Plate movement

In 1912 Alfred Wegener published his theory that a single continent existed about 300 million years ago. It was called Pangaea, it then split into Laurasia and Gondwanaland in the south.

Geological evidence for the theory

• the puzzle fit of south america and africa
• evidence of the last glaciation found in south america, antarctica and india.
• rock sequences in northern scotland closely agreed with canada.

Biological evidence for the theory

• fossil brachiopods found in indian limestone comparable to fossils in australia
• fossil remains of the reptile Mesosaurus found in south america and southern africa

Beyond the 1940 the mid atlantic ridge was discovered and explored. Examination suggested the sea floor was spreading. Iron particles erupted on the ocean floor and were aligned with the earths magnetic field, as it solidifes it provides evidence of the earths polarity at the time of eruption (paleomagnetism)

However it reverses every so often and creates magnetic stripes with rocks aligned towards north and south pole.

They then found ocean trenches where ocean floor was being pulled downwards.

• Core: dense rocks containing iron and nickel alloys and is divided into a soid inner core and molten outer core
• the mantle is made up of molten and semi molten rocks containing lighter elements such as silicon and oxygen
• the crust is even lighter because it is made up of silicon, oxygen, aluminium, potassium and sodium
• Lithosphere: crust and upper section of mantle and divided into 7 large plates and a number of smaller ones, these are divided into oceanic and continental
• Hot spots within the core of the earth generate thermal convection currents within the aesthenosphere which causes magma to rise towards the crust and then spread before cooling and sinking

Where plates move apart in oceanic areas they produce mid oceanic ridges. where they move apart in continental crust they create rift valleys. The space between is filled with basaltic lava.

Oceanic ridges

• total length of 60,000 km
• volcanic activity occurs along the ridge forming submarine volcanoes- surtsey iceland

Rift valleys

at constructuve margins in continental areas such as east africa, the brittle crust fractures as sections of it move apart. Areas of crust drop down between parallel faults to produce rift valleys.

Oceanic/ continental convergence

The denser oceanic plate is forced beneath the continental, known as subduction. The nazca plate is subducting underneath the south american plate forming the peru chile trench.

Sediments and the edge of the plate are uplifted to form fold mountains such as the andes along the pacific side of south america.The increase in pressure of these can trigger major earthquakes.

The Benioff zone- where the friction heats up the rock, begins to rise and plutons of magma reach the surface making volcanoes.

Oceanic/ oceanic convergence

Ocean trenches and island arcs are the features of this.

Continental/ continental convergence

There is not much subduction when they meet so their edges and sediments are forced up into fold mountatins. There is little volcanic activity but there are shallow focus earthquakes. Best example is the indo australian plate being forced into the eurasian plate.

Where two crustal plates slide past each other. The san andreas fault in california where the pacific and north american plates move parallel to each other.

A concentration of radioactive elements inside the mantle may cause hot spots to develop. A plume of magma eats into the plate above, where the lava breaks through, active volcanoes occur. The hot spot is stationary so as the pacific plate moves, a line of volcanoes is created. Hawaii is the best example.

• along ocean ridges where plates are moving apart - mid atlantic ridge
• associated with rift valleys- african rift valley
• on or near subduction zones- ring of fire in pacific ocean
• over hot spots such as hawaiin islands

When magma is forced to the urface, only a small amount actually reaches that level. Most of the magma is intruded into the crust where it solidifies into a range of features. These are often exposed by erosion.

Batholiths are formed deep below the surface when large masses of magma cool and solidify. They are often dome shaped and can be several hudred metres in diameter. If small injections of magma force the strata to arch upwards it forms a laccolith.

Dykes are vertical intrusions with horizontal cooling cracks. they cut across the bedding plane of the rocks in which it intruded, sills are horizontal intrusions which have vertical cooling cracks.

• lava plateau
• shield volcano
• dome volcanoes
• ash and cinder cones
• composite cone
• calderas

• solfatora- small volcanic areas without cones produced by gases escaping to the surface
• geysers- occur whe water heated by volcanic activity explodes onto the surface
• Hot springs/ boiling mud- water does not explode onto the surface, when the water mixes with surface deposits it makes boiling mud

• tephra
• pyroclastic flows
• lava flows
• volcanic gases
• lahars
• flooding
• tsunamis
• volcanic landslides
• climatic change

When pressure builds up within the rocks, it is suddenly released and parts of the surface produce an intense shaking motion that lasts for a few seconds. The point at which this is released occurs is known as the focus, and the point immediately above that is the epicentre. The depth of the focus can be shallow, intermediate or deep.
There are 3 types of seismic waves:
Primary: travel fast and are compressional, vibrating only in the direction they're traveling in
Secondary: half the speed of P waves and shear rock by vibrating at right angles to the direction of travel
Surface L waves: slowest and nearer to ground surface. Some surface waves shake the ground at right angles to the direction of wave movement and some have a rolling movement.

The vast majority occur along plate boundaries, the most powerful at destructive margins. Some are associated with the deactivation of old fault lines

Magnitude is measured on two scales:
The Richter scale is a logarithmic scale and an event measured at 7 has an amplitude of seismic waves ten times greater than one measured 6 on the scale. The energy release is proportional to the magnitude.
The mercalli scale measures the intensity of an earthquake and its impacts.

Secondary affects:
Soil liquefaction
Landslides or avalanches
Collapsing buildings, destruction of road systems and communication as well as infrastructure, flooding, food shortages

The effects of a tsunami will depend on:
The height of the wave and distance it has travelled
The length of the event that caused the tsunami
The extent to which warnings can be given
Coastal geography
Coastal land use and population density