Plate tectonics

Outer core

made up of hot, electrically conducting liquid in a semi-molten state, creates the Earth's magnetic field

Surface waves

move through the crust like ripples on water (Love and Rayleigh)

Natural hazard

a natural hazard is a threat of a naturally occuring event that will have a negative effect on people and/or the environment. A natural hazard becomes a natural disaster when it affects people, officially causing more than 10 deaths, injuring more than 100 people and/or causing $16,000,000 of damage

Tsunami

a series of waves generated in a body of water by an impulsive disturbance that vertically displaces the water column. Around 90% of tsunamis are generated within the Pacific basin and are associated with the tectonic activity taking place at its edges. Most are at convergent plate boundaries where subduction takes place, particularly off Japan-Taiwan island arc (25%)

Secondary (S) waves

slower (2-5km/s), transverse waves, able to move through solid rock only, move rock materials up and down, or side-to-side, perpendicular to the direction of wave propagation which causes a lot of damage at the surface

Solfatara

small volcanic areas without cones, produced by gases (mainly sulphurous) escaping to the surface, e.g. around the Bay of Naples, Italy. Crater is 2km in diameter, larger than Vesuvius

Hot springs

sometimes the water, heated below, does not explode onto the surface. If this water mixes with surface deposits, boiling mud is formed. These are common in Iceland.

epicentre

the point immediately above the focus on the earth's crust

Reverse fault

Destructive boundaries - compressional forces drive hanging-wall up the fault plane known as thrust fault in angle is less than 30. Depth of earthquake varies from very shallow to 700km depth. Largest earthquakes found here, can exceed 9 on Richter scale.

Ocean ridges

Elevated regions with a central valley on an ocean floor at the boundary between two diverging tectonic plates where new crust forms from upwelling magma. They are the longest continuous uplifted features on the surface of the planet and have a total length of 60,000km. In some parts, they rise 3000m above the ocean floor.

Run-up

Except for the largest tsunamis, such as the 2004 Indian Ocean event, most tsunamis do not result in giant breaking waves - rather, they come in much like very strong and fast-moving tides. The first part of the wave reaching the local shore is a trough, which will appear as the sea receding far from the shore which is a common natural warning sign for tsunamis.

Dykes example

Isle of Arran in Scotland. 'dyke swarm' - 500 such features in a 20km stretch of coastline. Most are more resistant to erosion than surrounding sandstones so where they cross beaches, they stand up like groynes. Average 3m but vary from 1-15m width.

Fold mountains

Large mountain ranges with very high mountains which are rocky, with steep slopes. There is often snow and glaciers at the highest points, with lakes in the valleys between them.

Caldera

Large, deep craters filled with water. Occur when the build up of gases becomes extreme and a huge explosion removes the summit of the core, leaving an opening several kilometres in diameter. Frequently emit smoke and ash but erupt irregularly and violently. The caldera may become flooded by the sea, or a lake may form within it. E.g. Krakatoa, Indonesia and Thera, Santorini (Greece)

Ash/cinder

Layers of ash and cinder from steep, symmetrical sides which are slightly concave. Formed from ash, cinders and volcanic bombs ejected from the crater. E.g. Paricutin, Mexico - formed in the 1940s.

Ocean trenches examples

Peru-Chile trench off the coast of South America where Nazca plate subducts beneath S.American plate. Western Side of Pacific Ocean where Pacific plate subducts beneath Philippines plate, forming the Marianas trench and a line of volcanic islands - the Marianas. The Challenger Deep, at the southern end of the Marianas Trench is the deepest part of the ocean at nearly 11000m.

Sills example

The headlands at Drumadoon, Arran = 50m thick. The Great Whin Sill runs across large distances in the north of England, forming an upstanding cliff-like feature. Many rivers produce high waterfalls as they plunge over it, for example High Force in the Tees Valley in the Pennines.

Intrusive features

They occur where there a few lines of weaknesses or faults in the Earth's crust. Magma enters these lines of weakness, cools and solidifies underground. They also occur where magma rises slowly which forms geomorphological features called plutons, bodies of intrusive igneous rock crystallised from cooled magma.

Split stage (tsunamis)

Within several minutes of the earthquake, the initial tsunami is split into a tsunami that travels out to deep ocean (distant) and one that travels to the nearby coast (local). The distant tsunami travels faster than the local one.

Oceanic crust

associated with volcanic activity, constantly being renewed or destroyed at constructive and destructive boundaries, shallow (5-12km) as renewal and destruction prevents build-up, less than 200 million years old, higher density, mainly basalt

Faults

fractures along which the opposite sides have been displaced. The surface of displacement is the fault plane. The footwall block is below the fault plane and the hanging-wall block is above it.

Tephra (volcanic bombs and ash falls)

fragments of volcanic rocks and lava that are blasted into the air by explosions or carried upward by hot gases in eruption columns, range in size from less than 2mm (ash) to more than 1m in diameter. Large tephra falls to the ground on, or near the volcano and small fragments are carried away from the vent by wind, volcanic ash can travel hundreds of thousands of kilometres downwind from a volcano.

seismicity

frequency or intensity of earth movements and vibrations in an area. This movement is caused by the release of seismic waves.

Distance between buildings

further away - less destruction as won't collapse onto eachother, buildings close together = more deaths

Continental crust

generally made up of various types of granite although surface rocks may be sediment, e.g. sandstone or limestone, relatively old as it does not subduct

Volcanic gases

including CO2, CO, hydrogen sulphide, sulphur dioxide and chloring, rarely lethal but can reach great heights in the atmosphere and spread around the globe. E.g. 1986 0 CO2 from Lake Nyos in Cameroon suffocated 1700 people

Volcanoes - secondary hazards

lahars, tsunamis, climate change

Lava flows

streams of molten rock, can erupt non-explosively and more slowly at a few metres to a few hundred metres per hour (cooler and less gas-enriched), can move rapidly down quick slopes (higher gas content and temperature), slow enough not to threaten human life and follow a predictable course, can cause extensive damage by burning, crushing and burying everything in their paths. Controlling them can avoid such destruction.

Inner core

temperature of about 6200 degrees, solidifies under pressure, made up of solid iron and nickel

Island arcs

A curved chain of volcanic islands located at a tectonic plate margin, typically with a deep ocean trench on the convex side.

Fissure

A long crack in the Earth's surface through which magma erupts. They form as two plates move apart, constructive e.g. near mid-ocean ridges. Basaltic lava is ejected and flows a considerable distance, forming large plateaus, gently sloping areas either side of a large crack. E.g. Antrim lava plateau in N. Ireland, the edge of which can be seen at Giant's causeway.

Earthquakes

A series of shocks and tremors resulting from a sudden release of pressure along active fault lines and in areas of volcanic activity

Hotspot

A small area of the Earth's crust where unusually high heat flow is associated with volcanic activity...most are located away from plate boundaries. "constant relative motion" supported by volcanic chains associated with hot spots.

Laccoliths

Are exposed at the surface as a range of small hills, e.g. the Eildon Hills on the Scottish Borders. They form as an igneous intrusion injected between two layers of sedimentary rock. The pressure of viscous magma at this point is so high (as it cannot spread easily) that the overlying strata are forced to arch upwards, forming a dome shaped intrusion. They generally form at relatively shallow depths.

Andesitic/ryholitic magma

Andesitic (hot - 8000 to 1000, silica content 52%, doesn't extend more than 8km from vents), Ryholitic (highest viscosity, less hot - 650 to 800, high silica 66%, flows very short distances as cools and solidifies quickly), less frequent but violent eruptions due to build up of gases, produces steep-sided but more localised features, found at destructive margins where oceanic crust is destroyed, melts and rises, e.g. subduction zones (Mt St Helens) and island arcs (Mt Pelee, Martinique)

Formation of island arcs

As oceanic plate subducts, an increase in pressure causes it to dehydrate which promotes partial melting of the crust in a part of the subduction zone known as the Benioff zone. It is less dense than the surrounding asthenosphere and so molten material rises. Weaknesses in the crust above act as vents through which these materials pass to form volcanoes. The andesitic lava is viscous and creates complex, composite, explosive volcanoes. If the eruptions take place offshore, a line of volcanic islands known as an island arc will appear. The descending plate also provides a source of stress as the two plates interact, leading to frequent, moderate to strong, earthquakes

Amplification stage (tsunamis)

As the local tsunami travels, amplitude increases and wavelength decreases = steepening of lead wave. Tsunami run-up occurs when a peak in tsunami wave travels from near-shore region onto shore. Run-up is a measurement of the height of the water onshore observed above a reference sea level.

Where does volcanic activity occur?

At divergent boundaries where plates are moving apart (e.g. Iceland represents a large area formed by volcanic activity), at convergent boundaries, on subduction zones (e.g. Pacific Ring of Fire - >50% of world's active volcanoes above sea level, most violent volcanoes), where the Earth's crust is thinning or spreading (non hot-spot intra-plate volcanism) - e.g. East African Rift Valley, over hotspots such as Hawaiian Islands

Fold mountains at different boundaries

At oceanic-continental (destructive) boundaries, the oceanic plate subducts further causing an increase in pressure which can trigger major earthquakes. At continental-continental (collision) boundaries, shallow-focus earthquakes may occur and material is forced downwards to form deep mountain roots.

Basic/shield

Basaltic lava erupts from a vent and cools slowly, spreading over a large area. The result is a cone with long, gentle sides made up of layers of lava from repeated flows. E.g. Mauna Loa, Hawaii. Hotspots or conservative boundaries. Low, gently sloping volcanoes which erupt regularly and gently. Lava is runny and flow long distances because of low silica content.

Foundations

Building foundations through liquefied layers is more stable, deep foundations are more stable as move with the crust

Transform (strike-slip) fault

Conservative boundaries - block of rock is sheared along its sides and slicken sides are orientated sub-horizontally. Shallow earthquakes generally smaller than 8.5

Normal fault

Constructive boundaries and intra-plate volcanism - extensional force causes hanging-wall block to slide down fault plane - shallow earthquakes generally smaller than magnitude 8

Why do volcanoes erupt?

Due to changes in density, buoyancy, temperature and pressure...magma melts due to an increase in temperature, a decrease in pressure or by the addition of water (lowers melting temp). This melted rock is less dense so rises, leading to a decrease in pressure = additional melting and further decrease in density. Magma rises until it erupts or enters material with the same density where it will form a magma chamber. Throughout its ascent, bubbles can form gas in magma which increases its pressure. If pressure become great enough, overlying rock will fracture = eruption. Volcanoes stop erupting because all trapped gases have escaped and there is no longer sufficient pressure to drive the magma out of the Earth or because enough heat is lost so that magma cools and is no longer buoyant

Dykes

Generally occur as small ridges in the landscape because they are more resistant than the surrounding rocks. They from after vertical sheet intrusions cut across the bedding plane of sedimentary rocks into which they have intruded. They have horizontal cooling cracks which have formed parallel to the surface as magma has cooled, contracted and solidified. The parts that come into contact with the surrounding rock will cool more rapidly to produce a chilled margin with smaller crystals. They usually form by forcing open existing fractures as well as opening up channels through cracks created by magmatic injection.

East African Rift Valley

Has been forming for 30 million years where African plate is splitting into Iubian and Somalian plates and eastern Africa is splitting away from the rest of the continent. The rift valley extends 4000km from Mozambique to the Red Sea. In places, the sides are over 600m in height while its width varies between 10 and 50km.

Sills

Horizontal intrusions along the lines of bedding planes between the existing sedimentary rocks, with vertical cooling cracks. Created by magma being intruded along bedding planes which provides a weakness for the flow of magma to exploit along.

Characteristics of tsunamis

In open ocean...long wavelength (>100km), low height (<1m), long periods (around 1 hour), high speeds (>700kmph). On reaching shallower water, they slow down and rapidly increase in height (energy flux, dependent on speed and height, remains constant)

Batholiths

In southwest England the top of exposed batholiths can be seen in areas such as Dartmoor. These were formed deep below the surface when large masses of magma intruded into the crust and cooled and solidified. As the magma cools, large crystals are formed in the rock (e.g. granite - which can be seen in the Grampians in Scotland). They can be several hundreds of kilometres in diameter and are often dome-shamed, exposed by later erosion. The area surrounding the batholith is altered by the heat and pressure of the intrusion to form a metamorphic aureole. For example, limestone can be transformed into marble.

Deep sea trenches

Long, narrow, deep depressions in the ocean bed, typically running parallel to a plate boundary and marking a subduction zone. They are the deepest parts of the ocean and some of the deepest natural spots on Earth. Denser oceanic crust subducts beneath lighter continental crust = the downwarping of oceanic plate forms a trench.

Rift valleys

Lowland regions that forms on constructive boundaries with steep sides (Horsts) and flatter valleys (Grabens). As sections of crust move apart, it becomes weak and areas of crust drop down between parallel faults to form rift valleys. They often fill with water to form new ocean basins.

Initiation stage (tsunamis)

Near the source of submarine earthquakes, the seafloor uplifts and down drops, pushing the body of water above it up and down. The potential energy from the uplift is transferred as kinetic energy to the horizontal movement of the tsunami wave.

Formation of ocean ridges

Plates move apart, stretching the already thin oceanic crust. Magma forces its way up and erupts to form submarine volcanoes which will eventually rise above the surface. The volcanoes which are formed as part of oceanic ridges have gently sloping sides due to the low viscosity of the basaltic magma.

Composite

Pyramid-shaped, resulting from alternate eruptions of ash (violent) and andesitic lava. Besides main crater, it has many little craters on its slope. E.g. Mt Etna, Sicily and Mt Pinatubo, Philippines.

Asthenosphere

Remainder of the mantle kept in a semi-molten state. Extends to a depth of 2900km where temperatures reach 5000 degrees, generating convection currents

Example of fold mountains

The Himalayas - Indo-Australian plate is moving northwards into Eurasian plate. Previous intervening ocean known as Sea of Tethy has had its sediments forced upwards in large overfolds to form Himalayas, an uplift which is continuing today. Range is 350km wide and extends for 3000km. The highest mountain is Mt Everest at 8848m. Indian plate is being pushed under Tibet to form mountain roots up to 70km deep. Violent, destructive earthquakes - Gujurat in northern India, 2001 (30,000 deaths in an earthquake lasting 45 secs), Sichuan in south-west China, 2008 (magnitude 7.9 and more than 80,000 deaths)

Acid/dome

Viscous, fast/slow-flowing, high-silica, rhyolitic lava solidifies near the crater forming steep-sided convex cones which can reach great heights. E.g. Mt Pelee, Martinique - lava solidified up the vent producing a spine. Sticky - plugged with lava and erupt violently

Formation of fold mountains

When an area of sea separates two plates, sediments settle on the sea floor in depressions called geosynclines. These sediments gradually become compressed into sedimentary rock. When the plates move towards each other again, the layer of sedimentary rock on the sea floor becomes crumpled and folded until it appears above sea level as a range of fold mountains. When the rocks are folded upwards they are called anticlines and when they are folded downwards they are called synclines.

Tide gauges

a device for measuring sea level and detecting tsunamis, those close to an earthquake would be able to detect rise in sea level that a tsunami would produce - expensive!

Destructive boundaries - lava

andesitic or ryholitic, violent and very explosive, low frequency (average 500 years dormacy), lava bombs, ash, dust, large amounts of fragmented material released, composite and acid dome volcanoes

Constructive boundaries - lava

basaltic, non-violent, gases can easily escape, regular and continuous, lava fountains and flows, low volatility and less viscous, lava plateau and shield volcanoes

Distribution of earthquakes

can occur at convergent, divergent or conservative boundaries, the most powerful are at destructive margins. Pacific Ring of Fire accounts for 80% of all global seismicity and 91% of major seismic activity. Can also occur away from plate boundaries due to reactivation of old fault lines or because of human activity (buillding of large reservoirs in which water puts pressure on surface rocks)

Mantle

composed mainly of silicate rocks, rich in iron and magnesium, rigid top layer = solid fragments are sometimes found in volcanic eruptions and eroded mountain belts

Lithosphere

consists of crust and rigid upper section of mantle, made up of tectonic plates

Love (L) waves

fastest surface wave formed by interaction of S waves at surface, transverse and can only travel through crust, move ground from side-to-side perpendicular to direction of wave, no vertical movement. Responsible for most damage to buildings and foundations. Only recorded on seismometers that measure horizontal ground movement.

Primary (P) waves

fastest type of wave (4-7km/s), compressional (push and pull material they travel through) and vibrate in the direction they travel in (direction of wave propagation), can travel through both solid rock and fluids and are rarely the cause of damage, longitudinal and are first to be recorded by seismometers. Speed varies depending on density and elastic properties of material it passes through.

Seismometers

fixed base and mass. Base moves with ground and mass stays still during earthquake. The movement is transformed to an electrical voltage to draw a seismograph. A greater interval between first P and S waves = greater distance from earthquake

Base isolation

involves floating a building above its foundation on a system of bearings, springs or padded cylinders. Lead-rubber bearings contain a solid lead core wrapped in alternating layers or rubber and steel. The lead core makes the bearing stiff and strong in the vertical direction, while rubber and steel bands make the bearing flexible in the horizontal direction. When the earthquake hits, the bearings allow the foundation to move without moving the structure above it. As a result, the buildings horizontal acceleration is reduced and it suffers far less deformation and damage.

Volcanoes - primary hazards

lava flows, tephra, pyroclastic flows and gases

tall buildings

less stable (high centre of mass), shaking of ground shifts centre left and right

Richter Scale

logarithmic scale of magnitude related to amplitude and duration of ground wave, most useful as it is easily comparable. an event measured at 7 on the scale has amplitude of seismic waves ten times greater than one measured at 6 on the scale. The energy release is proportional to the magnitude, so that for each unit increase in the scale, the energy released increases by approximately 30 times.

Basaltic magma

lowest viscosity, hottest (1000-1200), lower silica content (45%), takes a long time to cool and solidify so flows considerable distances, frequent but gentle eruptions producing extensive but gently sloping landforms, found at constructive margins where magma rises from the mantle, e.g. fissures along the Mid-Atlantic Ridge (Heimaey), hot spots (Mauna Loa)

Mercalli Scale

measuring intensity from I ( (detected by seismometers but felt by very few people - approximately equivalent to 2 on the Richter scale) to XII (total destruction with the ground seen to shake - approximately 8.5 on the Richter scale), relatable to physical destruction and ground shaking

Lahars (mud flows)

mixtures of water, rock, ash, sand and mud that originate from the slopes of a volcano, can travel over 80km and reach speeds of 35-65km/h, initiated by large landslides, heavy rainfall eroding volcanic deposits, the melting of snow and ice, or the breakout of water from glaciers

GPS

navigation and positioning system measures ways in which the Earth's crust is moving. Maps and models can be made to show direction and speed of crustal movements. E.g. Southern California Integrated GPS network - one of newest GPS arrays, produces detailed info about movements in S.California to reduce damage and loss

Formation of hotspots

originate as disturbances at the core-mantle boundary. The zone develops a small bump which protrudes into the mantle from the core. The bump transmits the intense heat of the core into the adjacent mantle creating high rates of convection which raises the mantle plume. As the plume rises, pressure reduces allowing the material to become molten and penetrate the rigid crust above. Chains of volcanoes such as Hawaii or Yellowstone form as a plate (Pacific and N.American) move over the hotspot. The hotspot is stationary so a line of volcanoes is created, with the one above the hotspot active and the rest form a chain of extinct volcanoes

Deformation

result of strain = elastic strain, plastic deformation and rupture. Lithosphere has elastic properties and will return to its shape once force is released. Every elastic material has an elastic limit - the point at which deformation is irreversible and sometimes there is plastic deformation which means shape has changed an additional amount beyond elastic limit. If substance is brittle it breaks before plastic deformation.

Causes of tsunami

shallow-focus underwater earthquakes (secondary hazard), landslides, volcanic eruptions

Rayleigh waves

slowest of all seismic waves, move in an elliptical motion, producing vertical ground movement, only surface wave to affect water, due to vertical movement, formed by interactions of P and S waves at the surface

Stepped buildings

stable, low centre of gravity and can evacuate onto roofs = easy to rescue

The nature and frequency of volcanic activity depends on

the plate boundary and the type of lava which influence explosivity, the features that are formed, the types of hazards posed

Focus

the point at which the pressure release in an earthquake occurs. Shallow (0-70km deep) cause the greatest damage and account for 75% of all earthquake energy released, intermediate (70-300km), deep (300-700km)

high viscosity

thick consistency, flows short distances, low temperature, more explosive

Earthquake bracing

transfer seismic forces to the ground = more stable, steel rods have greater elasticity than concrete = can move more when strain is applied to them and bounce back, shear walls = vertical walls that stiffen structural frame of a building and help resist rocking forces

Body waves

travel through the Earth's inner layers (primary and secondary)

Pyroclastic flows

very hot (800 degrees), gas-charged, high-velocity flows made up of a mixture of gases and dry rock fragments (tephra), consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow

Geyser

water in the lower crust is heater by rocks and turns to steam; pressure increases and the steam and water explode onto the surface. E.g. Old Faithful, Yellowstone National Park which blows every 85 minutes.

seismic waves

waves of energy that are released when rocks break or explosions occur in the mantle. They radiate outwards from the focus.

Stress and strain

when a force is applied to an object, it is under stress which produces strain (actual deformation), stress and strain are related so easy to determine from each other when you know the value of proportionality, movement and interaction between plates at margins builds up stress and creates strain and deformation