Topic 4/F4: Earth Structure and Global Tectonics Flashcards
What are the layers of the earth?
Crust
Upper Mantle
Lower Mantle
Outer core
Inner core
What are the two types of crust?
Oceanic and continental
What is the difference between the oceanic and continental crust? (4)
Oceanic:
- Made of Basalt
- More dense (2.9g/cm3)
- Younger rock (200Ma)
- Thickness (5-10km)
Continental:
- Made of Granite
- Less dense (2.7g/cm3)
- Older rock (4000Ma)
- Thickness (90km)
What is the oceanic crust composition? (4)
- Made of Basalt, Mafic (Mg and Fe)
- Basalt Pillow Lavas
- Dolerite Dykes
- Gabbro in layers
What is the continental crust composition? (3)
- Made of Granite, Felsic (Al and Si)
- Granite rocks
- Igneous, Metamorphic and Sedimentary rocks
What are the different types of seismic/surface waves?
- P waves
- S waves
- L waves
- R waves
What are the features of P waves?
- Move through solids and liquids
- Longitudinal waves
- Vibrate back and forth
- Higher frequency than S waves
- Move fastest, and arrive first
What are the features of S waves?
- Move though solids only
- Transverse waves
- Vibrate at right angles
- More damaging but move slower than P waves
What are the features of L waves?
- Surface wave
- Most damaging, but the slowest
- Energy focused at the surface
- Cause a side to side motion
What are the features of R waves?
- Surface waves
- Like water waves, have a rolling motion (vertical & horizontal)
- Amplitude decreases with depth
- Most destructive
What is the shadow zone?
An area in which seismic activity isn’t detected (P and S waves can’t pass through) due to a discontinuity in the earth
What happens at the shadow zone?
- P waves and S waves are refracted
- Cannot be detected between 103-142 degrees
- Beyond 142 degrees s waves aren’t received, whereas p waves arrive late
- This is all evidence for a liquid outer core, as S waves can’t travel through and P waves move slower
What is the boundary between the inner and outer core and what happens to seismic waves?
- The Guttenburg Discontinuity
- At a depth of 2900km
- P waves decrease in velocity
- S waves can’t pass through as it is a liquid outer core
Why can’t S waves travel through liquids?
- They are transverse
- Slower than P waves
- Travel through solids only
- The outer core is liquid
What happens at the moho boundary?
- Rigidity and incompressibility increases, so seismic velocity increases
- Depth also increases, further increasing wave speed
- Waves are refracted differently due to different density in layers
What detects seismic activity?
Seismometer
What is direct evidence for the earth’s composition?
- Meteorites
- Mining and Boreholes
- Ophiolite Suites
- Geothermal Gradient
- Magnetism
What is indirect evidence for the earth’s composition?
- Density
- Seismic Waves
- Gravity surveys
- Geomagnetism
How do ophiolite suites show direct evidence of the earth’s composition?
- Ancient sections of crust can be found on land without drilling
- Thrust upon land due to plate movement
How do meteorites show direct evidence of the earth’s composition?
- Rock samples from the solar system that have fallen down to earth
- Two different compositions: iron which is similar to the earth’s core and stony which represents the earth’s mantle
What is the asthenosphere made up of?
- Upper mantle, below the lithosphere
- Below the MOHO discontinuity
- Where magma generates
- Semi-molten or plastic
- P and S waves lose velocity here
3 different types of seismic reflection?
- Seismic refraction which shows crustal layering
- Seismic reflection showing the internal structure of the crust
- Seismic tomography shows the deep structure of the mantle
What are features of the lithosphere?
- Made of crust and uppermost mantle
- Rigid and brittle
- Fractured by plate boundaries and faults
- Earthquakes generated here
What is continental drift?
Evidence for the plates moving over a period of geological time
What is the low velocity zone and what happens here?
- Occurs close to the boundary between lithosphere and asthenosphere
- In the asthenosphere the seismic velocity decreases due to it’s plastic like composition
Who created the theory for continental drift?
Alfred Wegener
What was Alfred Wegener’s evidence for continental drift? (3)
- Appalachian mountains and the Scottish Highlands have similar mineral composition and age determined by radiometric dating
- Fossil dating of the ‘Mesosaurus’ found across West Africa and South America
- Glacial striations tracing the glacier movement of Southern Africa (moved from the southern pole)
What is slab pull?
The force of a cold, dense oceanic plate plunging into the mantle due to it’s own weight drives the movement of the plate
What is ridge push?
Rigid lithosphere slides down the hot, raised asthenosphere below mid ocean ridges, pushing plates away from each other
What is seafloor spreading?
The continuous input of magma forming a mid ocean ridge, for example the Mid-Atlantic Ridge
What is paleomagnetism?
Paleomagnetism is the record of the Earth’s magnetic field in rocks, sediment and archaeological materials
How does paleomagnetism provide evidence for sea floor spreading?
Zones of magma ‘lock in’ the Earth’s magnetic polarity, when it changes every million or so years, so scientists can use this to create a ‘geo-timeline’
What happens at divergent plate boundaries?
Two plates are moving away from each other (can be continental or oceanic), leading to new crust being created by magma being pushed up from the mantle
What happens at convergent plate boundaries? (oceanic-continental)
Where two plates meet, the denser oceanic lithosphere is forced down and under the more buoyant continental lithosphere in the process of subduction. The friction between the plates prevents the subducting oceanic plate from sliding smoothly, so it drags over the overlying plate causing both to fracture and deform.
What happens at transform plate boundaries?
Two plates are sliding past each other, often at different speeds. This causes a build up of friction as they drag against each other.
What is an accretionary wedge?
A zone of deformed sediment made up of thrust slices scraped off a subducting oceanic plate and added onto the over-riding plate.
How do convection currents drive plate movement?
Heat derived from the Earth’s core rises within the mantle to drive rocks upwards. Near the crust, these rocks drag along the tectonic plates, slowly moving them before sinking as they lose heat from the Earth’s core. This creates a convection current cell, heat constantly rising and falling to power plate movement.
What is the relationship between seismicity, volcanicity and plate boundaries?
Plates rip apart at a divergent plate boundary, causing volcanic activity and shallow earthquakes.
At a convergent plate boundary, one plate dives or “subducts” beneath the other, resulting in a variety of earthquakes and a line of volcanoes on the overriding plate.
What is the curie point?
The temperature at which certain magnetic materials undergo a sharp change in their magnetic properties
What is radiometric dating?
A technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities
What is the rock cycle for igneous rocks?
Form when either magma or lava cools down and turns from liquid to solid. When this happens, igneous rocks form crystals and are said to crystallise.
What is the rock cycle for sedimentary rocks?
- Rocks on the Earth’s surface are gradually broken down into smaller pieces by water ect
- Now sediment, will be transported by water or eroded. They collect at the bottom of lakes/rivers
- Over time they are squashed and compacted to form sedimentary rock
What is the rock cycle for metamorphic rocks?
Rocks that have been changed over time. When rocks are pushed deep down into the Earth, grains and minerals can become squashed and slightly melted from the pressure and heat. (Metamorphism)
How does andesite and granite form at continental to continental plate boundaries?
How does andesite, granite and rhyolite form at oceanic to continental plate boundaries?
How does basalt, gabbro and dolerite form at divergent plate boundaries?
What is regional metamorphism?
Metamorphism affecting rocks over an extensive area as a result of the large-scale action of heat and pressure.
What is dynamic metamorphism?
Metamorphism which is the result of very high shear stress, such as occurs along fault zones.
