Chapter Four- The Earth’s Interior
WHAT CAN WE LEARN FROM THE STUDY OF SEISMIC WAVES?
Seismic reflection-return of some of the energy of seismic waves to the Earth’s surface after the
waves bounce off a rock boundary.
Waves reflect off of the boundary between two layers of rocks and reflects back to a recording
device called a seismogram. The seismogram measures the amount of time it took the waves to
reflect and come back to it to measure the depth of the boundary.
Seismic refraction- the bending of seismic waves as they pass from one material to another. As
the seismic wave strikes a rock boundary, a lot of the wave travels through I, and changes
Seismograph stations receive the direct wave before the refracted one
Many different seismographs can be set up in a row to record the velocities and depths of the
In a thick layer of rock, the increasing pressure with depth changes the wave velocity, making
the waves curve.
WHAT IS INSIDE THE EARTH
The crust is the outer layer of rock which forms a thin skin on the Earth
The mantle is a thick shell of rock that separates the crust from the core
The core is the centre of the earth made of metals and making the magnetic field
Crust is thinner beneath oceans than continents and seismic waves travel faster through oceanic
crust (7km/sec) vs 6km per sec through continental.
The continental crust is made of granite and other rocks covered in a layer of sedimentary rocks.
Felsic-rocks high in feldspar and silicon for the continental crust
Mafic-rocks high in magnesium and iron for oceanic crust
Mohorovicic discontinuity- the crust that separates the crust from the mantle
Due to the way that the seismic waves travel, it is believed that the mantle is made of solid rock
with magma chambers of melted rock in the upper mantle
Ultramafic rock is dense igneous rock made up of ferromagnesium minerals. It is believed that
this is what the upper mantle is made up of.
The crust and uppermost mantle together form the lithosphere that is brittle
The lithosphere is the basis of the plate tectonics theory and its lower boundary is marked by the
place that the seismic waves slow down.
Asthenosphere-the rocks in this zone are closer to melting point than the rocks above or below
the zone. Some of the rocks may even be melted, forming a crystal and liquid slush zone.
If the rocks are close to their melting point, the zone may be important because it may represent
a zone where magma is likely to be generated, and the rocks here may have relatively little
strength and therefore are likely to flow. This can cause plasticity between the asthenosphere
and mantle rocks.
There is argument as to whether the asthenosphere even exists under continental crust as it does
in oceanic areas.
Scientists think that the chemical make up of the mantle rock is about the same throughout, but