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9.2 Tectonic impacts: 1. Lithospheric plates and their motion

Syllabus reference (October 2002 version)
1. lithospheric plates and their motion	*Students learn to:* describe the characteristics of lithospheric plates identify the relationship between the general composition of igneous rocks and plate boundary type outline the motion of plates and distinguish between the three types of plate boundaries (convergent, divergent and conservative) describe current hypotheses used to explain how convection currents and subduction drive plate motion	*Students:* gather and analyse information from secondary sources about the forces driving plate motion

Extract from Earth and Environmental Science Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW
[Edit: 1Mar05]

Prior Learning: Preliminary modules 8.2 (subsection 2), 8.5 (subsections 2, 3

Background: The lithosphere is an outer layer of the Earth that includes the continental crust, the oceanic crust and the upper, most rigid layer of the mantle. The lithosphere lies above a less rigid layer of the earth called the asthenosphere.

The lithosphere is not a uniform layer all around the globe. It consists of a series of plates, called lithospheric plates, which ride and move on the partially molten asthenosphere.

The lithospheric plates move relative to each other. They are created at mid-oceanic ridges and destroyed at subduction zones.

Photo glossary of terms US Geological Society, Volcano Hazards Program.

describe the characteristics of lithospheric plates

The upper layer of each lithospheric plate is composed of crust
The crust is composed of either continental crust or oceanic crust.
Continental crust is typically made up of relatively less dense rock like granite.
The oceanic crust is typically made up of more dense rock like basalt. Oceanic crust carries sediments that have been deposited on the oceanic floor.
Some lithospheric plates have just oceanic crust. Others have some oceanic crust and some continental crust.
Oceanic crust is generally thin, usually between 5 and10 kilometres thick. Continental crust is generally between 25 and 50 kilometres thick. Under large mountain ranges the crust can be over 80 kilometres thick.
The lithospheric plates are up to 70 km thick, where there is oceanic crust, and up to 150 km thick, where there is continental crust.
Earth cutaway , Colin Rose, Dorling Kindersley, Think Quest

outline the motion of plates and distinguish between the three types of plate boundaries (convergent, divergent and conservative)

Plates move slowly, at speeds of up to 12 centimetres per year.
Plates are created at divergent boundaries, slide past each other at conservative boundaries and are absorbed back into the earth at convergent boundaries.
At divergent zones, the plates are moving away from each other and new oceanic lithosphere is created to fill the gap. Divergent boundaries are usually found at the mid-ocean ridges. Some special cases of divergent boundaries can be found in rift valleys on continental crust where the continent is beginning to divide e.g. the African Rift Valley, the Dead Sea Rift Valley.
At conservative plate boundaries, crust is neither created nor destroyed. The plates slide past each other along faults. Near mid ocean ridges these faults are called transform faults. On continental crust, these boundaries are the cause of many earthquakes e.g. Alpine Fault System in N.Z and the San Andreas Fault in California, USA.
At convergent zones, the lithosphere is consumed. This occurs when one plate, usually consisting of oceanic material, is subducted beneath another plate. Deep ocean trenches are usually found along the continent’s edge when subduction occurs. Quite often, the upper surface of the subducted plate is shaved off, creating folded sediments at the edge of the overlying plate. As the subducted plate moves deeper into the asthenosphere, it partly melts and this molten rock rises because it is less dense than the material above it, creating magma chambers. From these magma chambers, volcanoes are produced in the overlying plate.

identify the relationship between the general composition of igneous rocks and plate boundary type

At divergent boundaries the dominant types of igneous rocks are the mafic igneous rocks like basalt, gabbro and peridotites. Mafic rocks are dark coloured because they contain minerals richer in magnesium and iron like olivine, pyroxene, amphibole and biotite. The mafic rocks at divergent boundaries form as a direct upwelling of dense magmas from the asthenosphere.
At convergent boundaries the dominant types of igneous rocks are felsic rocks like andesite, rhyolite and granite. Felsic rocks are light coloured because they contain more feldspar and quartz, minerals with relatively more silica than the dark coloured minerals. Felsic rocks are produced from magmas with higher water content.
Occasionally at conservative boundaries a variety of igneous rocks occur as molten rock fills cracks to form intrusions, such as dykes and sills.

gather and analyse information from secondary sources about the forces driving plate motion

Try to gather information from a range of resources, including popular scientific journals, digital technologies like CD-ROMs and the Internet. Focus on collecting the descriptions and explanations of models of plate motion and the evidence given to support the models.
Once you have collected it, summarise and collate the information to describe and explain each of the models of plate motion.
Analyse the information by identifying the evidence used to support each hypothesis. Assess each hypothesis by explaining how the evidence supports or refutes each particular hypothesis.

Useful web site starting point is:

Plate Tectonics: The Mechanism , Museum of Palaeontology

describe current hypotheses used to explain how convection currents and subduction drive plate motion

The following are three hypotheses used to explain plate motion.

Idea 1: The plates move because of convection currents in the asthenosphere which transfer heat from the lower mantle towards the crust. As the currents move, they drag the plates with them. (shear traction).
Idea 2: The higher density of cold rock compared to that of hot rock causes the lithosphere to be dragged by gravity (ridge-push) from the relatively high mid-ocean ridge to the subduction zone by the sinking denser lithosphere (slab-pull).
Idea 3: A tensional force is placed on an upper plate caused by subduction of the lower plate. The subducting zone moves away (called roll-back) from the upper plate to create secondary volcanic arcs (trench suction).