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