1. Plate Tectonics: Continental Drift. a relatively new scientific concept introduced some 40 years ago. revolutionized our understanding of the dynamic planet upon which we live. drawing together many branches of the sciences, from palaeontology (the study of fossils-biology), to seismology (the study of earthquakes- physics) and petrology (the study of rocks-chemistry). provided explanations to questions that scientists had speculated upon for centuries- why earthquakes and volcanic eruptions occur in very specific areas around the world? how and why great mountain ranges like the Alps and Himalayas formed?

2. Plate Tectonics: Continental Drift. Evidence for continental drift was firstly compiled by Alfred Wegener. Realised that the edges of continents separated by thousands of kilometres of ocean have: Shapes that fit very closely e.g. Africa and South America. Similar patterns of rocks. Similar patterns of fossils.

3. Plate Tectonics: Continental Drift. Such observations suggested that the continents were once part of a single landmass (Pangaea) which has since moved apart. Consequently, the Earth is ‘cracked’ into a number of large pieces, termed tectonic plates, that move across the Earth’s surface. Satellite monitoring has shown they move at speeds of a few cm per year.

4. Plate Tectonics: Continental Drift. Two fatal weakness existed in Wegener's theory: It could not answer what kind of forces could be strong enough to move such large masses of solid rock over such great distances. Wegener suggested that the continents simply ploughed through the ocean floor, but it is physically impossible for a large mass of solid rock to plough through the ocean floor! Consequently, his ideas were dismissed for some 50 years until scientists Started to explore the oceans and discovered the process of: SEA FLOOR SPREADING.

5. Sea Floor Spreading: The Evidence. Measurements of temperature variation at the Earth’s surface shows that heat is escaping from the hotter core (4500 o C). Evidence from various sources tells us that heat is escaping from the Earth’s centre: Volcanoes Geysers/Hot Springs Black Smokers.

6. Sea Floor Spreading: The Causes. Heat is produced inside the Earth primarily due to radioactive decay. This heat escapes from the core through the mantle to the base of the lithosphere by convection (solids can flow when hot!). These convection currents drag the overlying lithospheric plates apart- a process termed seafloor spreading.

7. Tectonic Plates. The brittle solid lithosphere, which is dragged across the Earth’s surface by the convection currents in the mantle, is split into a number of tectonic plates. These plates can be distinguished because earthquakes and volcanoes occur at the boundaries between these tectonic plates.

8. Types Of Tectonic Plate Boundaries. Three main types of tectonics plate boundary exist: Conservative (e.g. San Andreas). Destructive (e.g. Andes) Constructive (e.g. mid-Atlantic Ridge). Conservative Destructive Constructive

9. Conservative Tectonic Plate Boundaries. http://www.crustal.ucsb.edu/ics/understanding/elastic/rebound.html Tectonic plates slide past each other e.g. San Andreas Fault. Tectonic plates are neither destroyed nor created. Earthquakes are generated by the sudden movement of the tectonic plates.

10. Destructive Tectonic Plate Boundaries. Occurs when two tectonic plates move towards each other e.g. Andes. The thinner and denser oceanic plate is forced under (subducted) beneath the thicker less dense continental plate. Due to the increased temperature with depth, the oceanic plate partially melts. The magma may rise to form volcanoes. The Earth’s mountain ranges, originally thought to be caused by shrinkage of the crust as the Earth cooled, are now known to formed by compression as the plates collide, causing folding, faulting and metamorphism.

11. Constructive Tectonic Plate Boundaries. Occurs when two tectonic plates move away from each other e.g. mid-Atlantic Ridge (Iceland). Convection currents in the mantle rise and move laterally dragging the overlying lithospheric plate. This causes fractures to form which fills with magma generating new oceanic (basalt) crust http://lamar.colostate.edu/~gnelson/nazli.html

12. Constructive Tectonic Plate Boundaries. The seafloor is formed when lava, which contains iron-bearing minerals, is erupted from volcanoes and solidifies to form basalt at oceanic ridges, e.g. mid-Atlantic Ridge. The evidence for seafloor spreading comes from ‘magnetic stripes’ preserved in these basaltic lavas. These ‘stripes’ are formed when the iron minerals in the magma line up like iron filings in the direction of the prevailing Earth’s magnetic field as the lava solidifies (much like a giant tape recorder!). Occasionally the Earth’s magnetic field ‘flips’ poles, and these reversals are recorded as ‘stripes’ parallel and symmetric to the oceanic ridges. http://lamar.colostate.edu/~gnelson/nazli.html

13. Predicting Earthquakes & Volcanoes. It is possible to forecast which areas are most at risk to earthquake hazards. However, predictions using techniques such as foreshocks, change in ground elevation and strain, radon gas seepage and even animal behaviour have yielded little success because of the inconsistent nature of earthquakes. It is also possible to forecast which areas are most at risk to volcanic hazards e.g. lava and pyroclastic flows. Although predictions using techniques such as foreshocks, change in ground elevation and strain, sulphur dioxide gas seepage and ground resistivity have enabled evacuations to be organised, the inconsistent nature of volcanoes prevent the exact timing of eruptions to be predicted.

14. Conclusions. The Earth is a dynamic planet. The surface of the Earth is comprised of a number of thin tectonic plates which move at speeds of a few centimetres a year. Plate tectonic boundaries are delineated by the positions of earthquakes and volcanoes. Plate tectonic boundaries come in three flavours: Conservative (crust neither destroyed nor created). Destructive (crust destroyed). Constructive (crust created).