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Transcript of Geography Revision
Temp = Over 6000 degrees
Semi-molten outer core contains solid
inner core Mantle = Silicate rocks /w iron and magnesium
Temp = nearly 5,000 degrees
Semi Molten (asthenosphere)
Convection currents due to high temps Crust - Very thin
Coolest elast dense rocks
Oceanic Crust = 6-10km thick
Continental crust= up to 70km
Seperated from mantle by Moho discontinuity Theory of Plate tectonics Developed in late 1960's
People had always noticed the continents either side of the Atlantic seemed to fit Francis Bacon was aware of this as
early as 1620. Alfred Wegner published his theory that continents
were once joined together in a formation known as
Pangea. He proposed they had drifted apart, but his
theory was dismissed due to lack of explanation of how this
occured. Evidence for continental drift: Continental fit - some contients (esp. Africa and S. America)
seem to fit together like a jigsaw. Particularly if you take continental shelf shape into account Geological evidence - Rocks of the same type and age
are fond in mounts of eastern USA NW Europe. Suggesting they were once in simmilar positions. Simillar glacial deposits are found in Antartica, South Amerca and India. Climatological evidence - Antarctica, N. America, Svarlbard and the UK all contain coal deposits of similar age - al formed under tropical conditions. They must therefore have all once beenin tropical zones. Biological Evidence - Similar fossil formations are found on each side of the Atlantic. The Mesoasaurus is only found in South African and South American sediments. Marsupials are onlyfound in Australia because it drifted from Pangea before predators could move in and wipe them ot as they did elsewhere Evidence from Palaeomagnetism Wegner did have convicing evidence but there was no clear explanation of how the continents could possibly move. In the 2nd half of the 20th century 3 discoveries helped to show how it moved. In 1948 a survery of the Atlantic Ocean discovered the Mid Atlantic Ridge
Survery of the floor in the 50s showed regular patterns of palaeomagnetic striping either side of the ridges. This is due to the way iron minerals in the lava allign themselves to magnetic north when they cool The Earth's polarity reverses every 400,000 years so clearly the sea floor is expanding. Furthermore surveys of the sea floor's age show that the oldest crust is near continents and the newest crust is near the Mid Atlantic Ridge, with Iceland's ccrust being very young.
Older crust is continuously being pushed aside by new crust. Old oceanic crust is subducted under continental crust.
Higher temperatures found in the Earth's core create convection currents. These circulatory movements allow for the sea floor to be spread. Plate Margins Landforms associated with constructive margins Oceanic Ridges Longest continuous uplifted feature on Earth. Total comined length of 60,000km Weaker crust where the plates pull apart as well as increase in surface heat.
This hotter expanded crust forms a ridge This ridge may include a valley where crust has subsided into lavaa below. A split like
this may form a low pressure zone where liquid lavas can erupt to form submarine volcanoes. If eruptions like these persist they may reach the surface and form islands, tlike Iceland Rift Valleys Spreading and heating of crust leads to updoming and fracturing of crust. As sides of a rift move apart central sections drop down forming rift valleys. The Great East African Rift Valley indicates where such crust has begun to pull apart. It is currently 4000km long 50km wide and 600m deep. If it widens further the sea may inundate it Deep ocean trenches Friction of descent generates massive amounts of heat - leads to partial melting of crust
magmas from this are less dense and rise up through fissures. This will usually result in
a volcanic eruption. The marginally denser or faster moving plate is subducted beneath the other.
The processes are the same but eruptions that occur may form island chains - e.g. the Aleution Islands.
They will also form deep ocean trenches - some up to 10km deep.
Some island chains may erupt over millions of years to become major landmasses e.g. Japan.
This subduction can causes massive Earthquakes - the Indonesian earthquake of 2004 measured 9 on the richter scale. Landforms associated with destructive plate margins Oceanic/Continental Convergence Oceanic plate is denser than continental crust
Therefore it is subducted when the two meet. Friction from descending continental crust builds up and causes major earthquakes
Rocks scraped off the descending plate and folding of continental crust create fold mountain chains
on the leading edge of contienetal crust. E.g. the Andes in South America Oceanic/Oceanic Convergence Continental/Continental convergence Continental plates has such similar density that they cannot be subducted beneath one another.
Instead they collide to form fold mountain chains.
The Indian subcontinent was propelled by the spreading of the sea floor until it collided with the Eurasian plate.
This collision formed the Himalayan mountain chains.
These continue to grow due to continual plate movement (isostatic lift) but due to extreme weathering it is only about 2.5cm a year Landforms associated with conservative plate margins Occuring when two plates meet side by side conservative plate margins are sometimes known as slip margins. They can cause earthquakes along their fault with those found on the San Andreas fault being well known.
Vulcanism is not associated with conservative plate boundaries. Landforms associated with Hotspots A hotspot is a seemingly stationary area of vulcanicty unrelated with plate boundaries - The Hawaiian islands were created by a hot spot. As mentioned before islands may be created due to a plume of magma erupting through oceanic crust. Eventually the islands become part of the crust and move away from the hotspot. Newer volcanoes erupt over the hotspot and the process is repeated - in this way volcanic island chains cna be formed. Vulcanicity Case studies Lava types Basaltic (Basic) - come from upward movement of mantle material. Common along spreading ridges. Also found @ hotspots. Andesitic (intermediate) - typically found on destructive plate margins where crust is destroyed. Rhyolitic (acid) - found at desructive
collision margins Pyroclasts - Volcanic fragments from fine ash and lapili to volcanic bombs. Usual of gaseous eruption phase of eruption due to gas build up creating violent explosion. Nuées ardentes are glowing gas clouds - melded fragments form ignimbrite. travel much further than lava and can cause lahars. Volcano Shape
(major extrusive features) Fissure eruptions - elongated cracks on crust allow lava to spill out over a large area. Found at spreading ridges.
Rock type: basaltic
Location: Rifts/early constructive margins
Eruptions: gentle, persistent Shield volcanoes - gently sloping cones made from layers of less viscous lava.
Rock type: basaltic
Locations: hot spots where oceanic meets oceanic crust.
Eruptions: Gentle, predictable. Composite volcanoes - most common type. created by layers of ash from explosive phase of eruptions and susequent lava layers. e.g. Mount Etna.
Rock type: andesitic
Location: destructive margins
Erutpions: explosive, unpredictable Acid/dome volcanoes - steep sidede, formed from very viscous lava. As lava cannot travel far it builds up convex cone shape. Lava may solidify in vent and be releavled by later erosion.
Rock type: rhyolitic
Location: contiental crust
Eruptions: Explosive, unpredictable
Calderas - When gasses build up under a blocked vent a catestrophic explosion will at least destroy the volcano summit which leaves an enormous crater. Later eruptions here may cause smaller cones - the crater may fill with water or even inundate the volcano's remains.
Rock type: andesitic
Location: destructive margins
Eruptions: very explosive and unpredictable Geysers & hot springs - even where there is no active vulcanism water heated at depth can periodically escape as steam and hot water. Geysers discharge superheated water which can mix with mud near the survace forming a mud volcano. Intrusive features The majority of lava will never reach the survace, instead it forms coarse igneous rock beneath the surface Minor extrusive features Furmaroles - Superheated water turns to steam and condeses on the surface as the pressure drops when it emerges from the ground. Solfatara - gases, mainly sulphorous escape to the surface. Batholiths form when large masses of magma cool very slowly.
Magma can be squeezed between two strata in the rock forming a sill.
A vertical intrusion formed in a similar way is called a dyke Volcano frequency Estimates for the number of active volcanoes around the world varies alot.
Active volcanoes have erupted in living memory
Dormant volcanoes have erupted within historical record
Extinct volcanoes will never erupt again Volcano hazard management Eruptions cannot be prevented, but are often predicted.
This involves accurate hazard mapping (mapping previous lava and pyroclast flows. Analysing seismic shockwave patterns. Sampling gas and lava emissions and sensing changes in topograhy, heat and gas emissions by satellite.
Not all volcanoes are very predictable.
Protection involves reducing the risk of damage via preparation. the USGS are supported by FEMA on how to react before during and after an eruption. Seismicity Earthquakes Occur continously over the surface of Earth.
18 earthquakes of over magnitude 7 each year. Caused by a build up of pressure in the
earths crust until pressure is too much so is released. General Facts Point at which pressure is released = focus point
Shallow focus: 0-70km deep
Intermediate: 70-300km deep
Deep: 300-700km deep
Surface point directly above focus is called epicenter Seismic wave types:
P (primary) waves: fastest wave shake Earth back and forth. Travel through solids and liquids.
S (secondary) waves: slower, move with sideways motion, shake Earth at right angles to direction of travel. Cannot move through liquids but do much more damage than P.
Surface waves: travel much nearer the surface and move more slowly than both S or P waves. Most destructive wave type. Include L waves which move the ground sidewys and Raleigh waves which move it up and down Magnitude and frequency An earthquake's magnitude is how much energy it releases - usually measured on the Richter scale The Richter Scale is logarithmic - each full number is
10x more powerful than the previous one. The subjective intensity of an earthquake can be measured onthe 12 point Mercalli scale Frequency of earthqyuake events vary between region.
Seismometers measure and record shock waves created by earthquakes.
Aftershocks are earthquakes that follow on from the main event. They may last form months after the original earthquake as the earth settles. Effects of earthquakes Wide range of geomporphological effects from ground shaking to landslides, avalanches to tsunamis. Their severity depends on the magnitude of the earthquake distance from epicenter and underlying geology. Tsunamis - Enormous sea waves generate by disturbances on the sea floor. Triggered by earthquakes and submarine landslides. E.g. 2004 Indian Ocean Tsunami Liquefaction - Violent disruption of the ground causes it to become like a liquid. Shakingcauses increased pore water pressure reducing the rigidity of the soil and the shear strength of it. This makes it more likely to collapse. Liquified soil may flow and the ground can crack and move. This effect occurs most commonly on unconsolidated land such as land reclaimed from the sea and land build on top of old rubble. Landslides and avalanches- slope failure occurs as a result of ground shaking Human inpact - this depends on the population density and distance from the epicentre. Strong shaking can cause buildings roads and bridges to collapse cause disruption to power gas and water supply lines. Human impacts termed primary effects such as death occur immediatley after the earthquake. Other secondary impacts such as fires from ruptured gas mains, disease from polluted water and so on are caused indirectly by the earthquake event and may last for an extremley long time. Distribution of earthquakes Earthquakes are not evenly distributed. They occur along plate boundaries and the most powerful events occur along destructive boundaries.
Constructive margins produce submarine earthquakes far from civilization and therefore present little threat.
Conservative margins can produce serious earthquakes when plates slide past each other.
Earthquakes can occur in regions away from active platemargins, earthquakes in china and cenral asia occur along lines of weakness related to India's collision with Eurasia. Earthquake management Prediction There is no surefire way to predict earthquakes accuratley but areas likely to suffer from them can be indicated.
In these risk areas imminent earthquakes may be predicted by:
Seismic Gap Theory - this involves studying patterns of past records and using these to predict when an earthquake is due in an area.
Radon gas emissions - radon is released when granite is rapidly fractured before a quake.
Ground water - deformation of the ground causes water levels to rise or fall independantley of atmospheric conditions.
Remote sensing - there is evidence that electromagnetic (EM) disturbances in the atmosphere above areas about to experience an earthquake.
Low frequency EM activity - A satllite detecting EM activity has made observations and strong corellations between low frequency EM activity and seismically active zones on earth. A sudden change in ionospheric electron density and temperature was recorded a week before a 7.1 magnitude earthquake in Japan.
These methods are not entirley reliable and many earthquakes still occur with no prior knowledge of them.
Seismographs When the earth shakes the seismograph does so with it except for a mass on a spring which stays motionless due to inertia. The device thereby records the motion of the unit as apposed to the mass and produces a graph. Earthquake Protection Since they cannot be prevented often damage levels are minimised. These Japanese authorities attempt to this in 3 major ways.
1- Making buildings more earthquake resistant
2- Raising public awareness about disaster prevention via education programmes.
3- improving prediction of earthquakes.
Evacuation routes and evac sites are improved to make cities less vulnerable. Reducing the risk of fire is also important, better fire resistant buildings and fire service are being created all the time. All new buildings must meet strict earth quake resistant standards - since 2007 new laws were in effect to force buildings to be double checked against earthquake regulations. Many hotels documents were falsified to save builders money. There is a lot of observation in high vaule and earthquake prone areas such as Kobe and Tokyo.In densley packed areas special building techniques are used including computer controlled weights and rubber bearings in the foundations.
Citizens are advised to keep earthquake kits and police info sheets ensure everyone knows what to do in the event of an emergency. Weather and climate and
associated hazards Major climate controls The structure of the atmosphere Nitrogen - 78.09% Needed for plant growth
Oxygen - 20.95% Produced by photosynthesis
Water Vapour - 0.2-0.4% Forms clouds reflects incoming radiation
Carbon Dioxide - 0.03% Absorbs radiation contributes to greenhouse effect
Ozone - 0.00006% Absorbs incoming UV radiation reduced by CFCs