Early view was that the earth was flat, but it’s generally regarded as being round: actually an oblate spheroid as it is slightly flattened at the poles. (Polar diameter is 0.33% less than equatorial diameter) At its scale, the spherical globe is correct.
Until 1930's, some US Church schools still taught that the earth was flat.
PLATE TECTONICS
General theory was only developed in the last 35 years. First use of phrase was in 1965, when a Canadian geologist called J. Wilson linked together 2 earlier ideas:
Alfred Wegener was one of the main people involved, and had to fight against people who didn’t believe his theory of continental drift: that the continents moved, proposed in 1915.
Developed the idea that at one time all the continents were in one huge mass called Pangaea: (meaning ‘all the land’) surrounded by one large ocean Panthalassa.
Harry Hess: developed the idea of ‘sea floor spreading’: that there were mid-ocean ridges where the crust was cracked and through which material emerged to create new crust. This had to be the case as if continents were subducted in one place and therefore destroyed the earth would be getting smaller.
Structure of the Earth (Tectonics means structure)
Familiar with the concept of crust, mantle and core.
Crust: split up into plates (we’ll come back to that…)
Thin layer usually less than 1% of earth’s radius (equatorial diameter is around 40,000 km) Continental crust is between 40-50km. Deeper beneath mountains, due to concept of isostasy (buoyancy) – largely made of granitic rocks. Mainly silica and aluminium (sial)
Oceanic crust rarely thicker than 6km. It is much more recent, and made of denser material (basaltic lava), mainly silica, iron and magnesium (sima)
The break point between the crust and the layer below is marked by the Mohorovicic Discontinuity
Another division is between the lithosphere and the asthenosphere: separates the crust and rigid uppermost layers of the mantle from the weaker more plastic rocks at a depth of around 100km. Asthenosphere goes down to around 240km. It is hot and relatively fluid: called ductile, rather than the lithosphere which is brittle or rigid. Plates are rigid and move as one unit. Lithosphere is created and destroyed at 2 points on the ocean floor, and is therefore recycled.
Density increases with depth: controlled by temperature and pressure.
Mantle makes up 82% of the volume of the earth.
At a depth of around 2700-2900km, there is the Gutenberg discontinuity, which marks the division between the mantle and the outer core.
How do we know what’s inside the earth ?
Evidence for Continental Drift
Past and present distribution of earthquakes
Changes in the earth’s magnetic field – ‘polar wandering’ is recorded in the rocks of each plate
The jigsaw ‘fit’ of the continents, noticed nearly 400 years ago - there are other matches as well as those between South America and Africa.
Glacial deposits in Brazil match those in W. Africa, as do the routes of ancient mountain ranges
Fossil remains in India match those in Australia
Fossils of Mesosaurus (Palaeozoic creature like a lizard) match between Argentina and West Africa. The creature is not a good swimmer, so unlikely to have crossed the Atlantic.
Similar rock sequences. Mountain belts link up if we reassemble Pangaea.
Glacial remains are found in areas which are warm today, along with deposits linked to glaciers e.g tillite found in S. Africa.
Why did people not believe him ?
Had no mechanism to explain continental drift. Forces he thought of were much too small to have moved continents.
Comparison of Continental and Oceanic Crust
| Examples | Continental | Oceanic |
| Thickness | 35-70km | 6km |
| Age | over 1500 million years | under 200 million years |
| Rocks | Average
density 2.6 Light in colour |
Average
density 3.0 Dark in colour |
| Nature of rocks | Granitic | Basaltic |
Core:
Outer Core: Liquid, mainly iron – movements of liquid are said to be the cause of the earth’s magnetic field
Inner Core: Solid, iron and nickel – denser than outer core, and five times as dense as crust
Outline the evidence for Plate Tectonics
What is a convection current ? a thermal plume ? a craton ?
What happens at a subduction zone ? (Benioff Zone)
How fast do the plates actually move ? What causes them to move ?
How many plates are there ?
PLATES
Pacific
North American
South American
Eurasian
Cocos
Nazca
Caribbean
Antarctic
African
Indo-Australian (or Australian-Indian)
Philippine Plate
Scotia
Juan de Fuca
Anatolian
Arabian
Hellenic
Iranian
Caroline
Gorda plate
