Plate Tectonics This Dynamic Earth pubsgs/gip/dynamic/dynamic.html

1. Plate TectonicsThis Dynamic Earthhttp://pubs.usgs.gov/gip/dynamic/dynamic.html and the distribution of major landform features

2. I. The discovery of Plate Tectonics –Continental Drift • 17-18th centuries: outlines of the continents similarity of the shape of the coast lines of the Atlantic Ocean • 19th century: geology of southern continents: Africa, S. America, Australia, India “super-continent – Gonwanaland”

3. I. The discovery of Plate Tectonics –Continental Drift • 1915 Alfred Wegener (German scientist) All the continents were jointed together “Pangaea” The single ocean “Panthalassa Ocean” - For reasons yet unclear: 200 m.y.a. Cracks began to divide Pangaea into 2 supercontinents, north: Laurasia south: Gondwanaland They then began to be broken apart by additional cracks, finally the continents today

4. I. The discovery of Plate Tectonics –Continental Drift

5. I. The discovery of Plate Tectonics –Continental Drift • Many geographers and geologists did not support Wegener’s theory at that point • as many points not adequately explained • The idea of continental drift took on new meaning in the 1960s and 1970s • when geophysical evidence from palaeomagnetic and ocean floor studies supports the theory. • The idea was combined in plate tectonics, the leading theory now.

6. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 1. Ancient Ice Sheet • 2. Fossils • 3. Sea-floor Spreading • 4. Palaeomagnetism

7. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 1. Ancient Ice Sheet(discovered in late 19th C) • Evidence suggesting that an ice age existed about 200 million years age (~ same age) in 3 continents in the northern hemisphere – South America, Australia and Africa. Rock outcrops with a smooth appearance and a series of parallel grooves, probably made by a huge mass of ice of about thousands metres thick moving very slowly over solid rock. Glacial deposits can also be identified, looking quite different from fluvial deposits by rivers and streams. What does the direction of the glacial grooves tell us?

8. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 2. Fossils • Presence of identical fossil plants on different continents separated by hundreds of kilometres of ocean. • Similar rock layers also occur on different continents Could it be the strongest kind of evidence to support the theory?

9. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 3. Sea-floor Spreading • The discovery of a mid-oceanic ridge system some 40,000 km long extending through the major ocean basins • Observations: rate of sea-floor spreading ~ 2 – 18 cm / yr • New oceanic crust is continuously being formed as the molten materials rises to fill the cracks, solidifies and forces it way out in opposite direction from the ridge • Could it be the strongest kind of evidence to support the theory?

10. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 3. Sea-floor Spreading • Evidence of sea-floor spreading? • Fossils showing the age of rock to increase from the mid-oceanic ridges outwards • Increasing thickness of marine sediment away from the mid-oceanic ridges, absence of sediment at the ridge crest • Arrangement of alternate reversal polarity pattern found at either side of the mid-oceanic ridges – almost symmetrical in rock bands • Alignment of volcanoes of different ages from hot spots

11. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 3. Sea-floor Spreading • The concept is defended successfully only if: a. the earth is expanding (vs: + 2% in past 200 m.y.) b. other surface material is somewhere returning to the earth’s interior (vs: major bending only evident within some mt. belts and ocean) - How can we account for all of the newly formed crust?

12. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 3. Sea-floor Spreading • How can we account for all of the newly formed crust? • Some crustal materials is being “consumed” by downward currents at the deep oceanic trenches • At the oceanic trenches, the older crust sinks towards the mantle, resulting in earthquakes and volcanic activity. • What was the mechanism of spreading?

13. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 3. Sea-floor Spreading • What was the mechanism of spreading? • It seems probably that there is a convective movement of material within the mantle. • A series of convective cells, driven by internal energy produced by decay of radioactive elements appears to exist within the asthenosphere. • So, the asthenosphere behaves in a plastic or semi-fluid manner over a long period of time.

14. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 4. Palaeomagnetism • The several reversal of the earth’s magnetic field in the North and South Poles • When molten rocks spread away from the central ridges, rocks with metallic materials would adopt a magnetic orientation reflecting that of the earth’s magnetic fields.

15. I. The discovery of Plate Tectonics –Continental Drift • Evidence • 4. Palaeomagnetism • The transect across the ocean floor suggests that: a. palaeomagnetism of rocks varies as if the ocean floor had been formed at different times. b. The pattern on either side of the mid-oceanic ridge: almost identical!

16. II. The Theory of Plate Tectonics • Introduction • Tectonics means the study of tectonic activity. • The study of deformation (e.g. breaking & bending) of earth materials and the structures that result from deformation. • It is the study of geological activity on a global scale.

17. II. The Theory of Plate Tectonics • History - 4 major scientific developments • demonstration of the ruggedness and youth of the ocean floor • confirmation of repeated reversals of the Earth magnetic field in the geologic past • emergence of the seafloor-spreading hypothesis and associated recycling of oceanic crust • precise documentation pointing to the concentration of the world's earthquake and volcanic activity along oceanic trenches and submarine mountain ranges Further reading: http://pubs.usgs.gov/gip/dynamic/developing.html

18. II. The Theory of Plate Tectonics • The development • Plate tectonoics is a scientific concept developed in the 1960s to explain the pattern of the earth’s structural components and the mechanisms by which they were formed. • The lithosphere is broken into segments called “plates”. They float upon the soft and plastic asthenosphere below.

19. II. The Theory of Plate Tectonics • Basic assumptions • 1. Large areas of the earth’s outer portion act as a rigid caps on a sphere. These rigid plates undergo no significant internal deformation. 2. Each plate is in relative motion with respect to the other plates on the surface deformation only at their boundaries with each other. [except for surface geologic processes, most geologic activity is concentrated along the plate boundaries.]

20. II. The Theory of Plate Tectonics • Plate movement • Due to convection currents generated by heat from the centre of the earth • Plates movement: moving towards, away from or sideways along adjacent plates • It is at plate boundaries that most of the world’s major landforms occur, and where earthquakes, volcanic and mountain zones are located.

21. II. The Theory of Plate Tectonics • Plate movement – the rules • 1. Continental crust does not sink due to its relatively low density • 2. Continental plates may consist of both continental and ocean crust, e.g. Eurasian Plate • 3. Plates do not overlap • 4. No ‘gap’ may occur on the earth’s surface. • 5. New oceanic crust being formed at one place, elsewhere older oceanic crust must be destroyed • 6. Plate movement is slow but is usually continuous. • 7 Most significant landforms are found at plate boundaries. Very little change occurs in the centre of plates.