang, chemical plants collapsed, killing hundreds and spilling toxic ammonia. About 100 km from the epicentre, power and water supplies were cut, but there were few deaths. About a total of 5 million buildings collapsed. Some 374,000 people were injured.
3) Give the Immediate Responses to the Earthquake (7)
Troops parachuted into Wenchuan to asses the situation, and hiked on foot as landslides had made the area inaccessible. The production of tents was increased, as 3.3 million were needed to house the homeless and protect them from the spring rains. Areas of land were flattened so they could be erected. More than £100 million was donated to the Red Cross, which went into running camps, providing food, medicine and doctors, and providing tents with blankets and matresses. Twenty helicopters were assigned to rescue and relief efforts in Wenchuan (cut off by landslides). Large scale efforts were made to free survivors from trapped buildings, which the army and teams from Japan, South Korea and Russia helped with.
4) Give the Long-Term Responses to the Earthquake (3)
One millions temporary homes to house those that were made homeless were expected to be put up in the next 3 years. Banks wrote off debts of survivors who didn’t have insurance. The Chinese government pledged $10 million to the rebuilding fund, to go towards those temporary homes and rebuilding collapsed edifices.
Kobe Earthquake
1) When was the Kobe Earthquake and how strong was it?
5:46am on 17th January 1995. It measured 7.2 on the Richter Scale, and tremors lasted for 20 seconds.
2) How was the Kobe Earthquake Caused?
The Philippine Plate was subducted under the Eurasian Plate, along the Nojima fault line.
3) What were the Effects of the Kobe Earthquake? (8)
6,434 people were killed (4,600 of them Kobe residents). 40,000 people were seriously injured and 300,000 people became homeless. The city’s infrastructure was badly damaged, gas mains were ruptured, water mains fractured and elevated roads collapsed, like the Hanshin Expressway.. Railway lines, and 30% of the Osaka to Kobe tracks were unuseable. 2 million homes lost electricity, and 1 million lost water. Fires engulfed the city, especially to the west of the port, devouring the traditional wooden houses. Damage to roads made it harder for them to be reached and extinguished. The damage cost more than $220 billion. Companies such as Panasonic had to close temporarily.
4) What were the Initial Response to the Earthquake? (3)
Friends and neighbours searched through the rubble for survivors, joined by the emergency services when possible (because roads were damaged). Hospitals struggled to cope with the damage, treating people and operating in corridors, due to the masses hurt in the quake (40,000 seriously injured) Major retailers such as 7-11 helped to provide essentials as people lost their possessions and money. Motorola maintained telephone connections free of charge so people could contact their friends, relatives and emergency services.
4) What were the Long-Term Response to the Earthquake? (3)
New buildings were built farther apart, to lessen the domino effect. High-rise buildings had to have flexible steel frames; others were built of concrete frames built with steel instead of wood. Rubber blocks were put under bridges to absorb shockwaves.
Prediction, Protection and Preparation
1) Why are the Three Ps Important?
They provide the key to trying to reduce the impact of an Earthquake.
2) How can Earthquakes be Predicted?
Seismographs can record foreshocks, radon gas counters measure the levels of dissolved radon gas in the groundwater. Animal behaviour, e.g. toads disappearing before L’Aquila Earthquake. Magnetometers measure slight changes in the Earth’s magnetic field. Monitoring water levels in wells shows changes in the level of the underground water table.
3) What does Protection Involve?
Building design, e.g. underground rollers, better spacing, flexible steel frames and lead dampers to reduce shaking.
4) What does Preparation Involve? (3)
Community preparedness - e.g. the Early Warning System in Japan, Great Shake Out Drills, and Japan’s Disaster Prevention Day
Volcanoes
1) What is a Volcano?
A volcano is a vent in the earth, through which magma or other volcanic materials are ejected. They are proof that the Earth is active and that the plates are in constant motion. They are an example of a natural hazard. Their spread closely relates to plate margins. The area in the Pacific Ocean specifically prone to volcanoes is known as the Ring of Fire.
2) Give some features of a Cone Volcano (9)
Steep slopes and a narrow base; often secondary cone, layers of thick lava and ash; infrequent, but violent eruptions; usually found at destructive plate boundaries; made of alternating layers of lava and ash; may erupt a pyroclastic flow - hot steam, ash, rock and dust, which can roll down the sides of a volcano at very high speeds and temperatures of 400 degrees C. Thick magma means it doesn’t travel very far before solidifying, giving the volcano its cone shape.
3) Give some features of a Shield Volcano (4)
Wide base and gentle slopes; low, rounded peak; layers of runny lava with little ash - which is why there’s a wide base, as it can travel far before solidifying; eruptions frequent and non-violent.
5) What is Pyroclastic Material?
Magma can be forcefully ejected into the atmosphere as pyroclasts. Pyroclastic material is all material formed by explosive eruptions including bombs, blocks, lapilli and ash.
6) What is Ash?
Rock, mineral and volcanic glass fragments, it’s hard so abrasive and doesn’t dissolve in water. Smallest particles are less than 2mm in size.
7) What are Lapilli?
Small pieces of material, similar to pumice, ranging in size from 2-64mm across.
8) What are Bombs and Blocks?
Volcanic bombs are lava fragments ejected whilst molten that are larger than 64mm in diameter. Volcanic blocks are lava fragments larger than 64mm that were ejected whilst solid. Blocks commonly consist of solidified pieces of old lava flows that were part of the volcano’s core.
9) How do Volcanic Eruptions Trigger Tsunamis?
Submarine, coastal or island eruptions can trigger tsunamis.
10) How do Volcanic Eruptions cause Lower Temperatures?
Volcanic dust absorbs solar energy, so lowers atmospheric temperatures.
11) How are Lahars Created? (2)
By the volcanic melting of snow. Dust emissions can seed torrential rainstorms resulting in wet ash and mud lahars.
12) Give the Benefits of Volcanoes (8)
Lava flows can create land (Hawaii); hot rocks can lead to the development of geothermal power; igneous rocks contain valuable mineral deposits (gold, silver, copper and diamonds); sulphur is an important ingredient in the pharmaceutical industry; igneous rocks like granite are good building materials; extinct volcanoes form excellent defensive sites (Edinburgh Castle) lava and ash weather quickly into fertile soils; tourism (geothermal)
Mount St Helens
1) Where is Mount St Helens?
It’s one of the five volcanoes in the Cascade Range in Washington State, USA.
2) What Created Mount St Helens?
The Juan de Fuca Plate subducting under the North American Plate, and being destroyed and rising to the surface as magma.
3) When Did Mount St Helens Erupt?
8:32 am on the 18th May 1980, but residents had been evacuated in March due to earthquakes and an explosion of steam and ash. Visitors were not allowed within the 8km exclusion zone around the crater.
4) What were the Major Effects of the Eruption? (6)
The 5.1 earthquake caused a landslide on the north-east side of the mountain, (biggest ever recorded), and a sideways blast of pulverised rock, glacier ice and ash wiped out all living things 27km north of the crater. In the ‘blow-down’ zone, trees like 500 year old cedars, were uprooted and tossed about like matchsticks. 57 people died.
5) What were the Effects of the Eruption on Rivers and Lakes?
Ash fell into rivers and lakes, raising temperatures while sediment and mud choked channels. This resulted in the death of all fish, including those in a hatchery and those 250km of top-class trout and salmon rivers. Spirit lake was filled in.
6) What were the Effects of the Eruption on Communications and Services?
Floodwaters washed away several roads and railway bridges. Falling ash hindered the smooth running of car engines in three states! Electricity supplies were interrupted and telephone lines cut.
7) What were the Effects of the Eruption on Forestry and Wildlife?
Every tree in the 25km blast zone was totally flattened and destroyed, and nothing in it survived.Trees were carried down the river caused a log jam 60m away. Some 10 million trees had to be replanted.
8) What were the Immediate Responses to the Eruption? (3)
Helicopters were mobilised to search and rescue those in the vicinity of the blast, who could have been trapped in their homes or cars, been asphyxiated by ash, or tourists caught in mudflows. Many people were stranded as roads were blocked by ash, so they were cleared in 3 days to allow traffic to flow. The ash posed a health risk, so President Carter promised to send 2 million face masks.
9) What were the Long-Term Responses to the Eruption? (3)
The ash improved soil fertility, allowing vegetation to return and letting animals and bird to survive there also. 10 million trees were replanted to replace those knocked over or killed by wind and heat. Buildings and bridges had to be rebuilt as many were damaged by falling trees and heat.
Monitoring Volcanoes
1) How can Volcanoes be Monitored to Predict Eruptions? (6)
Tiltmeters can detect small, subtle changes in the landscape as ‘swelling’ can show that an eruption will occur (e.g. Mount St Helens) GPS use satellites to detect movements of as little as 1mm. The change in surface temperature can be seen on satellite images. Digital cameras placed on the rim of craters can photograph events relatively safely. Monitoring SO2 levels in the gas emitted as there are increased levels of it in before an eruption. Robots called spiders are used to monitor changes - the past frequency of eruptions, the gaps between them; the pattern of lava flows, ash movements and lahars can tell us how a volcano is likely to behave.
2) Why is Monitoring Volcanoes Essential?
Because it allows people to prepare for an eruption, and organising people and preparing supplies.
Supervolcanoes
1) What is a Supervolcano?
A mega colossal volcano that emits at least 1000km3 of material. They have large depressions called calderas which are often marked by a rim of higher land around the edges.
2) How is a Caldera Formed?
First rising magma cannot escape and a large bulge appears on the surface. Then, cracks appear on the surface as gas and ash erupt from the magma chamber. The magma chamber collapses, leaving a depression called a caldera.
Yellowstone
1) Give some features of Yellowstone and how big is the magma chamber?
Old Forest, Norris Geyser Basin &c. The magma chamber beneath Yellowstone is believed to be 80km long, 40km wide and 8km deep.
2) What are some possible signs that Yellowstone will erupt soon?
The magma is shifting, as the caldera is bulging up beneath Lake Yellowstone. Signs of increased activity at Norris. The ground has risen 70cm in places.
3) When have Eruptions Previously Occurred?
2 million years ago, 1.3 million years ago and 630,000 years ago.
3) What would be the Consequences of an Eruption?
It would potentially destroy 10,000km2 of land and kill 87,000 people. 15cm of ash would cover buildings within 1000km and ⅓ of people affected would die. This would affect buildings, transport and electricity. Lahars are a possibility. Global climates would change, crops would fail and people would die.
Tsunamis
1) Are Tsunamis Primary or Secondary Hazards?
They’re secondary hazards, caused by primary hazards such as landslides, earthquakes or even asteroid strikes.
2) What is the Most Common Cause of a Tsunami?
The most common cause of a tsunami is a submarine earthquake.
3) How are Tsunamis Made?
An earthquake vertically displaces the sea bed, which displaces the water upwards, which generates a tsunami at the ocean surface. Tsunamis cannot be seen with the naked eye in the deep ocean because wave heights are under 1 metre.
4) How fast and long are Tsunami waves?
They have wave lengths of 200km and travel at up to 800km an hour.
5) Describe Shoaling
The ocean becomes shallower as the coast is neared (shoaling). As the wave depth decreases, friction between the tsunami and the sea bed slows the wave down. As the wave slows, its wavelength dramatically decreases by the wave height increasing. This produces a series of metres high waves. When tsunamis hit the land the effect is much more like a flood than a wave. The volume of water and long wavelength means that water is pushed onshore continually, without backwash. Most tsunamis are not just one wave, but several over a period of many minutes (a tsunami wave train)
2004 Boxing Day Tsunami
1) How was the 2004 Boxing Day Tsunami caused?
The Indo-Australian plate slid below the Eurasian Plate, triggering an earthquake that measured 9.1 on the Richter scale. This displaced the sea bed and the water upwards.
2) What were the General Effects of the Tsunami?
More than 220,000 people died, 650,000 seriously injured and 2 million made homeless. Public buildings including schools and hospitals were wiped out in some areas. Many people posted photos in affected areas in the vain hope that a loved one had survived. Emergency teams and rescue services were swamped by the scale of the disaster. Injured people were left untreated for days as wounds turned gangrenous and conditions worsened. Bodies littered the streets before being buried in mass graves.
3) What were the Effects on Sri Lanka?
Wide stretches of eastern and southern coastline devastated by massive waves. Districts of Mutur and Trincomalee inundated by waves as high as 6m. Colombo, the port of the capital was shut by flash floods.
3) What were the Effects on the Maldives?
High floods and waves inundated the islands and ⅔ of Malé was underwater.
3) What were the Effects on Indonesia?
The Province of Aceh on the northern tip of Sumatra was badly hit, including the provincial capital Banda Aceh, where 1,500 settlements were believed to have been wiped out. Dozens of buildings were destroyed in the initial earthquake before flood waters rushed over the region.
4) What were the Immediate Responses? (3)
Fresh water, water purification tablets, food, sheeting and tents poured in as aid. Medical teams and forensic scientists arrived. The UK government promised £75 million and donations of £100 million followed.
5) What were the Long-Term Responses? (4)
A year later £372 million had been donated by the British public. Rebuilding is progressing, and the Disasters Emergency Committee has spent more than £40 million on projects in Sri Lanka and Indonesia, and plans were made to spend a further £190 million building 20,000 houses. The Indian Ocean Tsunami Warning System was set up in June 2006. Ensuring people know how to respond and that local people have plans in place are essential to its success.