2. Crust Deformations Cracks in rock formations are due to cooling of molten rocks and mechanical stresses. Motions occur along fracture lines called faults . Faults form in rocks when the stresses overcome the internal strength of the rock resulting in a fracture. These motions usually take place as a series of small displacements with intervals of years to centuries. Immediate motions create cliffs ( fault scrapes ), which can be leveled off by erosion before the next motion. The Earth’s crust looks firm, but it is in a constant motion. Strong forces act in the crust to move the rocks around. Such forces and the changes they cause are called tectonic (Greek for “carpenter”)

3. Crust Deformations Slow, continuous, motion produces rocks folding . Folding may produce hills and depressions directly, but erosion usually erases them quickly. Indirectly folds produce parallel ridges and valleys, which are resistant to the action of water streams. Large-scale motions in the crust may involve the entire continents or their large parts (coastal features).

5. Continental Drift The continental coastal lines suggest that the continents may have been joined together in the past. The first detailed theory of continental drift, based on biological and geologic evidence, was proposed by Alfred Wegener in 1912. Wegener tried to explain the parallel evolution of animals and plants in many now-distant regions. Only during the last 200 million years living organisms in different continents developed in different ways. Wegener suggested that the continents were once part of one large landmass called Pangaea .

6. Plate Tectonics

7. Plate Tectonics The Earth’s lithosphere (the crust + the uppermost part of the mantle) is broken into a dozen plates . The lithosphere gradually turns into the softer asthenosphere (~100 km thick). Most major earthquakes and volcanic eruption occur along plate boundaries. Plate tectonics carries rock from the mantle, transports it across the seafloor, and returns it back. New crust is spread through mid-ocean ridges. The old crust is returned back through trenches ( subduction ).

8. Tectonic Plate Collisions Ocean floors, mapped only recently, consist of much younger (~200 million years) rocks than continents (up to 3.8 billion years). It was found that ocean floor is continuously spreading by rising molten rock along mid-ocean ridges. However, the Earth does not expand. Therefore, the ocean floor spread should be balanced by other large-scale processes in the lithosphere, which is split along the ridges, trenches, and fracture zones. Tectonic plates move apart, creating new lithosphere, and collide, destroying the old one.

9. Plate Tectonics

10. Moving Plates The plates move with speeds of a few centimeters per year. The continents merged together ~200 million years ago and formed a single continent Pangaea ( Historical perspective ). Volcanoes usually exist near plate boundaries. Many islands emerged at places of subduction.

11. Summary The Earth’s crust looks firm, but is being deformed by strong forces over long periods of time. The crust is broken in a dozen tectonic plates that are constantly, but very slowly, moving causing the continental drift. Plate tectonics, a unique phenomenon among the planets of the Solar system, is responsible for the changing appearance of the Earth’s surface.