8.5 Dynamic Earth: The Crust 1

Plate Tectonics8.5 Dynamic Earth

Students learn to: Students:

2 Crustal plates move and their edges are marked by tectonic activity

describe similarities and differences between oceanic and continental crust

gather information from secondary sources to identify the major world plates, their positions and boundaries, on a map

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The Theory of Plate TectonicsThe Theory of Plate Tectonics

The plate tectonic theory states that the Earth’s surface has broken into rigid plates. These plates are 100-120km thick, and include the crust and a small part of the upper mantle. Together, this layer is termed the lithosphere.

Use the arrows below to move

forward and back.

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Earth’s Tectonic PlatesEarth’s Tectonic Plates

There are 12 major plates plus a number of minor ones. The plates are named after the regions in which they are located.

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Earth’s Tectonic Plates – Challenge!Earth’s Tectonic Plates – Challenge!

See if you can find the Caribbean, Arabian and Indian plates – click on them and see if you are correct.

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Earth’s Tectonic PlatesEarth’s Tectonic Plates

The plate boundaries have been identified through research into earthquake activity, the locations of volcanoes, and mapping of the sea floor.

Can you find the subduction zone near

Indonesia?

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Plate MovementPlate Movement

Over time, the plates have moved through the process known as continental drift. Australia is still moving, at a rate of 73mm per year in a north-east direction.

Click here for a timeline of the concepts to this point.

Click here to access a number of web based animations and interactive activities for this topic.

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The Earth’s CrustThe Earth’s Crust

Continental Crust Continental crust is on average older, more silica-rich and thicker than oceanic crust, but is also more variable in each of these respects. The oldest parts of the continental crust, known as 'shields' or 'cratons', include some rocks that are nearly 4 billion years old. Most of the rest of the continental crust consists of the roots of mountain belts, known as 'orogens', formed at different stages in Earth history. Over large areas, however, these orogens are covered by younger sedimentary rocks. New continental crust is still being generated by processes operating at subduction zones. However, sediments eroded from the continental crust and deposited on the ocean floor are also recycled into the mantle at some subduction zones. The average thickness of the continental crust is about 40 km, but beneath parts of the Andes and the Himalaya mountain ranges the crust is more than 70 km thick.

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The Earth’s CrustThe Earth’s Crust

Oceanic Crust Oceanic crust underlies most of the two-thirds of the Earth's surface which is covered by the oceans. It has a remarkably uniform composition (mostly 49% ± 2% SiO2) and thickness (mostly 7 ± 1 km). The ocean floor is the most dynamic part of the Earth's surface. As a result, no part of the oceanic crust existing today is more than 200 million years old, which is less than 5% of the age of the Earth itself. New oceanic crust is constantly being generated by sea-floor spreading at mid-ocean ridges, while other parts of the oceanic crust are being recycled into the mantle at subduction zones. Maps of oceanic crustal ages have been produced as a result of studies of the record of reversals of the Earth's magnetic field which is preserved in the crust as it forms.

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Mechanisms for Plate MovementMechanisms for Plate Movement

It is believed that convection currents, caused by heat from the interior of the Earth, drive processes of plate tectonics much like a conveyor belt. Other ideas include ridge-push, gravity-slide and slab-pull mechanisms.

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Convection CurrentsConvection Currents

Mantle convection is the slow movement of Earth's rocky mantle in response to variations in its density. Material near the surface of Earth, particularly oceanic lithosphere, cools down by conduction of heat into the oceans and atmosphere, then contracts to become dense, and sinks under its own weight at plate boundaries.

This subducted material sinks to some depth in the Earth's interior where it is stopped from sinking further. This stoppage creates a thermal boundary layer where sunken material soaks up heat via thermal conduction from below, and may become buoyant again to form upwelling mantle plumes

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Further ReadingFurther Reading

Mechanisms for Plate Movement: http://webspinners.com/dlblanc/tectonic/mechansm.php

Some unanswered questions:http://pubs.usgs.gov/gip/dynamic/unanswered.html

The Earth’s Crust:http://www.antarctica.ac.uk/Key_Topics/Geological_Evolution/earths_crust/index.html

Continental Drift:

http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Continents.shtml

Structure of the Interior:

http://www.geo.lsa.umich.edu/~crlb/COURSES/270/Lec16/Lec16.html

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