Sciences - Simone Damiano · Sciences Plate tectonics, Earthquakes and Volcanoes ... 1- continental vs continental plate 2- oceanic vs ... The arc is formed from volcanoes which erupt

S c i e n c e sPlate tectonics, Earthquakes and Volcanoes

Key words: lithosphere, continental and oceanic plates, convective movements, plate boundary

The Structure of the Earth

The surface of the Earth is covered by plates which are part of the lithosphere and are about 100 km thick. The oceanic plates are mainly composed of basalt (high density igneous rock) whereas the continental plates are made of granite, which is less dense. The difference between the plates density have an important effect on the results of the plates converging.Inside the mantle there are convective movements (transfer of matter as a result of differences in temperature. Hot material is less dense, so it rises and viceversa).

The Earth’s engine: Convective movements

Heat is constantly dispersed from the core trough the Earth’s surface. The quantity of energy that escape from the Earth is called Planetary Heat Flux.

Mantle convection is the driving force that causes tectonic plates to move around the Earth's surface and is the engine for the heat and mass transfer on the Earth.

The Earth’s engine: the Planetary Heat Flux

The Mantle movements heat and push the rocks up to the Crust, which rise upward due to the lower density.

As the molten rocks reach the surface, they flow outward with volcanic processes with the formation of new Crust.

These movements pulls the plates along until they meet another plate.

The process trigger the movement of the plates known as the Continental Drift.The process is similar to the motion of the ice plates above the water.Eventually the rocks cool, increase in density and sink down, back to the Mantle.

The Earth’s engine: tectonic movements

The Earth’s engine: Plates boundaries

Earth's outer shell, the lithosphere is a fluid mosaic of many irregular rigid plates. Composed of rocks, these enormous blocks vary in size and shape, and have definite borders that cut through continents and oceans alike. There are 9 large plates and a number of smaller plates and most plates are comprised of both continental and oceanic crust. Of the nine major plates, six are named for the continents embedded in them: the North American, South American, Eurasian, African, Indo-Australian, and Antarctic.

The Earth’s engine: Plates boundariesThe interfaces between plates are called boundaries. There are three main types of plate boundaries and they differ by the relative movement of the plate each other.

I - Diverging boundariesAre formed when the plates move away from each other. Magma from below rises up and forms a mid-oceanic-ridge, areas reach in volcanic activities. As plates pull apart, hundred years by hundred years, a space between them called rift valley is created. From an oceanographic point of view a rift valley is a young expanding ocean.

The Earth’s engine: Plates boundariesExamples of diverging boundaries are found in the Mid-Atlantic Ridge and the East-Pacific Ridge. It separates the Eurasian Plate and North American Plate in the North Atlantic, and the African Plate from the South American Plate in the South Atlantic.Although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it have enough elevation to extend above sea level as in the island of Iceland.

The Earth’s engine: Plates boundaries

The East African Rift Valley is a divergent boundary above sea level and is a forming ocean created by the separation of two parts of the African Plate

The Earth’s engine: Plates boundariesII - Converging boundariesAs two plates come together there is the formation of a converging boundary and several scenarios can result (depending on the type of plates that collide):1- continental vs continental plate2- oceanic vs continental plate3- oceanic vs oceanic plate

Continental-continental plates collisionBoth plates crumple upward, forming a single land mass. An example is the boundary between China and India. India is moving north more than 5cm per year, creating the Himalayan Mountains.

The Earth’s engine: Plates boundaries

Asian Plate

Indian Plate

Himalaya chain

The Earth’s engine: Plates boundariesOceanic-continental plates collision

When a continental plate collide with an oceanic plate, the second one, made of basalt (which has a density of 3g/cm3) plunges downward, while the continental plate, made of granite (less dense), stays at the surface. The oceanic plate is considered the subducting plate (which forms a deep sea trench) while the continental one is called the overriding plate and forms mountain chain. An example of this type of collision is the area along the Andes where the oceanic Nazca Plate is being subducted beneath the continental South American Plate

South American Plate(uplifting)

Nazca Plate(subducting)

Andes chain

The Earth’s engine: Plates boundariesOceanic-oceanic plates collision

When two oceanic plates converge, they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it.The Mariana Trench is the deepest part of the world's oceans and results from the convergence of the Pacific Plate (oceanic-subducting) and the Mariana Plate (overriding).

Mariana Plate(uplifting)

Pacific Plate(subducting)

The Earth’s engine: Plates boundariesIII - Sliding boundariesWhen the plates slide past each other, a sliding boundary is formed. This is also called a Transform boundary. On the contrary of the other kinds of margins, at the transform boundary there is no production of crust volumes but only the friction of huge amounts of rocks, this is the reason why they are also called conservative boundaries.

The Earth’s engine: Plates boundariesIII - Sliding boundariesThe most famous San Andreas Fault in California moves about 5 cm per year. This is part of the larger sliding boundary between the Pacific and the North American Plates.

North American Plate

Pacific Plates

The Earth’s engine: Plates boundaries

The Continental Drift Theory

At the beginning of last century, Alfred Lothar Wegener, a German scientist, geophysicist and meteorologist, proposed his theory of “Continental Drift” in 1912, which hypothesized that the continents were slowly drifting around the Earth.

Wegener found the same fossils in Brazil and South Africa (both dated back about 270 million years ago), and also noted that there were similar rocks and mineral deposits found on both these continents in corresponding locations.

The Continental Drift Theory

From 1912, Wegener advocated the theory of "continental drift", arguing that all the continents were once joined together in a single landmass and have drifted apart. He supposed the cause might be the centrifugal force of the Earth's rotation or the astronomical precession.

Unfortunately Wegener was unable to demonstrate a mechanism for continental drift. It was not until the development of the theory of plate tectonics in the 1960s, that a sufficient geological explanation of that movement was found.

The Continental Drift Theory

Evidences for the “Continental Drift”

1- Similar plant and animal fossils are found around different continent shores, suggesting that they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile, found both in Brazil and South Africa, are one example

2- The complementary arrangement of the facing sides of continents as South America and Africa

3- Widespread distribution of similar glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia. This was one of the major pieces of evidence for the theory of continental drif

The Continental Drift Theory