Tectonics EQ 1 Flashcards
Asthenosphere
Is the area of the upper mantle found 200km to 700km down. This layer can move slowly carrying the lithosphere. Due to the balance of material and temperature and pressure rock have little strength and are viscous.
What is continental drift
Alfred Wegner had an idea that 300million years ago was a single continent called Pangea.
Evidence of continental drift
- jigsaw puzzle
- geological evidence
- fossils
- coal deposits: coal is found in Antarctica have it is only formed under warm wet climates.
- glacial evidence: evidence to show that glaciers in Antarctica, Africa, South America, India and Australia were once together
Sea floor spreading
Harry Hess a US geologist explained how continents drift, known as sea floor spreading.
He studied rocks and found that rocks in the middle of the oceans where young while rocks USA and Europe were older. This is evidence that the floor was spreading out from the centre a 5cm a year. Along ocean ridges is where oceans crew and along trenches is where the sea floor was recycled.
Palaeomagnetism
Is the study of the changes in the earths magnetic field, every 400,000 years.
When material from the mantle rises through the ridges a record of polarity is preserved. Iron in basaltic lava will align with the magnetic pole.
Vine and Matthews noticed there was a symmetrical patters of magnetic strips on either side. They are the same age and at similar distances away from the ridge.
Convection currents
The mantle is heated from hot spots from the earth, these produce convection currents. Convection currents produce a horizontal motion, as it creeps along it creates frictional drag on the lithosphere, so plates move very slowly. It is suggest that there are 2 layers in the mantle.
Push and pull forces in the mantle
The push force is convection currents and the pull force is subduction and slab pull.
What is slab pull
Is when subducted crust cools and becomes thicker and denser. This causes the crust to sink into the mantle under its own weight which pulls the rest of the plate further down. This is a major driving force in plate movement.
When does Subduction occur
Is when two plates collide and the denser plate will subduct. However if they are roughly the same density the one which is moving faster or the thicker crust will subduct.
Benioff zone
An area where many deep earthquakes occur, situated under a destructive plate boundary.
Focus
The point inside the crust from which the strain is released.
Epicentre
Point on the surface directly above the focus
Magnitude
Is the amount of energy released at the epicentre in moment magnitude scale.
Intensity
Shows the amount of damage and effect on people, structures and the natural environment. The modified mercalli intensity scale is uses which takes observations from people who experienced the earthquake.
Primary waves
Are the fastest waves at 8kms. They travel though solids and liquids and push in a forward and backward motion.
Secondary waves
They are slower waves due to their longer wavelength at 4kms. However they only move through solids and in up and down motion. They are more damaging than p waves.
Name the two surface waves
Rayleigh waves
Love waves
Rayleigh waves
Travel through the surface in a rolling motion, moving the ground up and down and side to side.
Love waves
Travels through the crust, fastest of all surface waves and moves side to side as it moves forward.
What does the moment magnitude scale measure
-total amount of energy released by all shockwaves at the moment it occurs
-amplitude and size of the seismic waves
-amount of slippage or rock movement
-area of fault
-resistance of rock affected
Is a logarithmic scale.
List 2 primary effects of an earthquake
Ground shaking: which causes buildings, bridges, roads and infrastructure to collapse
Cristal fracturing: when energy released during an earthquake causes the earth crust to crack leaving
Secondary effects
Liquefaction: when water is brought to the surface by shaking. Buildings often topple over or sink into the ground.
Landslides: ground shaking puts stress on slopes so they fail and can bury houses or damage them. People also become buried.
Avalanches
Tsunamis
Mercalli scale
Measures observations from people who experienced the earthquake and rates them from 1 hardly noticed to 12 catastrophic. Usually stronger magnitude earthquakes have a higher intensity but local conditions can affect this.
Genesis of a tsunami
Is caused by a submarine earthquake at subduction zones. When the plates subduct there is a build up of stress and when the overriding plate breaks free it displaces water upward and forwards. This is known as water column displacement.
How are tsunamis formed
- caused by displacement of large quantities of water
- when a plate subducts there is a build of stress and when the stress is released, the plate recoils back in place.
- this displaces water upward and forward and is know as a water column
- when the tsunami reaches shallower water the waves slows down which decreases wavelength but increases wave height.
Run up of a tsunami
- bathymetry: In deeper waters energy is retained, In shallow waters waves slow down and increase in height.
- Topography and orientation
- Distanced travelled: energy lost over greater distances.
What is lava flow
Is streams of lava reaching temperatures of 1170*c
What are Pyroclastic flows
Is a mixture of dense hot rock, lava, ash and gases travelling at 100km/h and are 700*c
What is a tephra
Pieces of volcanic rock and ash that blast in the air during an eruption. Larger pieces fall near the volcano and can cause harm and death while ash travels for thousands of km. Causing poor visibility, slippery roads, engines may clog, roofs may collapse.
What is Gas release
Gas released during a volcanic eruption. This can be water vapour, carbon dioxide and sulfur dioxide. These gases can accumulate in valleys and kill people people.
What is Flank collapse
When a chunk of a volcano is blown off, leaving a crater. This results in a landslide.
What is Lahars
Mud and water that travel quickly down the side of volcanoes. This is caused when an eruption melts snow and ice.
What is a Jökulhlaup
Volcanic eruption rupturing melts the snow and ice in a glacier causing heavy floods. Floods release large amounts of water, rock gravel and which harm people
