Unit 2 - Plate Tectonics and the Origins of Resources Flashcards
What does the theory of plate tectonics say about the Earth’s crust?
According to the plate tectonics theory, the Earth’s crust comprises a number of irregularly shaped plates that “float” on top of the asthenosphere. The asthenosphere behaves like plastic, so the plates can migrate. During the Earth’s history, the plates have moved and changed in shape and size.
What are divergent plate boundaries and convergence zones?
Divergent plate boundaries are zones where crustal plates move apart. Such regions are characterized by basaltic magma welling up at spreading submarine ridges. Typically, as fresh magma comes to the surface, segments of crust formed earlier move outwards laterally (on either side of the ridge) to form new oceanic crust. Thus, the crust displays a proportional increase in age with distance from the ridge.
Convergence zones are regions where plates meet or collide. Depending on the nature of the plates that are coming together, the convergence can result in subduction, which is when oceanic crust that has cooled is forced downwards into the mantle.
What process moves the plates in the plate tectonics theory?
A number of theories have been posed to explain plate motion in the plate tectonics theory. They include
A. Convection cells: Heat from radioactive decay in the Earth’s interior results in the development of convection cells in the mantle. The movement of these cells carries the overlying plates away from spreading centres at mid-ocean ridges to come together at convergence zones.
B. Ridge push: As plates move away from spreading centres (mid-ocean ridges), they cool and increase in density. Further out from the ridges, the newly formed ocean floor slopes away from the spreading centres. The cooling also leads to an increase in the thickness of the lithosphere, proportional to the distance from the ridge, resulting in a slope forming at the lithosphere-asthenosphere boundary, which slants away from the ridges. The lithosphere is believed to slide down this inclined boundary (away from the ridges) towards the zones of convergence.
C. Slab-pull: Through gravitational pull, subducted segments of the crust may effectively “drag” that portion of the plate that is still at the Earth’s surface. This pulling may be stronger where the plates dip into the mantle at steep angles, resulting in a phenomenon called trench-suction.
Which two main geological factors have controlled the distribution of Earth’s resources?
The two main geological factors that have controlled the distribution of Earth’s resources are 1) the processes of plate tectonics and their influence on the creation of major geological settings and 2) localized processes that result in the development of particular resources.
What is the difference in composition between granitic and basaltic rocks?
Granites are light-coloured, coarse grained rocks characterized by high silica content. Basalts, on the other hand, are dark-coloured, fine-grained rocks characterized by low silica content.
Which processes generate granitic magma?
Granitic magma is generated when rocks in the Earth’s crust are heated until temperatures are high enough to melt the minerals that make up the rocks of the crust. Because minerals have different melting points, when rocks are heated adequately, they begin to melt differentially, a process called partial melting. The exact temperature at which the melting of a particular mineral begins depends on factors such as pressure and water content, but generally, light-coloured minerals (that make up granites) melt first. Once melted, the minerals with high silica content form granitic magma. This magma can move upwards or sideways, incorporating other rocks into the melt in the process, bringing about diversity in the content of granite magma.
Settings where rocks of the Earth’s crust are heated enough to generate granitic magma include convergence zones where oceanic crust collides with continental crust resulting in subduction of the oceanic crust into the asthenosphere. Partial melting of the asthenosphere generates magma which, after rising, can underplate the continental crust. Magma pools at the bottom of the continental crust, in turn, result in partial melting to yield magmas of granitic composition. The granitic magmas may then rise for emplacement in the crust as plutons.
Which mineral deposits are associated with granitic rocks?
Magma usually contains significant portions of water. Most of the water, called hydrothermal fluid, is released when the magma solidifies. This takes place through crystallization of individual minerals. Ions of elements such as copper, lead, zinc, silver, and sulfur and compounds such as sodium chlorite are usually dissolved in the hydrothermal fluids. As released fluids migrate through the surrounding rocks, they cool, and pressure subsides. Dissolved compounds may be deposited in cavities and fractures within the rocks to form minerals such as sulfide ores of copper, lead, zinc, silver, and gold (in some cases).
Pegmatites, which are very coarse-grained rocks that form on the peripheries of granitic intrusive bodies, sometimes contain fluorine, berrylium, lithium, or rare Earth elements. Gemstones can also occur in pegmatites.
Where are basaltic magmas generated?
Basaltic magmas are generated when mantle material undergoes partial melting. The magmas migrate upwards to the crust by melting the overlying rocks.
Which mineral deposits are associated with basalts and other silica-poor rocks?
Basaltic magmas that extrude from the surface are not usually associated with major mineralization because they cool too rapidly. When they cool at depth, however, basaltic magmas crystallize slowly, and can significantly alter surrounding rocks. This crystallization process can also follow a sequential process in which some mineral species concentrate in layers. This takes place with minerals such as olivine; sulfide ores of iron, nickel, and copper; as well as chromite, platinum group metals, and magnetite. Magnetite may contain vanadium.
What is regional metamorphism, and how does it differ from contact metamorphism?
Regional metamorphism is the large-scale alteration of rocks in a region due to increasing temperature and pressure at depths of up to 10 km or more. Contact metamorphism, on the other hand, occurs when minerals in a rock are transformed by heat and fluids that come from intrusive igneous bodies. Thus, contact metamorphism occurs next to intrusive structures, and the magnitude of the intrusion determines the thickness of the metamorphic zones.
What is the origin of fluids in contact metamorphism, and what role do they play in the formation of deposits?
Fluids in contact metamorphism can emanate from water released by cooling igneous bodies. Alternatively, the fluids can also come from the rocks into which the intrusive body is emplaced. Heat from the intrusion generates convective cells that circulate in the rocks.
Which mineral deposits are associated with metamorphic rocks?
Mineral deposits associated with regional metamorphic rocks include slates (from shales), marbles (from limestones), and quartzites (from sandstones). All these rocks are important industrial resources.
Hydrothermal fluids in contact metamorphism can generate a broad range of metalliferous ores called skarns. Skarns often contain iron oxides as well as sulfides of copper, lead, zinc, and iron. They can also host gold and silver in variable quantities.
What is weathering, and what factors control it?
Weathering is the modification of rocks and minerals by geomorphic agents such as rain, wind, frost, and by biological activity.
What is erosion?
Erosion is the movement or transportation of Earth’s materials downstream or downslope in response to gravitational pull. Such transportation can also result from wind action. Thus, erosion is capable of either concentrating or dispersing materials.
Where does soil come from?
Soils form when minerals that occur in rocks are altered into mineral species that are stable at the Earth’s surface. For instance, ferromagnesian minerals break down to silica and metal ions; feldspars release some of the ions to form clay minerals. Quartz, on the other hand, is relatively stable at the Earth’s surface, and undergoes minor changes apart from reduction in size.
