Wave Type 1: Body Waves
P Waves
The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air.
The arrow shows the direction that the wave is moving.
S Waves
The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock. This wave moves rock up and down, or side-to-side.
The arrow shows the direction that the wave is moving.
Wave Type 2: Surface Waves
A surface waves is a seismic wave that travels along the surface of the Earth. Rayleigh waves and Love waves are surface waves.
The arrow shows the direction that the wave is moving.
The arrow shows the direction that the wave is moving.
CHAPTER TWO – Earth’s Structure
Both P and s-waves travel through the earth but only p-waves can travel through the core. The seismometers within the earth can be used to research the internal structure of the earth. S-waves leave a shadow and provide evidence that the core of the earth is molten. At the plate boundaries of an oceanic and continental plate, the oceanic plate subducts under the continental plate causing an earthquake. This subduction causes off shore trenches, which is where the sea is at its deepest. As the oceanic plate subducts beneath the continental plate, it begins to melt as it enters the hot mantle, producing magma. Volcanic chains can be formed from the fold mountains due to the subduction but that is only if there is a way for the magma beneath the earth surface to reach the ground surface, for example a crack. We know that due to the waves travel times, they cannot be moving through the earthy in a straight line.
At a constructive or transform plate boundary, two plates slide side by side past each other in opposite directions. The forces exerted on either plate contribute to the rock style found there. The earth is structured as follows:
The crust is very thin (about 20km or so), and the mantle has the properties of a solid but can flow very slowly. The core accounts for just over half the earth’s radius and it is made from iron and nickel. This is where the earth’s magnetic field originates. The iron and nickel sank to the centre of the earth long ago because they are denser materials. The core has a solid inner part and a liquid outer part. Radioactive decay creates all heat in the earth and this heat causes the convection currents, which cause the plates of crust to move. We can tell how dense the earth is by measuring seismic waves and the earth’s motion. Scientists have found that the inner core is much too dense to be made out of rock. Meteorites, which crash into the earth, are often made of iron and nickel, which are both magnetic and very dense. If the core of the earth were made of iron and nickel it would explain a lot: iron and nickel are both the right density, and being metals this would explain the earth’s magnetic field (it being like a giant electromagnet). Also, by following the paths of seismic waves as they travel through the earth, we can tell there is a change from solid to liquid about halfway through the earth. There must be a liquid outer core of iron and nickel. The seismic waves also indicate a solid inner core. The earth has 4 main shells; the crust, the mantle, outer core and the inner core. The earth’s structure drawings have been established by many independent analyses of measurements of earthquake waves.
CHAPTER 3 – Plate Tectonics
The Earth’s crust is divided into a number of different plates moving around on a layer of magma. The map below shows the different plates:
When the plates move, a number of things happen at the plate boundaries.
- Constructive plate boundaries: Constructive plate boundaries occur when two plates move away from each other. When this happens, earthquakes and volcanoes occur.
- Destructive plate boundaries: Destructive plate boundaries occur when an oceanic plate is forced under (or subducts) a continental plate. When this happens, volcanoes and earthquakes occur. Trenches and fold mountains are also formed.
- Conservative plate boundaries: Conservative plate boundaries occur when two plates slide past each other. When this happens, severe earthquakes can occur.
- Collision plate boundaries: Collision plate boundaries occur when two continental plates move towards each other. When this happens, earthquakes occur and Fold Mountains are formed.
CONCLUSION
Seismic waves can be both very useful and very dangerous at the same time. Seismic waves can teach us so much about the Earth we live in. Plate tectonics can also teach us so much but can also be dangerous as they can cause earthquakes, volcanoes, and other natural disasters. Plate tectonics can teach us about the earths past and also what will happen to the earth in the future.
Seismology has given many scientists ideas on how the Earth is structured. They have used the seismographs to figure out the differing effects of P and s-waves, and how they go through the earth, determining their behaviours. Scientists to discover the effects on and at plate margins used theories about plate tectonics. Because of volcanic eruptions and sever earthquakes, the Earth’s crust is now considered to consist of several large slabs of rock and some smaller ones, called tectonic plates. Seismology is an advanced scientific research.