2. Tectonic Theory

Describe the core of the Earth?

• Hottest part of the Earth with temperatures reaching 5000℃

• Made up of the inner core (solid due to pressure and made up of iron-nickel alloy) and the outer core (semi-liquid and mainly iron)

• Source of heat is the mostly natural radioactive decay of isotopes like uranium and other elements. However, some of the heat energy was primordial (heat left other from the Earth’s formation)

• Heat from the core creates convection currents within the mantle which spread through the asthenosphere and may be partly responsible for movement of tectonic plates

What parts of the Earth does the mantle contain compared to the lithosphere?

Mantle - Mesosphere, asthenosphere and upper most solid mantle

Lithosphere - crust and upper-most solid mantle

Describe the composition of the mantle?

• Made of molten and semi-molten rocks containing lighter elements like silicon and oxygen. The rocks become denser with depth due to heat and pressure

• 2900km thick

Describe the asthenosphere?

• This is what the plates float on

• Under the lithosphere and is semi-molten

Describe the lithosphere?

• Crust and rigid/solid upper section of the mantle

• Approximately 80-90km thick

• Divided into 7 large plates and some smaller plates

Describe the oceanic crust?

• 6-10km thick

• Less than 200million years old

• 3.0 density (heavier than continental)

• Layer of basaltic rock called SIMA (silicon, magnesium, oxygen) but mainly basalt

Describe the continental crust?

• 30-70km thick

• Over 1,500 million years old

• 2.6 density (lighter than oceanic)

• Layer of granite rocks known as SIAL (silicon, aluminium and oxygen) but mainly granite

Explain the overall ideas by Bacon and Wegener?

• Bacon noticed in the 17th Century that the continents either side of the Atlantic Ocean fit together well - the bulge of South America fitting into the indent below West Africa

• In 1912, Alfred Wegener published the theory that there used to be a single continent 300 million years ago. He called this the super-continent called Pangaea

• He said Pangaea later split into 2 more continents called Laurasia and Gondwanaland and today’s continents where formed by these 2 splitting more

• This theory is called continental drift

What is geological evidence for the theory of continental drift?

Geological

• The bulge of South America fitting into the indent below West Africa

• Evidence of late-Carboniferous glaciation deposits (290million yrs ago) found in South America, Antarctica and India which can’t be explained → continents must have been formed together than moved. Also, striations on rocks in Brazil + West Africa supporting this

• Rock sequences in Northern Scotland as similar to Eastern Canada indicating they were formed in the same conditions in 1 location

What is biological evidence for the theory of continental drift?

Biological

• Fossil remains of the reptile Mesosaurus were found in South America and Southern Africa. It is unlikely the reptile could have developed in both areas or migrated across the Atlantic

• Fossil brachiopods found in Indian limestone are comparable with similar ones in Australia

What did Wegener’s theory lack and how did Harry Hess make up for it?

• Wegener’s theory didn’t explain how the plates moved but Hess’s theory did through the idea of sea-floor spreading

• The discovery of the mid-Atlantic Ridge suggested the sea floor was spreading

• Evidence for this came from Palaeomagnetism, where symmetrical magnetic stripes were found on either side of mid-ocean ridges aligned with the North and South Poles.

• The rocks are formed when iron particles in lava erupt on the ocean floor and align with the Earth’s magnetic field → the rocks then solidify making the rock

• The Earth’s polarity reverses every 400,000 years making the striped pattern

• These magnetic patterns show that new crust forms at the ridge and moves away over time.

What did sea-floor spreading imply and suggest about oceanic crust?

• Suggests the oceanic crust gets older with distance from the mid-oceanic ridge

• Implies the Earth is getting bigger → evidence of huge oceanic trenches linked to the idea of subduction where the ocean floor was being pulled down by hot spots which generate thermal convectional currents causing magma to rise up and spread before cooling and sinking with the plates floating on top

Name the 7 main tectonic plates?

Pacific Plate, North American Plate, Eurasian Plate, African Plate, Antarctic Plate, Australian Plate, and South American Plate

Explain an alternative idea to convection currents making plates move at constructive boundaries?

Ridge push/gravitational sliding

• Upwelling of hot material at ocean ridges creates a buoyancy effect making an ocean ridge 2-3km above the ocean floor

• Ocean plates experience the force ridge push which acts down the slope of the plate pulling the plates away

• The occurrence of shallow earthquakes result from the repeated tearing apart of the newly formed crust. This indicates that there is a frictional resistance to this force

Explain an alternative idea for making plates move at destructive boundaries?

Slab pull

• The downward gravity force acting on the oceanic plate as it subducts is as a result of the negative buoyancy of the plate

• The pushing of the subducting plate against the overriding plate creates frictional resistance → shallow and deep earthquakes

Evaluate slab pull and ridge push as an explanation for tectonic movements?

Each plate moves at their own rate so forces vary plate to plate → unlikely 1 single agent is solely driving a plate’s movement → must be a combination of forces