Tectonic Plates and Their Properties

Tectonic Plates and Their Properties

1. Constructive Plate Boundaries

   • Oceanic Constructive Boundary
     • E.G THE MID-ATLANTIC RIDGE
     • North American and Eurasian plates
     • Plates are moving away from each other at a rate of 2.5cm per year
     • Shield volcanoes form because the basaltic magma has low viscosity and is hot so it flows long distances before solidifying so volcanoes have shallow sides + wide base
     • Small magnitude earthquakes occur on Mid-Atlantic Ridge because the magma rises and puts pressure on rocks causing them to vibrate
     • A fissure eruption is an eruption that occurs along fault and fracture lines, long, thin openings in the earth's crust are known as fissures
     • Landforms found on the Mid-Atlantic Ridge are: Ocean ridge, shield volcanoes, volcanic islands
   • Continental Constructive Plate Boundaries
     • E.G. EAST AFRICAN RIFT VALLEY (EARV)
     • The Somalian, Nubian and Arabian plates are involved in the EARV
     • Horsts are the raised up areas of land. A graben is the dropped down bit in the centre because it has fault lines at either side of it
     • Rift lakes are formed as the graben stretches and a dip forms on the surface. Rain water collects in this to make a lake e.g. Lake Tanganuka
       • Check Lake Tanganuka -  it may be called something different
     • Volcanoes occur on the edges of the rift valley because the magma rises to the surface through fault lines. Where magma rises through thick continental crust it is andesitic (viscous) giving composite volcanoes
     • It is expected that Africa will split in two and a new ocean will be created
     • Lanforms found on the East African Rift Valley are hursts, grabens, lakes and composite volcanoes

2. Destructive Plate Boundary

   • Two plates become stuck together, this causes the pressure to build up, when this pressure is released an earthquake occurs
   • Increased volume of magma creates increased pressure and magma is forced to rise through the crust through cracks
   • Destructive margins occur where 2 plates move towards each other due to descending limbs of convection cell currents in the mantle beneath the lithosphere. Where oceanic crust and continental crust meet ata a convergent boundary a subduction zone results as the denser oceanic lithosphere is forced below continental lithosphere
   • E.G. INDONESIAN SUBDUCTION ZONE
   • Composite volcanoes also known as stratovolcanoes form at a subduction zone like Merapi on the Indonesian island of Java which is a stratovolcano with an active lava dome.
   • Lava generated by wet partial melting and so is andesitic with +50% silica content and a lower temp of 800C which makes it thicker and more viscous. So lava flows slowly and cools quickly giving volcano cones with narrow base and greater height
   • Sometimes stratovolcanoes have secondary or parasitic cones on their sides. These form when the passage of rising magma through the main vent is blocked.
   • Volcanoes have alternating layers of lava and ash. Ash is produced by highly explosive eruptions often after blocking of the vent, fragmenting parts of the cone
   • Indonesia contains over 130 active volcanoes more than any other country on earth. They form part of the Indonesian island arc system which is generated by north-eastward subduction of the Indo-Australian plate beneath the Burma and Sunda plates
   • Collision Zone on a Destructive Plate
     • e.g Himalayan Collision Zone
     • Convergence of 2 continental crust is known as a collision margin. This collision causes ground displacement which over time forms fold mountains
     • No subduction occurs as both plates are low in density. Instead, oceanic sediments trapped beneath the 2 converging plates are heaved upwards, resulting in uplift of major fold mountain ranges. Process of fold mountain building is known as ORGENESIS
     • Indo-Australian plate is moving northwards to collide with the Eurasian plate. Prior to their collision the 2 continental land masses seperated by the remainders of the Tethys Sea. Sedimentary rocks created beneath the Tethys Seas were thrust upwards to create the young fold mountain chain of the Himalayas
     • Mount Everest reaching a height of 9,000m and continuing to rise more than 1cm a year..
     • The enormous pressures involved in orogenesis fold continental rock into called anticlines and downfolds called synclines
     • No volcanic activity but deep focus earthquakes are common along with the Trans-Himalayan Batholith

3. Conservative Plate Boundaries

   • e.g. SAN ANDREAS FAULT, CALIFORNIA
   • Occur when 2 pieces of tectonic plate slide past each other (lateral crust movement) they are known as strike slip faults
   • No lithosphere is being being destroyed or created = no volcanoes
   • Earthquakes tend to be frequent, shallow focus (0-70km deep) and can be high in magnitude but usualy lower in mag. than destructive plate boundaries
   • As the plates are sliding past each other they can get stuck together. This causes pressure to build up as they continue to try and move past each other. When this pressure is released it causes shock waves to travel through the earth as an earthquake.
   • Few landforms associated with conservative plate boundaries but the most obvious is the presence of a fault line on the Earth's surface as seen in California