Geol 107 Final Exam Questions 1. What is the Earth’s magnetic field and what causes it?
Earth’s magnetic field is largely a dipole, meaning that it has two poles- a north pole and a south pole. 2. What are faults and how are different types defined?
A fault is a large fracture on which sliding occurs, that slopes gently to the west. Motion along a fault doesn’t happen continuously, but rather rocks along a fault slowly bend and eventually snap. The intersection between a fault and the ground surface is the fault trace, or fault line A) Normal faults form during stretching or extension of the crust. The hanging wall moves down B) Reverse faults form during squeezing and shortening of the crust. The hanging wall moves up and the fault is steep. C) Thrust faults also form during shortening of the crust, but the fault’s slope is gentle (<30 degrees) D) On a strike-slip fault, one block slides laterally past another, so no vertical displacement takes place E) As oceanic plates spread apart at mid oceanic ridges, they must slide past other plates, along transform faults 3. What is the lithosphere and asthenosphere? (defined by strength)
The lithosphere is a zone of strength, where if you apply enough stress, it will break causing an earthquake. The lithosphere is 0-100 km thick between oceans, and 100-150 km thick between continents. The lithosphere consists of the crust plus the uppermost part of the mantle.
The asthenosphere is a zone of weakness, where if you apply enough stress, the asthenosphere will flow. It is 200 km thick and occurs beneath the lithosphere. Lithospheric plates move on top of asthenosphere. The asthenosphere is the portion of the mantle in which rock can flow. 4. What are the internal chemical layers in the Earth and how are they defined?
Crust- there are two basic types of crust. The oceanic crust is thinner and consists of basalt and gabbro. Continental crust varies in thickness and rock type. Oceanic crust is only 7-10 km thick. On average upper continental crust is less mafic than ocean crust, so it is overall less dense than oceanic crust. Oxygen is the most abundant element in the crust
Mantle- In terms of volume, the mantle is the largest part of the earth. In contrast to the crust, the mantle consists entirely of an ultramafic rock called periodite, which is the most abundant rock in our planet. The upper mantle goes from the crust down to a depth of 660 km, the lower mantle from 660-2900 km, and the transition zone is the section between 400 km and 660 km deep.
Core- The core consists of iron alloy. The outer core consists of liquid iron alloy because the temperature is so high, and the inner core is a solid iron alloy. Even though it is hotter than the outer core, it is solid because it is subjected to greater pressure. 5. What are mid ocean ridges and what do they produce?
Mid-ocean ridges elongate submarine mountain ranges whose peaks lie only about 2-2.5 km below sea level. Seafloor spreading is the concept where new ocean crust is created at the mid ocean ridge, where it moves away, gets older, cooler, denser, and sinks. Depth increases away from the mid oceanic ridge. 6. What principles allow us to determine relative ages of rocks?
Isotopic dating is the process of determining the numerical age of rocks. The overall study of numerical ages is geochronology. Because an element’s half life is constant, we can calculate the age of a mineral by measuring the ratio of parent to daughter isotopes in the mineral. 7. Describe the differences between unconformity types
A surface representing a period of nondeposition and possibly erosion, is called an unconformity. The gap in the geologic record that an unconformity represents is called a hiatus.
Angular Unconformity: Rocks below an angular unconformity were tilted or folded before the unconformity developed. An angular unconformity cuts across the underlying layers, and the orientation of layers below an unconformity differs from that of the layers above.
Nonconformity: This is a type of unconformity at which sedimentary rocks overlie generally much older intrusive igneous rocks and/or metamorphic rocks. The igneous or metamorphic rocks underwent cooling, uplift, and erosion prior to becoming the substrate or basement on which new sediments accumulated.
Disconformity: 1) layers of sediment accumulate; 2) sea level drops and an erosion surface forms; 3) sea level rises and new sedimentary layers accumulate 8. How are magnetic reversals useful for dating rocks?
Times when the Earth’s field flips from normal to reversed polarity is called a magnetic reversal. When the earth has reversed polarity, the south magnetic pole lies near the north geographic pole, and vice versa. Geologists applied isotopic dating to determine the ages of rock layers in which they obtained their paleogmagnetic measurements, and thus determined when the magnetic field of the earth reversed. With this information, they constructed a history of magnetic reversals for the past 4.5 million years, and this history is called the magnetic reversal chronology. 9. Why did early scientists think that earth was < 1 mya 10. How do we identify a plate boundary?
The breaks between lithospheric plates are known as plate boundaries. We distinguish between active margins, which are plate boundaries, and passive boundaries, which are not plate boundaries. Along passive margins, continental crust is thinner than normal. We identify plate boundaries by looking at a map showing the location of earthquakes. Earthquake belts define the position of plate boundaries, because the fracturing and slipping that occur along plate boundaries as plates move generate earthquakes 11. Describe three types of plate boundaries
A boundary at which two plates move apart from each other is a divergent boundary. A boundary at which two plates move toward each other so that one plate sinks beneath the other is a convergent boundary. A boundary at which two plates slide sideways past each other is a transform boundary. 12. Describe the major features of a convergent boundary
At convergent boundaries, two plates, at least one of which is oceanic, move toward one another. One oceanic plate bends and sinks down into the asthenosphere beneath the other plate in a process known as subduction. At convergent boundaries, the downgoing plate grinds along the base of the overriding plate causing large earthquakes. A volcanic arc is a chain of volcanoes that develops behind the accretionary prism. 13. Why are transform boundaries required?
Transform boundaries are defined by a vertical fault on which the slip direction parallels the Earth’s surface. 14. What is a fold shaped like an elongate arch called?
An anticline is a fold that has an arch like shape in which the limbs dip away from the hinge. A syncline is a fold with a trough-like shape in which the limbs dip toward the hinge (the line which curvature is the greatest). A fold with the shape of an overturned bowl is called a dome 15. What types of rocks are produced from orgogenesis?
Various rocks form during orogeny. Seidment eroded from the orogeny fills a sedimentary basin next to the mountain; rocks buried deeply in the orogeny become squeezed, sheared, and heated to form metamorphic rocks; and igneous rocks intrude from below. 16. What is the difference between brittle and ductle deformation?
Deformation is a process by which rocks bend, break, or flow in response to compression, tension, or shearing. Deformation produces geologic structures including joints, folds, faults, and foliation. Brittle deformation occurs when something breaks and smashes into many pieces, and with ductile deformation, objects change shape without visibly breaking. Whether a rock deforms brittlely or ductilely depends on temperature, pressure, deformation rate, and composition. Warmer rocks tend to deform ductilely while colder rocks deform brittlely. Under great pressures, rock behaves more docilely than under lower pressures. A sudden change in shape causes brittle deformation, while a slow change in shape causes ductile deformation. Softer rocks deform ductilely while harder rocks are brittle. 17. How are stress and strain different?
We define stress acting on a plane as the force applied per unit area of the plane. Stress refers to the amount of force applied per unit area of a rock, whereas strain refers to a change in the shape of a rock. STRESS CAUSES STRAIN 18. Compare the motion of Normal, reverse, and strike-slip faults
Normal faults form during extension of the crust and the hanging wall moves down. Reverse faults form during shortening of the crust and the hanging wall moves up and the fault is steep. On as strike-slip fault, one block slides laterally past another, so no vertical displacement takes place. 19. Describe the difference between anticlines, sinclynes, and monoclynes?
Folds that have an arch-like shape in which the limbs dip away from the hinge are called anticlines. Folds with a trough-like shape in which the limbs dip toward the hinge are called synclines. A monocline has the shape of a carpet dragged over a stair step. 20. Describe the principle of isostacy
The lithosphere, which consists of relatively rigid crust and lithospheric mantle, “floats” on the softer asthenospheric mantle below. As a result, the elevation of the top surface of the lithosphere represents a balance between buoyancy force pushing lithosphere up and gravitational force pulling the lithosphere down. Geologists refer to the condition that exists when this balance has been achieved as isostasy. It exists where the elevation of the Earth’s surface reflects the level at which the lithosphere naturally floats.