Tectonic theory, Plate margins and Landforms
Tectonic theory, Evidence, Plate Margins, Processes, Associted landforms, Hot spots
The Earths structure
Plate tectonic theory: The idea that the earths surface is a series of moving slabs of lithosphere called tectonic plates.
4 Main opinions on why the plates move:
- Continental Drift
- Sea floor Spreading
- Convection Currents
- Slab Pull- most recent
Alfred Wegener suggested that the continents were all together about 300,000 called Pangea.
Evidence for this is:
- Continental fit- south Africa fits in Africa, more revealing when looking at continental shelves.
- Biological evidence- Fossil remains. e.g remains in India match those in australia (brachiopods.
- Geological evidence- Matching rock types and strata. e.e similar age type and formation in East Brazil and South West Africa.
- Climatological evidence- Red sandstones of desert origin found in Britain.
WEGENERS IDEAS WERE DISMISSED AS HE COULD NOT PROVE HOW THEY MOVED
Sea Floor Spreading
1948- US survey in the Atlantic discovered the Mid Atlantic Ridge
1960's gave way to the idea of sea- floor spreading
Rock dating and paleomagnetism was used as evidence.
In 1962 new dating techniques enabled the date of when rocks were formed to be accuratly calculated.
The youngest rocks found closest to the ridge.
Convection currents and slab pull
Convection Currents- The movement of the left over heat from the core as it rises and falls in the mantle.
The heat circulates in cells beacuse hot rock rises upwards from the lower mantle to the asthenosphere. The asthenosphere rock becomes more ductile or plastic and spreads sideways. As plates cool they drag plates down into the mantle helping with the movement of the plates this idea is called slab pull.
Plate margins- Boundaries between tectonic plates that move towards each other, away or sideways. Two processes can happen at margins, seismicity and vulcanicity.
Constructive: Ocean ridges and rift valleys
Destructive: Young fold mountains, Deep sea trenches and island arcs
Conservative: no land forms
Landforms: Sea floor spreading creating ocean ridges
Example: The mid atlantic ocean ridge
Process: Convection currents rise, this movement drags the lithosphere in different directions. Tensions and high temperature weaken the plate until it splits. The gap is filled by magma as rock in the asthenosphere melts due to a fall in pressure.
Landforms: Rift Valley
Example: The Great African rift valley
Process: Convection currents rise, this movement drags the lithosphere in different directions. Tensions and high temperature weaken the plate until it splits. The asthenosphere heats the plate above which expands and forms a bulge on the surface called a horst. The lithosphere stretches and fractures into parallel faults. The crust in between these faults collapses between the horsts and sinks forming grabens, these are the rift valleys.
Destructive- Continental meets oceanic
Landform: Oceanic Trench and Young Fold Mountains
Example: The Pacific coast of South America. The Nazca plate subducting the South American plate.
Process: When the continental plate meets an oceanic plate the denser, basaltic oceanic plate sinks below the less dense, granite based continental crust (subduction). The oceanic plate beging to patially melt and is destroyed by 7000km. Subduction creates a deep ocean trench on the seaward edge (e.g the peru-chile trench) and the edge of the continental plate buckles and gets compressed under the pressure creating younge fold mountains (e.g the andes).
Destructive- Continental meets oceanic
Destructive- Oceanic meets Oceanic
Landform: Deep Ocean Trench and Island Arcs
Example: The marianas Islands in the north western Pacific. The Pacific plate subducted under the smaller Philippine plate.
Process: The pacific plate is slightly denser and faster than the Philippine plate and is therefor subducted.The subducted plate partially melts and creates magma in the Benioff Zone. Magma rises through the gaps in the philippine plate and escapes as submarine volcanoes creating sea mounts in a crescent shape on the sea bed. Continued volcanic activity builds up the sea mounts and the eventually surface as volcanic islands. There are violent explosive andesitic magma due to partial melting and violent earthquakes due to subduction and friction in the benioff zone.
Destructive- Oceanic meets oceanic
Destructive- Continental meets Continental
Landform: Young fold Mountains
Example: Himalayan Mountains, Indian and Eurasian Plate
Process: The two continental plates are lower in density than the asthensphere below they DO NOT SUBDUCT. Instead they collide with each other. The collision buckles and compresses the two continental plates to form young fld mountains. This forms deep rooted crumpled rocks in the lithosphere and sedimentary rocks on the surface. The old dense oceanic crust gets seperated and sinks into the mantle. The Himalayas are getting higher as the rocks compress. This is called uplift and makes it an active earthquake zone. Highest peak is Everest at nearly 9000m and growing.
Destructive- Continental meets continental
Landforms: none- heat not sufficient enough to creat magam however ver violent earthquakes.
Example: Caribbean Plate and the North American plate both moving in a westerly direction. Causing earthquakes like Haiti.
Process: The Lithosphere is not subducted so magma is created so no volcanic activity. However they are extremely seismically active and the source of powerful earthquakes. When the plates lock together this builds up stress and tensions between the plates. When the plates slip this energy is released as seismic waves that form shallow focus earthquakes.
Landform: Island Chain
Example: Hawaiian Island Chain, the pacific plate moving in a north westerly direction over the hot spot
Process: Away from plte margins, areas of volcanic activity, only occure on oceanic plates where the lithosphere is thin. The heat is beleived to have been from a high concentration of radio active metals which rise from the inner core to the asthenosphere. This high concentration creats a hot plume which burns through the lithospere. The change in pressure causes decompression melting in the lithospere which creates the constant flow of magma. . this constant flow creates sea mounts on the sea bed eventually growing into and island on the surface. This island that is created moves away from the hot spot as the plate moves making the island extinct as they are o far from the plume. The older islands reduce in size due to erosion from the sea.
Vulcanicity And Seismicity
Intrusive and Extrusive activity, Types of volcanoes, Seismicity
Magam can force its way into the lithosphere by exploiting weaknesses such as fault. This usually happens at plate margins or hot spots.
When magma does intrude into the lithosphere it cools, the crystallises and solidifies to form igneous rock. When this happens intrusive features are formed and eventually due to weathering and being surrounded by less resistant rock they are exposed on the surface.
Forms of Intrusive activity
Dykes- Form when the magma solidifies in a vertical crack or fissure. They cut across the bedding planes of the surrounding rock at a right angle. The granite or dolerite that makes up the dyke is usually mor resisdent than the surrounding rock and the heat from the magma weakens the rock there for gets weathered and eroded to expose a ridge on the surface.
Sills- Form along horizontal or inclines strata kown as bedding planes.
Examples of Dykes ands sills: the isle of Skye. A famous sill on the isle of skye is Kilt rock.
Batholiths- Much larger in scale than dykes and sills. Form deep below the surfaceas large masses of magma cool, crystallise and solidify. Often domed shape, can be hundreds of km in diameter.
Examples of batholiths in dartmoor and the isle of Arran.
Extrusive activity- refers to the escape of water, steam and other gasses above the surface due to geothermal activity below the surface.
Geysers- Explosions plumes of water and steam erupting from the ground and occur when water in the crust is heated and erruption occur at regular intervals. The water is heated to boiling point by volcanically heated rock from magma intrusions and when the water reaches boiling point which creates large volumes of steam it pushes up through vents carrying water with it.
Hot springs- Pools of hot water, some are acidic, others alkali.
Boiling mud- If the water from hot springs mixes with surface sediments boiling mud is formed. Creted in places that have a large supply of steam and rock material of a type that breaks down into mud.
Fumaroles- Patches of escaping steam from small vents and sometime deposit sulfur around their edges. Formed when water boils below the surface to release steam or when magma releases vapours such as SO2 from degassing. These gasses escape to the surface through vents.
Examples of Extrusive Activity
Geysers- Iceland- Great Geyser- first erupted in the 14th century, erupts every 60 mins until the early 20th century. Dormant for 20 years until an earth quake triggered more activity for 10 years till 2010.
- Old faithful- 45/100 mins eruption intervals
- steamboat- 5- 50 years eruption intervals.
Hot springs- the blue lagoon, iceland
Types of lava
Basaltic- Runny low viscosity lava which is low in silica. Gas bubbles can expand as the magma rises , eruptions are fluid and none explosive.
Formed by partial melting of the mantle, constructive margins and hot spots, sheild.
Andesitic (acid)- Medium viscocity and silica content.
Formed by partial melting of subducting oceanic plates, destructive plate margins, Strato/composite
Rhyolitic- High viscous lava. Causes explosive eruptions as gas cannot escape easily, pressure builds up and can solidify and plug vents leading to more pressure building.
Formed by partial melting of continental lithosphere- continental and oceanic subduction margins, continental hot spots, dome/ caldera.
Types of volcanoes
Factors that affects the hazards:
- Energy of the blast
- Direction of blast
- Type of eruption, explosive or effusive
- Velocity and direction of the wind, effects on ash
- Lava viscisity
- Surface conditions- snow, ice
Lava flows- Very slow moving viscous flows. Lava + steep sides= many deaths e.g Nyragongo-congo fast flow of 60mph.
Tephra- solid material ejected into the atmospere. Lava bombs and ash, Ash clouds can get suspended in the air- slight change in the climate. e.g Iceland ash cloud 2011/ Pinatubo- phillipines- fine ash embeded in peoples lungs from 1991 erruption, people died one year later.
Poisonous gases- May be toxic or corrosive- CO2, sulphur dioxide and chlorine. e.g Lake Nyos cameroon- massive co2 emissions- 1700 died- 1986
Pyroclastic Flows- super heated gases and ash vaporise organs e.g MT pelee 1902
Lahars- mud flow- unconsolidated tephra and water and ash rapidly move down slope often along river valleys e.g navada del ruiz- 1985-23,000 killed
Tsunamis- Krakatoa- 1833- 36,000 poeple dead- formed by pyroclastic flow into the sea.
Flooding- Glacier bursts- lava melting large quantities of ice- melt water under glacier e.g Iceland, Grimsvotn- every 4-6 years- 1996 3.5 km3 of water.
Causes and characteristics of Earthquakes
Can be Physical and human causes. Occur at destructive, conservative and constructive plate margins when the slabs of lithosphere move on top of each other or next to each other and catch. This then builds up lots of pressure between the plates. When the pressure in the rock is released the rock fractures and releases seismic shockwaves.
Destructive: at subduction zones when less dense plates stick as they subduct- can cause displacement which is how much the rock has moved.
Conservative: Plates more prarallel to each other. e.g The San Andreas Fault Zone.
Constructive: As the plate smove apart they can stick and cause tremors anlong transform faults perpendicualr to the fault line. e.g MAR
Characteristics include the depth of the focus, the epicentre(rural,uraban damage depends on human interation with fault zone), Primary and secondary waves and rock type. All affect the characteristics and the magnitude of the Earthquake.
Three main types of seismic waves traveling in different directions and speed. They are shock waves released by the rupture of rock.
Primary- travel fastest and vibrate in the direction they are traveling in (longnitudinal waves).
Secondary waves- Half the speed of p waves and shear rock by vibrating at right angles to direction of travel (transform waves)
Surface waves- travel the slowest near to the ground.
-Love waves like secondary but on the surface
-Rayleigh waves in a rolling motion- most destructive, causes land to move up and down.
Richter scale- Charles Richter 1935. Measures the strength of the seismic wave at the focus. Uses a logarithmic scale, each unit shows a 10 fold increase in the seismic wave however the energy increases by 30 times per unit.
MMS- Updated version of Ritcher scale. Takes into acount the size of the rupture, amount of displacement and rigidity of the rock.
PROBLEM WITH THESE IS THEY DONT SHOW SHOW HOW MUCH DAMAGE ON THE SURFACE THE EARTHQUAKE HAS CAUSED.
Mercalli- Measures the intensity of the effects of the earthquake. Uses Roman numerals SO YOU SHOULD TO!!
Richter and Mercalli scale
Impacts of Earthquakes
- Ground shaking causes buildings to collapse, windows to shatter, powerlines to collapse
- Schools, colleges, universities destroyed
- Immediate deaths and injuries
- Liquefraction of saturated soil
- Panic, Fear, hunger
- Fires due to broken gas pipes and power lines, emergency services are hindered
- eduction suspended
- Bodies not buried
Impacts of the Earthquake depend on...
- Local geology type of rock and soil
- Shape of the land- Geomorphology
- Location- seabed can result in tsunami
- Distance of population from epicentre
- Population density
- Level of prepardness
- Building strength and design
- Time of day
A tsunami: a series of long waves triggered by a sudden displacement of seawater. A seismographic tsunami results from a rupture on the sea floor during an earthquake.
- Open ocean- Long wavelength more than 100km, low wave height under 1m, Travels fast more than 700km/h.
- Shallow water- the waves increase rapidly in height can be over 25m, Wave length decreases, Waves slow down to around 50km/h, The sea level drops because of draw down.