The lithosphere: (crust and upper mantle). The earths crust and the outermost part of the mantle make up the lithosphere. The crust is much thicker under the continents than it is on oceanic crust, continental crust is up to 90km thick in areas, whereas oceanic crust at most is 10km thick. The lithosphere overall is between 70 and 125 kilometers thick. The lithosphere s really just a thin skin as the earth’s diameter is about 12800km. The continental crust is made up of granite and of less denser rock types whereas the oceanic crust is comprised of dense rocks like basalt.
The earth’s plates: Plates are the pieces of crust that float on top of the mantle; they are moved by convectional currents, which move around in the mantle. Continental drift is the theory that the earth’s plates are constantly moving and have not always been in the same position since the earth was formed. The earth’s core temperature is 4000*c, the mantle, and crust act as insulators, which reduce the cooling process of the core. Convection currents happen because of the high temperature of the core and radioactive materials in the mantle breakdown and release energy this heats the mantle making it flow like plasticine. The lithosphere and crust are made up of fifteen large plates, they move on flowing mantle called the asthenosphere and move very slowly in different directions.
Types of plate margin: Destructive: Constructive: Collision: Conservative:
Colliding plates: When a continental plate meets an oceanic plate, the lighter continental plate moves of the more dense oceanic plate causing a trench to be formed, and over millions of years, mountain ranges are formed. When two continental plates collide, mountain ranges are formed, because neither descends because of the similarity in density. When two oceanic plates collide the slower moving plate is usually sub ducted underneath the faster moving plate. The areas where the oceanic plate descends beneath the continental plate are known as subduction zones. Zones like these have increased temperature and pressure this can cause metamorphism, which is when igneous and sedimentary rocks are changed into new types by recrystallization.
Separating plates: When two plates move apart, new crust is created by magma, which comes up from the mantle to fill the gaps. Sometimes continental plates split and continents move apart, causing a rift valley which if flooded will form a sea. The boundaries between separating plates and splitting plates are constructive boundaries because new lithosphere is being formed.
Conservative plates: At conservative plate margins the plates slide past each other, there is so much pressure building up until eventually the plate’s move with a large jerk causing earthquakes. This type of boundary is where most earthquakes occur.
Volcanoes: At ridges in the ocean, approximately 20 cubic meters of magma is produced each year, which eventually solidifies and form new oceanic plate. Most other areas of volcanic activity are along plate boundaries and where plates are destroyed, this means that most earthquakes and volcanoes are caused by plate movements.
When earthquakes occur, they produce shock waves or vibrations, these waves produced are called seismic waves. There are three types of seismic wave L, P and S waves, because they travel differently through the earth it allows us to deduce the structure of the earth.
Earthquake-surface waves: An earthquake is caused when two plates slide violently past each other. Minor ones happen most days, but major quakes, which result in a large loss of life, are rare.
The origin of an earthquake is the focus, this is where it starts, and above this point on the surface of the earth is the epicenter.
From the focus of the earthquake, three types of seismic wave are produced. L waves (love waves) move outwards from the epicenter as surface waves. They travel more slowly than the other waves and cause the damage to buildings.
Earthquake-body waves: Earthquakes also produce longitudinal P waves (primary or pressure waves) and transverse S waves (secondary or shake waves). These waves travel through the body of the earth, and because of their different properties, we can determine what is inside the earth. P waves are the faster moving body wave and travel through the whole earth, whereas S waves are slower moving and cannot travel through liquids. As these waves are transmitted through the earth, they are partially reflected at the boundaries between different materials and, also are partially refracted as the density of the materials changes. The waves are then detected on the earth’s surface by seismometers, which are machines, which measure seismic waves.
In some other areas, only P waves are detected. A shadow is cast by the absence of the S waves; this means that the outer core must be liquid. The size of the shadow means that the size of the outer core is about 7000km in diameter. Earthquakes also cause the earth to reverberate for a period of time after the earthquake, the reverberations we can deduce that inside the liquid outer core is a solid inner core about 2400km in diameter.
Seismometers: A seismometer consists of a solid base, which should be secured to a rock or the ground. A pen on the base of a heavy mass is suspended above a rotating drum and on which graph paper is attached. As the earth moves in an earthquake, the heavy mass stays still and a jagged trace is drawn on the paper. The different traces produced are analyzed and deductions about the earths structure can be made, the velocities of the waves can be worked out by looking at the time delay between the earthquake and when waves are received.
- OCR A Physics separate award
- GCSE Physics T.Ducan
- OCR a new introduction to Geography (GCSE)
- GCSE Geography Revision notes
- The Hutchinson Encyclopedia
- The internet
- Mr. Cocks’s vast seismological Knowledge!
