Earth science

Earth science

1. Plate tectonics

   • Movement of tectonic plates
     • Evidence:
       • South America and Africa were once joined
         • Common rock types and fossils but not common animals - suggests the split occurred a long time ago
       • Alfred Wegener
         • Associated with the original idea of continental drift - he was the first to provide precise scientific evidence
           • The reaction to Wegener's theory was hostile
             • He had no convincing mechanism for how the continents might move
               • Wegener thought the continents were moving through the earth's crust and that centrifugal and tidal forces were responsible for moving the continents
                 • People who opposed continental drift argued that plowing through oceanic crust would distort continents and centrifugal and tidal forces were too weak to move continents and continued to believe in static continents and land bridges
                 • He believed that about 300 million years ago, the continents formed a single mass called Pangea
                 • He noted a close fit between South America and Africa
                 • By the late 1960s plate tectonics was well supported but we now know that Wegener's theory was wrong on one major point: continents do not plow through the ocean floor
                   • Both continents and the ocean floor form solid plates which "float" on the asthenosphere, the underlying rock
         • In 1911, he found scientific papers listing identical fossils of plants and animals on opposite sides of the Atlantic - sparked an interest in the possibility of continental drift
         • Before Wegener developed his theory, it was thought that mountains formed because the Earth was cooling down, and, as it cooled down, it contracted. This process formed wrinkles, or mountains, on the Earth’s crust
           • If this was the case, then mountains would be spread evenly over the Earth's surface. We know that this is not the case.
     • How plates move
       • Both continents and the ocean floor form solid plates which "float" on the asthenosphere, the underlying rock
         • It is under such tremendous heat and pressure that it behaves as an extremely viscous liquid
         • Asthenosphere = upper mantle
       • Convection currents
         • The heat from the core causes convection currents to develop in the mantle. as these currents circulate, they drag the floating plates with them
         • Driven by the heat produced by the decay of radioactive elements and heat left over from the formation of the Earth.

2. Plate boundaries

   • Constructive plate boundaries
     • 2 plates moving apart and magma rises from the mantle to fill the gap
       • This causes volcanoes but as the magma can escape easily, it does not erupt with much force
       • Earthquakes are also found here
       • The Eurasian plate and the North American plate are moving apart by about 3cm a year
   • Destructive plate boundaries
     • Continental plate meets an oceanic plate
       • The oceanic plate descends under the continental plate because it is denser
         • As the plate descends, it starts to melt due to the friction caused by the movement between the plates
           • The melted plate is now hot, liquid rock: magma - it rises through the gaps in the continental plate
             • If this reaches the surface, it causes a volcano
   • Collisional plate boundaries
     • 2 plates of similar densities moving together
       • Causes the material to buckle and rise up causing fold mountains
         • The Himalayas are an example of a chain of fold mountains - they have been formed by the African and Eurasion plate
   • Conservative plate boundary
     • 2 plates don't collide but slide past each other along a fault
       • Earthquakes occur here
         • An example is the San Andreas fault in California
   • Subduction
     • A subduction zone is the biggest crash scene on Earth. These boundaries mark the collision between two of the planet's tectonic plates.
     • If the plates are moving towards each other, the edges of the plates crumple, and one plate ‘dives’ under the other.
   • At a tensional, constructive or divergent boundary the plates move apart. At a compressional, destructive or convergent boundary the plates move towards each other. At a conservative or transform boundary the plates slide past each other.

3. Earthquakes, volcano eruptions and tsunamis

   • Earthquakes
     • Seismic wave
       • The intensity from the focus point
     • Focus point
       • Where the energy is released from
     • Epicentre
       • directly above the focus point at the surface - most intense
     • An earthquake is the shaking and vibration of the Earth's crust due to movement of the Earth's plates (plate tectonics). Earthquakes can happen along any type of plate boundary
       • Earthquakes occur when tension is released from inside the crust. Plates do not always move smoothly alongside each other and sometimes get stuck. When this happens pressure builds up. When this pressure is eventually released, an earthquake tends to occur
     • Earthquakes are caused by the release of built-up pressure inside the Earth's crust. An earthquake's power is measured on the Richter scale using an instrument called a 'seismometer'.
     • On 17th January 1995, an earthquake struck Kobe, a heavily populated urban area in Japan. It measured 7.4 on the Richter scale and occurred as a result of plate movement along the boundary between the Philippines Plate, Pacific Plate and Eurasian Plate.
   • Volacanoes
     • Magma - volcanic rock underground
       • Magma = melted rock mixed with gas
     • Lava - volcanic rock on the ground
     • Volcanoes form when magma reaches the Earth's surface, causing eruptions of lava and ash. They occur at destructive (compressional) and constructive (tensional) plate boundaries
     • Volcanic eruptions can happen at destructive (plates moving towards each other) and constructive boundaries (plates moving away from each other), but not at conservative boundaries (plates moving past each other).Some volcanoes happen underwater, along the seabed or ocean floor.
     • Formation
       • Magma rises through cracks or weaknesses in the Earth's crust.
         • Pressure builds up inside the Earth. When this pressure is released, eg as a result of plate movement, magma explodes to the surface causing a volcanic eruption.
           • The lava from the eruption cools to form new crust. Over time, after several eruptions, the rock builds up and a volcano forms.
     • Magma chamber - collection of magma insde the Earth, below the volcano
     • Main vent - the main outlet for magma to escape
     • Crater - created after an eruption blows the top of the volcano off
     • Secondeary vents - smaller outlets through which magma escapes
     • Mount St Helens is on the plate boundary between the Juan de Fuca plate and North American plate. When it erupted in 1980, it permanently changed the surrounding landscape.
   • Tsunamis
     • A tsunami is a huge wave, usually caused by volcanic or earthquake activity under the ocean, which can eventually crash onto the shoreline
     • When an earthquake, volcano or landslide happens on the ocean floor, water is displaced. This water forms the start of the tsunami.
       • When the waves reach shallower water:
         • Their height can increase by several metres
         • The shallower water slows the bottom of the wave
         • The waves get closer together
     • It is hard to see that a tsunami is approaching. The most obvious sign is the coastal water retreats just before the waves reach the shore. This is actually the trough of the wave following behind.
     • On 26 December 2004 a tsunami occurred in the Indian Ocean. It was the result of the Indio-Australian Plate subducting below the Eurasian Plate. It was caused by an earthquake measuring more than magnitude 9. The earthquake caused the seafloor to uplift, displacing the seawater above

4. Structure of the earth

   • Geography- structure of the earth (see book)
   • Crust
     • This is layer we live on. It is a thin layer of rock, 0-60 km thick, around the earth.
       • The crust is made up of tectonic plates, which are in constant motion. Earthquakes and volcanoes are most likely to occur at plate boundaries
         • There are two different types of crust: continental crust, which carries land, and oceanic crust, which carries water.
   • Mantle
     • Convection current
       • The heat from the core
     • The mantle is the widest section of the Earth. It has a thickness of approximately 2,900 km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down the rock is soft and beginning to melt
   • Outer core
     • The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel. It is still extremely hot, with temperatures similar to the inner core.
   • Inner core
     • The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C
   • Shockwave data can be used to analyse the Earth's structure
     • Most of what we know about the interior of the Earth comes from the study of seismic waves from earthquakes. Seismic waves from large earthquakes pass throughout the Earth
       • These waves contain vital information about the internal structure of the Earth. As seismic waves pass through the Earth, they are refracted
         • Because the speed of the seismic waves depends on density, we can use the travel-time of seismic waves to map change in density with depth, and show that the Earth is composed of several layers
     • Earthquake's produce 2 types of waves
       • Primary waves
         • Longitudinal, fast moving and will travel through bothe solids and liquids
           • Longitudinal waves = vibrations in the same direction as travel
       • Secondary waves
         • Slower moving tranverse waves that can only travel through solids
           • Tranverse waves = vibrations at right angles to the direction of travel

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