Plate TectonicsSocial Studies for 10th E.G.B.Teacher: Mauricio Torres

Concept

It is a scientific theory that describes the large-scale motions of Earth's lithosphere. The theory builds on the concepts of continental drift, developed during the first decades of the 20th century.

How it works

The lithosphere is broken up into tectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and many minor plates. Where plates meet, their relative motion determines the type of boundary. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries.

The outer layers of the Earth are divided into lithosphere (outer) and asthenosphere (inner).

How do they move?

Tectonic plates are able to move because the Earth's lithosphere has a higher strength and lower density than the underlying asthenosphere.

Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. Therefore, in simpler terms:

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like asthenosphere.

Why do they move?

Plate tectonics is basically a kinematic phenomenon: Earth scientists agree upon the observation and deduction that the plates have moved one with respect to the other, and debate and find agreements on how and when. But still a major question remains on what the motor behind this movement is; the geodynamic mechanism, and here science diverges in different theories.

One of the theories says: different forces generated by the rotation of the globe and tidal forces of the Sun and the Moon. The relative importance of each of these factors is unclear, and is still subject to debate.

Who thought of this?

In 1912 the meteorologist Alfred Wegener amply described what he called continental drift.

Continental drift is the movement of the Earth’s continents relative to each other.

Starting from the idea (also expressed by his forerunners) that the present continents once formed a single land mass (which was called Pangea later on) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to "icebergs" of low density granite floating on a sea of denser basalt.

Formation and break-up of continents

The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Columbia or Nuna formed during a period of 2,000 to 1,800 million years ago and broke up about 1,500 to 1,300 million years ago.

This pieces later re-assembled into another supercontinent called Pangaea; Pangaea broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents).

Plate Boundaries

The location where two plates meet is called a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate’s Ring of Fire being most active and most widely known.

Plate Boundaries

Boundaries on the surface and underneath the ocean:

What happens in the boundaries?

Divergent BoundariesDivergent boundaries (Constructive) occur where two plates slide apart from each other. Active zones of rifting (such as Africa's East African Rift) is an example of divergent boundaries.

Subduction

In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These regions of mantle downwellings are known as "Subduction Zones". A subduction zone is an area on Earth where two tectonic plates move towards one another and one slides under the other.

Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately 2 to 8 centimeters per year (about the rate a fingernail grows).

Subduction

Effects

The strains caused by plate convergence in subduction zones cause earthquakes.

Nine out of the ten largest earthquakes to occur in the last 100 years were subduction zone events. This includes the 1960 Great Chilean Earthquake (which at 9.5 was the largest earthquake ever recorded), the 2004 Indian Ocean earthquake and tsunami, and the 2011 Tōhoku earthquake and tsunami.

Hot Spots

The places known as hotspots or hot spots in geology are volcanic regions thought to be fed b underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries.

Hot Spots

Hawaii, Réunion, Yellowstone, Galápagos, and Iceland are some of the most currently active volcanic regions to which the hypothesis is applied.

Pinnacle Rock, Galápagos Islands

Group Work

Get together in groups of five and research on the following sites:

Hawaii, Réunion, Yellowstone, Galápagos, or Iceland.

First, look up what they have in common. Then you may have a better idea of what you need to research on.

You will have to prepare an oral presentation of five minutes with multimedia.

Remember, it must all be related with GEOLOGY.

You have two weeks to have it ready.

Only the best and most outstanding presentation in the class, will have a 20 as a final grade on the project.