An Introduction to Physical Geology Questions with Answers

An Introduction to Physical Geology. 1. Metamorphism means to “change form.” Describe how a rock may change during metamorphism. Metamorphism is a change in mineralogy, texture, and sometimes chemical composition of a pre-existing rock. 2. What is meant by the statement, “Every metamorphic rock has a parent rock?" A rock metamorphoses when it is exposed to conditions different from those in which it formed. Most commonly, this is a change due to elevated temperature and pressure. The parent rock is the original rock upon which temperature and pressure caused changes, but did not melt the rock. 3. Define metamorphic grade. Metamorphic grade is the degree of metamorphism a rock undergoes; low-grade metamorphism involves low temperatures and low pressures, high-grade metamorphism involves high temperatures and high pressures. Rocks subjected to low-grade metamorphism generally retain some characteristics of the parent rock. Rocks that undergo high-grade metamorphism generally lose their original texture and have a different mineralogy of the parent rock. 4. List four agents that drive metamorphism. The four agents that drive metamorphism are heat, confining pressure, differential pressure, and chemically active fluids. 5. How is confining pressure different from differential stress? Confining pressure is a force that acts in all directions on a rock. Differential stress is force applied unevenly in different directions. 6. What role do chemically active fluids play in metamorphism? Chemically active fluids enhance metamorphism because they transport ions from one site to another in the crystal structure. These fluids provide a mechanism of transport for ions, allowing for the recrystallization of existing minerals and formation of new, stable minerals. 7. Define foliation. Foliation is the orientation of parallel or platy mineral grains in a metamorphic rock. 8. What is meant by nonfoliated texture? Name one rock that exhibits this texture. Nonfoliated textures are those that do not display layering or banding. These occur where deformation is minimal, or where the parent rocks are composed of equidimensional crystals. An example of a nonfoliated texture is the metamorphic rock marble; marble is formed when limestone (calcite) is metamorphosed around a plutonic body and the small calcite grains recrystallize into larger interlocking crystals. 9. What is an aureole? An aureole is the area around an intrusive igneous body that is metamorphosed by the heat of the intrusion. Aureoles are small around sills and dikes, and large around batholiths. 10. Which type of plate boundary is associated with regional metamorphism? Regional metamorphism is associated with continental collisional plate boundaries. Here, rock is folded and faulted, and shortened and thickened, through large-scale differential stresses resulting from the collision. As crust thickens, rocks are deeply buried, creating intense metamorphic activity. 11. List the common metamorphic rocks generated by regional metamorphism. Because regional metamorphism is large scale and created by high pressures, this form of metamorphism creates the sequence of metamorphic rocks slate, phyllite, schist, and gneiss as well as quartzite and marble. 12. How do geologists use index minerals? Index minerals give geologists clues about the intensity of metamorphism that a rock experienced. Different minerals form under different temperature and pressure regimes, and the occurrence of a specific mineral can help geologists to deduce the conditions under which that mineral, and its rock, formed. For example, chlorite and muscovite form under relatively low-grade metamorphism, while staurolite and sillimanite form under intense high-grade metamorphism. 13. What is rock deformation? How might a rock body change during deformation? Rock deformation is the changes in shape or position of a rock body in response to differential stress. Deformation can cause rock structures including folds (wave-like), faults (fractures), joints (cracks), and metamorphic structures like foliation and rock cleavage. 14. Identify the plate boundary that is most commonly associated with each of the three types of differential stress. Compressional stresses are associated with convergent boundaries and results in crust that is shortened and thickened by folding, flowing, and faulting. Tensional stresses occur where tectonic plates are pulled apart at divergent plate boundaries. Large-scale shear stresses cause movement at transform plate boundaries. 15. How is strain different from stress? Strain is a change in shape of a rock body in response to a force, called stress. Stress is the force; strain is the resulting distortion. 16. Distinguish between anticlines and synclines. Anticlines are folds with two limbs dipping in opposite directions from the fold axis. After erosion, older strata are exposed along the axial portion of the fold. Synclines are folds with two limbs dipping inward at the fold axis; after erosion, younger strata are exposed at the fold axis. 17. Contrast the movements that occur along normal faults and along reverse faults. What type of stress is indicated by each fault? Normal faults occur when the hanging wall block (rock surface immediately above the fault) moves down relative to the footwall block (rock surface immediately below the fault). Normal faults are associated with tensional stresses that pull rocks apart. Reverse faults occur where the hanging wall block moves up relative to the footwall block. Reverse faults are associated with compressional stresses that shorten the crust. 18. Describe the relative movement along a strike-slip fault. Along strike-slip faults, displacement is horizontal and parallel to the direction of the fault. Strike-slips are called right-lateral if the crustal block on the opposite side of the fault moves to the right and left lateral if the crustal block on the opposite side of the fault moves to the left. 19. Distinguish between the two measurements used to establish the orientation of deformed strata. Strike and dip are used to establish the orientation of deformed strata. Strike is the compass direction of a line produced by the intersection of an inclined rock layer with a horizontal plane. Dip is the angle and direction of inclination of a rock unit measured from the horizontal.