1.1 Structure of the Earth

A rheid is a .

The average thickness of the oceanic crust is .

The average thickness of the continental crust is .

Properties of the :
- It is made up of the crust and top part of the upper .
- High geothermal gradient
- It is .
- It is made of igneous, metamorphic and sedimentary rocks.
- P and S waves gradually with depth.
- Density 2.7 to g/cm3.

Properties of the asthenosphere:
- It is a
- It is 5% partially molten
- low geothermal gradient
- below the
- made of
- P and S waves slow down
- density g/cm3

Direct evidence for the composition of the crust:
B
- of igneous and metamorphic rock are brought to the surface from depths of about km (geological)
M
_ Rocks can be observed at depth in . The deepest mine is about 4 km.
V
- Crustal rocks brought up in such as fragments from volcanic vent or magma chamber
O
- Oceanic crust is and exposed by erosion allowing rocks such as , dolerite or gabbro to be seen

Direct evidence for the composition of the mantle:
Mantle xenoliths
- Peridotite is brought to the in volcanic eruptions.
Ophiolites
- Peridotite is sometimes found at the of ophiolites.
pipes
- Bring up of from the mantle from up to 250 km depth.

How are seismic waves used as indirect evidence for the state of the layers of the Earth?
Asthenosphere:
- in velocities of P and S waves due to reduction in caused by 5% partial melting.
Lower Mantle:
- Increase in velocity of P and S waves as pressure increases and rocks become more .
Outer core:
- P waves suddenly as they enter outer core as rigidity is .
- S waves stop completely as they cannot be transmitted in a .
Inner Core:
- P waves speed up at Lehmann Discontinuity as they enter inner core.
- S waves are by P waves.

Indirect evidence for the composition of the Earth:

- meteorites
- Earth has a magnetic field
- Earth has a high average density of gm/cm3 and the density of the oceanic crust is 2.9g/cm3 and the continental crust is 2.7g/cm3 so therefore the density of the core must be high.
Mantle
- meteorites which are dense like the mantle.