Internal structure of Earth
Earth’s Structure
Main Features of Plate Tectonics
The Earth's surface is made up of a series of large plates (like pieces of a giant jigsaw puzzle). These plates are in constant motion travelling at a few centimetres per year. The ocean floors are continually moving, spreading from the centre and sinking at the edges. Convection currents beneath the plates move the plates in different directions. The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building. Plate tectonics is a relatively new theory and it wasn't until the 1960's that Geologists, with the help of ocean surveys, began to understand what goes on beneath our feet.
How are Plates Created? As continental crust separates a divergent boundary has been formed, if this is within a continent it is referred to as rifting, and a plume of hot magma rises deep within the mantle pushing the crust upwards, the pressure exerted separates the crust. The mid- Atlantic Ridge is a key example of a divergent plate boundary, as the oceanic plates move apart new plate material forms. The mid-ocean rocks are formed from magma which contained magnetic materials, the reason for this is the composition of the core which is iron and nickel, as the magma solidifies the polarity of the rock will be fixed. The eruptions at the ridge occur and new plate is formed. Detailed research has shown there to be a symmetrical pattern of magnetism either side of the ridge. As the rift valley expands, water is collected forming a sea, and at present the mid-Atlantic Ridge is 2000 metres above the sea floor which is 6000 metres below sea level. Sea floor spreading is witnessed all over the world and creates as much as 17 sq kilometres of new plate every year.
How Plates are Destroyed
Plates are ultimately destroyed by a process known as subduction at convergent plate boundaries, plates move towards each other an example of this is the Nazca plate moving towards the South American Plate and being subducted beneath it. The diagram shows the oceanic plate the lithosphere sinks beneath the continent, an oceanic trench is formed water is brought in which triggers the formation of pools of molten rock, plate moves downwards by a rate of a few centimetres per year. It can take tens of thousands of years for the slow crystallisation of granites rocks or the eruption of lavas, which cool quickly to form basalts.
The collision of two oceanic plate leads to the formation of an island arc, and as the subducting island arc is melted, new magma rises forming volcanoes. It is apparent that the key to subduction is water as it acts as a form of lubricant.
Formation of the Himalayan Mountains
Millions of years ago India was situated on a tectonic plate, this plate was moving northwards by a rate of 10 centimetres per year, the Tethys Ocean was being subducted and when India met the Asian continent approximately 55 million years ago there was no longer any water to lubricate subduction. The welling upwards of the plates caused the formation of the fold mountain range we now refer to as the Himalayas.
The other plate margin example is referred to as a conservative plate margin, plate is neither created nor destroyed as two plates attempt to slide past one another,. A major example of such as boundary is the San Andreas Fault System in California, the strain energy that builds up as a result leads to the occurrence of earthquakes, as friction is overcome.