Unit 2
Plate Tectonics and the Origins of Resources

Overview

Have you ever wondered why, for instance, the geology of eastern Canada differs from that of western Canada, or why mineral resources that occur at one locality will not necessarily occur at another? In geology, the theory of plate tectonics is a unifying concept that explains geological processes taking place in the Earth’s crust. The theory essentially shows that geological processes are not random events. They result from identifiable processes that follow patterns that are somewhat predictable and that can be studied systematically. Plate tectonics theory provides explanations for a range of geologic phenomena, including the global distribution of earthquakes and volcanic activity as well as the formation of many mineral deposits. Hence, the theory of plate tectonics helps us identify which resources we can find where, and what processes to expect at a given locality.

The plate tectonics theory resulted from the merging of two earlier theories: continental drift and sea floor spreading. Towards the end of the 19th and beginning of the 20th century, some scientists identified evidence suggesting that continents could migrate across the Earth’s surface. Such evidence included

• fossils whose distribution suggested that the continents may have been contiguous at some stage;
• climate records suggesting that continents now in cold latitudes may have been located in warmer zones previously;
• the outline of continents resembling a jigsaw puzzle, suggesting they may have been one big structure at one time.
• the geology of parts of the west coast of Africa and Europe, which matches that of segments of the east coast of the Americas.

The theory of continental drift did not find universal acceptance. During the 1960s, attention shifted to ocean floors when scientists discovered that the oceanic crust displayed symmetrical bands of reversed and normal magnetic polarity around mid-oceanic ridges. Scientists also observed that on both sides of mid-oceanic ridges, the oceanic crust increased in age with distance from the ridge. These observations led to the development of the theory of sea floor spreading, which suggested that rift zones of mid-ocean ridge systems were spreading-centres where new oceanic crust was generated. Further investigations showed that Earth’s surface comprised a number of large continental and oceanic crust plates that glide on the plastic asthenosphere. More evidence emerged, showing that while spreading occurred at the midocean ridges, subduction took place at convergence zones, leading to the destruction of crust material. This evidence helped to refine the theory of plate tectonics.

There is no consensus about the driving forces behind plate motion, but a number of theories have been forwarded (your textbook offers only one). These include

1. Mantle convection: Heat from the Earth’s core raises the temperature of the overlying mantle, resulting in convective overturning. Such convection currents are believed to drag the crust with them as they move. Some have also modified this theory to include mantle plumes.
2. Ridge push: Lithospheric plates cool and increase in density as they move away from spreading oceanic ridges. As the distance from oceanic ridges increases, the sea floor cools and sags, creating slopes that rise towards the ridges. Lithospheric plate thickness also increases with distance from the ridge crest as the asthenospheric material cools and underplates the lithosphere. This results in a sloping boundary between the lithosphere and the asthenosphere, down which the plate slides.
3. Slab pull: As the cooler, denser lithosphere sinks into the hot mantle at subduction zones, it drags the plate with it. Where the angle of the subducting plate is excessively steep, this effect could drag the plates towards the trench; this process is called trench suction. This effect is believed to be more important than ridge push.

While each of these theories has strengths and weaknesses, it is possible that all three processes contribute in varying degrees to produce the final result observed in plate tectonics.

Unit 2 begins with a brief explanation of the principles behind the plate tectonics theory. The Unit then classifies processes through which Earth’s resources form the following classes, which are discussed in the Unit’s main sections:

1. subsurface igneous and metamorphic processes;
2. surface processes;
3. shallow subsurface and diagenetic processes; and
4. marine processes.

Objectives

After completing this unit, you should be able to

1. outline the composition of the Earth’s continental crust and oceanic crust.
2. describe the structure of the Earth’s interior, and outline the driving mechanism behind plate tectonics and plate motion.
3. describe the three types of plate boundaries and the processes that characterize each type.
4. explain the nature and location of geological processes responsible for forming mineral resources.
5. outline the role of hydrothermal fluids generated by igneous intrusions in the concentration of ore deposits.
6. describe the role of selective precipitation of metal oxides and sulfides from magmas as ore genesis mechanisms.
7. explain the role of regional metamorphism in modifying pre-existing rocks to form resources such as marble and slate.
8. describe mineral resources generated by contact metamorphism associated with heat from igneous intrusions.
9. describe how weathering and erosion influence the formation of residual minerals such as bauxite and placer deposits such as gold, tin, and titanium.
10. describe the formation of evaporite deposits in arid regions through the precipitation of soluble salts such as halite, potassium salts, gypsum, and nitrates.
11. describe the formation of fossil fuels.