OVERVIEW

This chapter introduces the grand cycle of plate tectonics. This cycle explains how the continents and ocean basins of the earth's surface slowly change as forces deep within the earth cause vast tectonic plates to converge, collide, split and separate.

• The solid earth has a layered structure: a dense, metallic core surrounded by a mantle of ultramafic rock which is covered by a crust with mafic rocks exposed on the ocean floor and felsic rocks exposed on the continents.
• Geologists use the term lithosphere to refer to the earth's outer layer of rigid, brittle rocks which extend through the crust to the upper mantle. Beneath it lies the asthenosphere; the layer of soft, plastic rock in the mantle.
• The history of the earth can be subdivided into various time intervals using the geologic time scale. Most of the landscape features of the earth's surface developed during the Cenozoic Era which began 65 million years.
• The major relief features of the earth's surface are the continents and ocean basins.
• The continents contain young, dynamic alpine belts and old, stable continental shields.
• The ocean basins consist of extensive, smooth abyssal plains marked by long, narrow midoceanic ridges.
• Shallow continental shelves are found beneath the ocean next to continental shields while deep oceanic trenches are found adjacent to alpine belts.
• The lithosphere is broken into six great plates and several lesser plates that move relative to one another.
• Spreading boundaries exist where plates move apart, converging boundaries where they collide and transform boundaries where they move past one another.
• Spreading boundaries are marked by midoceanic ridges on the ocean floor and rift valleys on continents. New ocean crust is formed along spreading boundaries.
• Subduction occurs where continents meet the ocean floor along a converging boundary and the denser rock of the ocean floor plunges beneath the continent. An oceanic trench marks the subduction zone.
• Subduction along continental margins leads to the formation of island arcs and alpine belts as subducted ocean crust melts and rises to the surface in volcanoes, and sediment from the ocean floor is folded and faulted as it accumulates on the continental margin.
• Plate movement is thought to be driven by convection currents in the plastic rock of the asthenosphere.
• The plate tectonics cycle ties together major relief features, volcanic and earthquake activity, and patterns of rock age and type at the earth's surface.

KEY TERMS

core              lithosphere        mountain arc       
mantle            asthenosphere      island arc         
crust             lithospheric       tectonics          
continental       plates             subduction         
crust             continental        plate tectonics    
oceanic crust     shields                               
                  continental                           
                  suture                                

STUDY QUESTIONS

1. Describe the structure of the earth's interior and its crust.
2. What are the lithosphere and the asthenosphere?
3. List in order, from youngest to oldest, the names of the eras of the geologic time scale since Precambrian Time. During which of these eras did the supercontinent of Pangea exist? During which did most of the present landscape features of the continents develop?
4. How do alpine belts and continental shields differ with respect to their topography, rock type and age?
5. Sketch a cross-section of an ocean basin with a passive continental margin on one side and an active continental margin on the other. Label the midoceanic ridge, axial rift, abyssal plain, continental rise, continental slope, continental slope, and oceanic trench.
6. Describe alpine belts of the earth's surface.
7. Describe the process of subduction and explain how it is related to island arc development and mountain-building.
8. How and where does new crust develop?
9. What is an orogen and what are the two types of orogens that are recognized?
10. How do accreted terranes form? Use the example of Wrangellia to illustrate your answer.
11. Describe the process of continental rupture by which new ocean basins can form. Which stages of this process are illustrated by the Rift Valley of East Africa and the Red Sea?
12. What is the Wilson cycle of plate tectonics and what is the net effect of a Wilson cycle on the continental mass?
13. What mechanism is responsible for the movement of lithospheric plates and what is the power source for this mechanism?