Earth Structure and Global Tectonics Flashcards
How many layers of the Earth’s core can be observed?
3.7%
What are the layers of the Earth? (top to centre)
Crust
Moho Discontinuity
Upper Mantle
Asthenosphere
Lower Mantle
Gutenberg Discontinuity
Outer Core
Lehmann Discontinuity
Inner Core
What evidence do we have to know all the layers of the Earth?
Geophysical measurements
Meteorite composition
Mantle Xenoliths
Mean Density Calculations
What are the geophysical measurements?
Seismology
Conductivity
Geomagnetism
Gravitational Anomalies
What is seismology?
The study of vibrations within the Earth using seismic waves
What are a few example causes of vibration?
Earthquakes
Extraterrestrial impacts
Explosions
Storm waves hitting the shore
Tidal effects
How can we measure seismology?
A seismometer measures seismic waves and produces a seismograph
- modern seismometers can measure movements smaller than one millionth of a millimetre
What are the kinds of seismic waves?
Body waves = P-waves and S-waves
Surface waves = Love waves and Rayleigh waves
What movements do the seismic waves make?
P-waves = longitudinal/compressional
S-waves = shearing motion (vertical)
Love waves = shearing motion (horizontal)
Rayleigh waves = elliptical/rolling
Why do waves bend as they travel through the Earth?
Because the pressure increases, density of the rock changes which alters the speed of the wave and refracts the path more so that it is not straight.
What happens to a seismic wave when it encounters a boundary between materials with different properties?
The energy splits into reflected and refracted (bent) waves
What happens to the seismic waves when they pass through a boundary into a more dense rock?
The velocity increases
The wave refracts (bends) towards the boundary separating the layers
What happens to the seismic waves when they pass through a boundary into a less dense rock?
The velocity decreases
The wave refracts (bends) away from the boundary separating the layers
What is conductivity?
The changes in temperature as you move from one layer of the Earth to the next
How is heat transferred within the Earth?
Conduction
Convection
Advection
What is conduction?
Give an example of where this occurs
Where heat is transferred through a substance with no movement involved
- In the lithosphere, through rocks
What is convection?
Give an example of where this occurs
Where heat is transferred by a substance due to buoyancy differences
- Currents in liquid mantle at hot spots or ocean ridges
What is advection?
Give an example of where this occurs
Where heat is transferred through a substance by a fluid
- Water or magma through the crust
Why is the interior of the Earth hot?
Formation of a proto Earth
Redistribution of siderophile elements
Radioactive decay in the core and mantle
How do we know the interior of the Earth is hot?
- Magma, geysers and other geothermal activity at the surface
- Heat is constantly transferred from its source in the Earth to the surface
How does heat get transferred from its source in the Earth to the surface?
- Geothermal Flux
- Geothermal Gradient
What is geothermal flux?
The rate of heat energy transfer through a given surface over a period of time ((milli)watts per square metre)
How does geothermal flux work?
Heat from interior of Earth moves towards surface through conduction (in solid rock) and convection (in molten rock or magma), from the hotter layers (core and mantle) to cooler layers (crust and surface)
Give examples of areas of high geothermal flux and explain why it is high
Mid-ocean ridges/tectonic plate boundaries - highest geothermal flux as new crust is constantly being formed and magma is closest to the surface, giving out a lot of heat
Volcanic regions - high geothermal flux due to volcanic activity and magma chambers near the surface e.g. Iceland, Hawaii
Give examples of areas of low geothermal flux and explain why it is low
Areas of stable continental crust - old continental crust e.g. large landmasses (cratons) have lower geothermal flux because mantle beneath is more stable and cooler
What is the significance of geothermal flux?
Plate tectonics - rising heat from the core drives convection currents in the mantle. Heat causes mantle rock to become less dense and rise while cooler, denser rocks sink, driving movement of plates
Geothermal energy - understanding GF allows us to safely tap into geothermal energy sources - renewable - creates electricity
Geological processes - flow of heat affects processes e.g. volcanism
What is geothermal gradient?
The rate at which temperature changes with depth per kilometre (Celsius per km)
What factors affect the geothermal gradient?
Types of rock - different thermal conductivities e.g. igneous and metamorphic better than sedimentary
Tectonic activity - in tectonically active regions GG is higher because magma is closer to the surface e.g. plate boundaries/rifts
Crustal thickness - steep gradient in thinner crust and lower in thick continental crust where the heat is spread out over a larger area
What is an area of high gradient?
Over 30 degrees per kilometre
What is the average geothermal gradient of the crust?
25-30 degrees per kilometre
Where are areas of high gradient found?
Active tectonic areas or where magma is close to surface e.g. mid-ocean ridges or volcanic areas
What is an area of low gradient?
Below 20 degrees per kilometre
Where are areas of high gradient found?
In areas with tectonic activity or where magma is close to the surface
e.g. mid-ocean ridges or volcanic areas
e.g. Pacific Ring of Fire
Where are areas of low gradient found?
In areas of stable continental crust where heat flow is less intense
e.g. cratons or shields
e.g. Canadian Shield
How does geothermal gradient change with layers of the Earth?
Crust - gradient is pronounced, significant heat increases with depth
Mantle - going deeper, gradient becomes more gradual because although temp still increases, the material ability to conduct heat also increases so there are less rapid temp change per km
Core - temp is extremely high (up to 6,000 degrees) but gradient is less important for surface geology since heat flow is slow compared to crust
What is the importance of geothermal gradient?
Metamorphism - GG controls conditions which rocks undergo metamorphism
e.g. high temp + shallow depth = contact metamorphism near magma bodies due to higher GG
Volcanism - magma from mantle can more easily reach the surface due to volcanic activities - high GG
Geothermal energy - understanding GG allows us to tap into geothermal energy - renewable - generate electricity
How is geothermal gradient measured? (NOT UNIT)
Drilling deep boreholes into the crust and measuring how temp changes with depth. Data then used to calculate rate of temp increase per km
