Download
1 / 16
180 likes | 384 Views
Geology. The earths structure. Earths history. Close examination of a globe results in the observation that most of the continents seem to fit together like a puzzle. In 1912 Alfred Wegner theorised that all the continents had once been joined together.
E N D
Earths history • Close examination of a globe results in the observation that most of the continents seem to fit together like a puzzle. • In 1912 Alfred Wegner theorised that all the continents had once been joined together. • It was not until the 1960’s when new data about the ocean floor revealed oceanic spreading ridges and deep trough-like subduction trenches, that his theory of continental drift was accepted as correct.
These discoveries and more led Harry Hess (1962) and R.Deitz (1961) to publish similar theories based on mantle convection currents, now known as "sea floor spreading".
Continental Drift Continental drift is caused by convection currents causing whole continents to separate over millions of years.
Convection currents • 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. • Convection currents beneath the plates move the plates in different directions. • Convection occurs because the density of a fluid is related to its temperature. Hot rocks lower in the mantle are less dense than their cooler one’s above. The hot rock rises and the cooler rock sinks due to gravity. • The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.
Plate Tectonics • The theory of plate tectonics (meaning "plate structure") was developed in the 1960's. • This theory explains the movement of the Earth's plates (which has since been documented scientifically) and also explains the cause of earthquakes, volcanoes, oceanic trenches, mountain range formation, and other geologic phenomenon.
Plate Tectonics refers to earths heat driven internal engine. • Boiling hot material is pushed up from deep inside the earths mantle. • This material pushes the earths crust upwards and out eventually ripping it apart. • The lightest of these materials accumulate and are not recycled back into the mantle, this is how earths first continents were formed. • This process is still happening today.
Earthquakes • Earthquakes can be huge natural disasters as we saw in Christchurch, but why do they happen? • As we know tectonic plates are always moving. Sometimes they just slide past one another, at other times they actually collide with one another. Plate movement causes the buildup of tremendous quantities of energy in the rock. When this energy is released, it produces vibrations that travel through the rock, leading to earthquakes. These occur at the boundaries of separate plates.
Faults and Folding During earthquakes, faults and folding are produced by the pressure of the moving rock. • Faults occur when the rock fractures and the two pieces move above above or below one another. • Folding occurs when rock bends without fracturing.
Volcanoes • A volcano is a landform (usually a mountain) where molten rock erupts through the surface of the planet. • In simple terms a volcano is a mountain that opens downward to a pool of molten rock (magma) below the surface of the earth. It is a hole in the Earth from which molten rock and gas erupt. • As pressure in the molten rock builds up it needs to escape somewhere. So it forces its way up through narrow cracks in the earths crust. Once the magma erupts through the earth’s surface it’s called lava. Volcanoes are like giant safety valves that release the pressure that builds up inside the Earth. • There are around 1510 'active' volcanoes in the world. We currently know of 80 or more which are under the oceans. Earthquakes, faults, folding and volcanoes all occur at plate boundaries, where the pressure can most easily be released.
P and S waves g P waves move in a compressional motion similar to the motion of a slinky S waves move in a shear motion perpendicular to the direction the wave is travelling.
Seismic waves are divided into two types: Body waves and surface waves. • Body waves include P and S waves, and these are the two types of waves that are used to determine the internal structure of the Earth. (There are other types of seismic waves, but they relate to surface damage and only travel through the Earth’s crust, not the entire Earth.) • These waves travel through the interior of the Earth and can be measured with sensitive detectors called seismographs. Scientists have seismographs set up all over the world to measure and record earthquakes and other changes in the earth.
Subduction • Subduction is a scientific Latin word meaning "carried under." Basically, it happens when one plate meets another and one of them goes down into the mantle. • Subduction happens only to oceanic plates. Continents (continental plates), are too buoyant to be carried much farther than about 100 kilometres deep. So when a continent meets a continent, no subduction occurs. When oceanic plates meet continental plates, the continent always wins and the oceanic plate subducts.
Subduction zones are often noted for their high rates of volcanism, earthquakes, and mountain building.
