Plate Tectonics Evidence ONACD –Editable Curriculum designed for teachers by teachers

Seismology Part 2 – Surface Waves There are two more types of waves that are released during an earthquake. These two waveforms travel along the surface of the earth (and thus cause the most damage to humans). Love Waves - In 1911, A.E.H. Love mathematically predicted the existence of waves that travel horizontally to the direction of motion. - Love waves are the typical sudden “jerking” waves people think of when they imagine or experience an earthquake. Love waves travel slower than both P-waves and S-waves. There are reports of animals acting strangely just before an earthquake is felt. Perhaps the animals are more sensitive and sense the P or S waves before the Love waves arrive? motion

Rayleigh Waves - In 1885, Lord Rayleigh, predicted the presences of this type of wave. -Rayleigh waves are the “rolling” waves people think of when they imagine or experience an earthquake. - Rayleigh waves are the slowest waveform (but still 10 times faster than sound) and are formed by the interaction of P and S waves with each other.

Plate Boundary Interactions As the lithospheric plates move their boundaries interact with each other in one of three ways: 1. Divergent Boundaries( ) – Two plates moving away from each other. Divergent boundaries are usually found in oceans (they can form on continents, but water flows in as the land pulls apart) and forms ridges.

Divergent boundaries and notes The typical example of a divergent boundary is the mid-Atlantic ridge. As the plates spread apart a long (“crack”) forms and magma is able to flow to the surface. This magma forms new oceanic crust, volcanoes (and islands like Iceland) and shallow-focus earthquakes. These divergent boundaries are not as common on the thicker continental plates. But if a divergent boundary does occur the gap is called a “rift valley”. Note: Symbols of Plate Boundaries: You should become familiar with the symbols which are used to represent the indicated information on a plate tectonics map.

Convergent 2. Convergent Boundaries ( ) – Two plates move towards each other. At the area of collision, one plate is goes down and the other is pushed up. This is dependent on density and thickness of the plates. The collision forms a long trench which can be 8 to 10 km deep. Convergence can occur in one of three combinations: An ocean plate and a continental plate Between two ocean plates Between two continental plates

Convergence: Ocean and continental Recall that oceanic plates are less dense than continental plates. As the oceanic plate is subducted under the continent the collision forms mountains and volcanoes. The mountains form as the continental plate is compressed and forced upward. The volcanoes form as the ocean crust melts into magma and as the Asthenosphere is Punctured. The high pressures are then able to break through the week spots in the crust above.

Convergence: Two Ocean Plates The Marianas Trench occurs between the Pacific plate and the Philippine plate and is 11km deep at a location called “Challenger Deep”. This is deeper than Mt Everest is tall! Volcanoes also form along the upper plate and parallel to the trench much like they do from ocean–continental collisions. When large enough, these volcanoes become islands.  Earthquakes are also frequent and powerfull as they are closer to the surface of the thinner oceanic crust.

Convergence: Two Continental Continental plates are relatively buoyant and tend to crumple and lift against each other instead of being subducted. This results in extreme plateaus and mountains.  The collisions between the Eurasian plate and the Indian plate form the world’s highest continental mountains called the Himalayas. The Tibetan plateau, formed next to the Himalayas, is higher than most of the mountains in Europe or North America. The depth of subducted plates can cause earthquakes to occur deep (300 to 700 km) within the earth. Earthquakes with a focus at these depths are called “deep-focus” earthquakes.

Transform Boundaries ( ) – Two plates move beside each other. Transform boundaries often form near ridges in the ocean. However, the most recognized transform boundary occurs along the west side of the North American plate. Notice: A “zig-zag” pattern forms as the two plates slide past each other.

Consequences of Plate Movement Look at the cross section below which shows the formation of a ridge like the mid-Atlantic ridge. The material being forced out at the ridge is distributed both to the left and the right simultaneously.  The diagram above reveals two factors that contribute to plate movement: Ridge “push” – the new Material added to the plate from the ridge increases the weight of the plate. The force of gravity will cause it to “slide” downhill. Slab “pull” – as a plate is subducted the convection currents help draw the plate in. Much like pulling on a roll of toilet paper.

Hot Spots There are many locations on the earth that appear to have continual volcanic or intense geothermic activity, but are not part of a plate boundary. These locations are called hot spots.  It appears that the source of the hot spot is dependent on the mantle. The hot spots may be a result of strong convection currents at one location, or they may be the results of magma plumes (like the blobs you see rise in a “lava” lamp).

Hot Spots As the plates drift over top the hot spot a “chain” of volcanoes are made. Hawaii is one of the best known hot spot chains. 1. Look at the diagram of the Hawaiian hot spot chain and answer the questions: a) Which island on the diagram is the oldest? Answer – Kaui as it is furthest away  b) Which island on the diagram is the youngest? Answer – Hawaii as it is still above the hot spot  c) If the hot spot is fixed in location, how are the islands spread apart? Answer – The Pacific Plate is moving.