Tectonics Geography A level 2027 spec

Characteristics of different types of plates

    Density

Oceanic crust is more dense than continental. 3kg/m^-1 compared to 2.7kg/m^-1. 

     Age

Continental crust is much older than oceanic. 4bn years compared to 200mn years. 

    Thickness

Continental crust is thicker. 40km thick for continental compared to 10km for oceanic

    Composition

Continental is mostly made of silicon and aluminium, (SIAL).  Oceanic has more silica and magnesium (SIMA). Continental is made of granite whereas oceanic is made of basalt

Evidence for tectonics

Continental drift theory, “jigsaw fit”

“Wegner’s theory of continental drift”. Based on how South Africa and West Africa seemed to fit together.

Geological and fossil records

Appalachian mountains match geological makeup of Caledonian mountains.

Mesosaurus fossils in South Africa and South America despite not being able to swim. 

Glossopteris (fern) is spread throughout many regions despite seeds being to heavy to carry through wind. Like Antarctica, Australia, Africa, South America

Evidence for sea floor spreading

Harry Hess discovered mid-Atlantic ridge. Suggested crust is pulled apart and magma fills the gap. Age of crust is older further away from the ridge. Rocks are dated using radiometric dating using things such as Potassium Argon

The iron in the eruptions aligned itself with the polarity of the earth, which flips. As it is locked in place they could determine symmetrical magnetic stripes were the cause of it being erupted out. 
GPS

Now, GPS from satellites can be used to track movement in real time.

Mechanisms for tectonic plate movement

Convection currents

Radioactive decay and heating from the inner core drives convection cells. This creates friction (basal drag) onto the crust, pulling it forward. a relatively weak force

Slab pull ridge push

Plates are less dense near divergent boundaries because they undergo thermal expansion from the magma. Plates are more dense new destructive boundaries because they are cooler. At destructive boundaries gravity acts on the oceanic plate pulling it down. This drags the rest of the plate along with it. This is Slab Pull. At a constructive (divergent) boundary the newly created less dense rock sits higher than the seafloor. This causes gravity to pull it down and “push” it

Divergent boundrary

When two plates are moving away, lava fills the gap and new land is created

“Decompression melting” occurs because as the plates pull apart the pressure on the asthenosphere decreases. This leads to the melting point to lower and for it to turn into magma without needing excess heat.

Oceanic-oceanic divergence (ridges)

Seafloor spreading occurs as the magma rises cools and then pushes the old crust aside. 

Volcanism occurs in the form of hydrothermal vents

This causes a mid ocean ridge. There are some earthquakes here

Examples include Pacific - Philippine plate and Pacific - North American plate

Continental-continental divergence (rifting)

Examples include great rift valley between African and Somali plate. This can cause fault scarps, which a cliff which has been displaced due to an earthquake

The crust stretches apart causing valleys.

These can fill up with water, such as lake Tanganyika, Red sea called linear lakes

Convergent (subduction)

Convergent plates are when two plates are moving into each other. They produce powerful earthquakes

Oceanic - Continental (subduction)

As the continental plate is less dense than the oceanic the oceanic is forced under the oceanic and subducts. This creates a deep oceanic trench. The Benioff zone is an area with a lot of earthquakes as the descending plate rubs against the overriding plate. As the slab sinks it carries water and sediment. This triggers flux melting that lowers the melting points and leads to the oceanic plate melting.

This causes fold mountains, as the continental crust is crumpled upwards (Andes)

It causes explosive composite volcanoes (Mt Cotopaxi) where South American meets Nazca

Oceanic - oceanic (subduction)

The older (more dense) plate will subduct under the younger one. The magma rises through the ocean floor resulting in volcanic island arcs. There is an accretionary wedge where the sediment build up of the subducted plate.

eg. Aleutians

Convergent (collision)

continental - continental (collision)

There is no subduction here as both plates are buoyant (Indo-Australian and Eurasian)

Accretion - the plates are crumpled upwards (and downwards) to form mountains. Mountain making is called orogeny. Mountains are eroded by wind. Fold mountains are created here aswell. Thrusting and shearing occurs. Fault scarps can occur here

eg. Himalayas

Transform (conservative)

When two plates slide past each other. There is no volcanism here but powerful earthquakes. Lithosphere is conserved

Plates move in the same direction but at different speeds or in opposite directions.

This causes transform faults, such as the San Andreas fault, where the Pacific plate moves at 6cm per year compared to North American at 1cm per year. Major earthquakes along the San Andreas fault. Fault scarps can occur as they are not perfectly straight and may create a pressure ridge.