The Structure of the Earth Plate Tectonics 1 The Structure of the Earth

The structure of the Earth The Earth is divided into three layers—the crust, the mantle, and the core—based on the compounds that make up each layer. These are compositional layers !

What is the internal structure of the earth?

The Crust There are two types of crust—continental and oceanic. Outermost layer of the Earth. The crust is 5 to 100 km thick, and is the thinnest layer of the Earth. There are two types of crust—continental and oceanic.

Continental crust Continental crust is thicker, older and lighter, and is composed mainly of granite. It is 35 km thick on average, but this increases under mountain chains, and is less dense than oceanic crust.

Continental crust Continental crust is more complex than oceanic crust in its structure and origin and is formed primarily at subduction zones at destructive plate margins. It is also very old, much of it being over 1,500 million years.  The rocks contain mainly silica and aluminium (SiAl) and granites are the most common.

Oceanic crust Oceanic crust is younger and heavier, and is mainly composed of basalt and Gabbro. This material contains Silica and Magnesium (SiMa) and is very young, most of it being under 200 million years old).

Oceanic crust It is mainly formed at constructive margins or spreading mid ocean ridges and tends to be between 6 and 10km thick. It is constantly destroyed and recreated because of the processes taking place along ocean ridges and destructive plate margins.

Ocean and Continental Crust Oceanic Crust primarily basalt 6-10 km thickness (thin relative to continental crust) denser (heavier) than continental crust Continental Crust primarily granite 30-70 km thickness less dense (will not undergo subduction)

The physical structure of the Earth The Earth is divided into five physical layers: The lithosphere The asthenosphere The mesosphere The outer core The inner core Each layer has its own set of physical properties. These are structural / mechanical layers !

The Lithosphere Lithosphere = Crust + Uppermost (rigid) part of the mantle Suggested activity Recap the internal structure of the earth with students with the website: http://www.learner.org/interactives/dynamicearth/structure.html Makes up the Earth’s rigid outer shell When the rocks in the lithosphere melt, hot molten rock called magma is formed.

The Asthenosphere Asthenosphere = Weak, plastic layer of the upper mantle on which the tectonic plates move. Suggested activity Recap the internal structure of the earth with students with the website: http://www.learner.org/interactives/dynamicearth/structure.html

The Mesosphere The strong, lower part of the mantle between the asthenosphere and the outer core. It has a greater density and is more rigid than the asthenosphere. The chemical composition is the same (since it is part of the mantle)

The Core The outer core is the liquid layer of the Earth’s core that lies beneath the mantle. The inner core is the solid, dense centre of our planet that extends from the bottom of the outer core to the center of the Earth

What is a tectonic plate? Plate tectonic theory says that the Earth’s lithosphere is broken into several pieces called tectonic or lithospheric plates. A tectonic plate is made up of the lithosphere (i.e. crust + uppermost part of the mantle). These plates move in relation to one another. The surface of the Earth is divided into 7 major and 7 minor plates. The largest plates are the Pacific, Eurasian, North American and African plates.

What is a tectonic plate? Tectonic plates contain: oceanic crust continental crust or a combination of both Those tectonic plates that contain only oceanic crust are called oceanic plates. Tectonic plates that contain only / mainly continental crust are called continental plates.

What is a tectonic plate? Some plates are large enough to consist of both continental and oceanic crustal portions (e.g. the African or South American plates) whilst the Pacific Plate is almost entirely oceanic Plates are on average 125 km thick, reaching maximum thickness below mountain ranges. Oceanic plates (50-100km) are thinner than the continental plates (up to 200km) and even thinner at the mid-ocean ridges where the temperatures are higher.

Why do tectonic plates move? 1. Convection currents Convection currents are movements of heat within the mantle. Material in the mantle is heated by the decay of radioactive isotopes in the core. This causes convection currents in the molten mantle material. Mantle expands, rises and spreads out beneath the plates. Plates are dragged along and move away from each other. Suggested activity: Demonstrate convection currents with the video: http://www.youtube.com/watch?v=PdWYBAOqHrk&feature=fvwrel

Why do tectonic plates move? 1. Convection currents Subsequently, the hot molten mantle cools slightly and sinks, pulling the plates along Hence plates move towards each other. The sinking mantle material heats up again as it nears the core and the whole process repeats.

Why do tectonic plates move? Plates moving away from each other Plates moving towards each other Plates Ocean floor Tectonic plates float on molten mantle, driven by heat energy/convection currents

Why do tectonic plates move? 2. Slab-pull force This occurs when an oceanic plate (denser) subducts under a less dense plate and pulls the rest of the plate along. The subducting plate drives the downward-moving portion of convection currents. While mantle material away from the subduction zone drives the rising portion of convection currents.

Effects of Plate Movement Plate movements occur very slowly, averaging about 5 cm a year. However, over several hundred million years, such movements have altered the distribution of the earth’s land masses. (see Wegener’s theory on Continental Drift)

Effects of Plate Movement Plate movement also causes plates to interact with each other along their margins (edges). These interactions have resulted in many phenomena (including earthquakes, volcanoes and mountain building) being found along these plate margins or boundaries. Differences in: the directions of movement and because some plates are continental and some are oceanic, result in different types of volcanoes and different types of earthquakes at the edges of the plates.