GOL106 QUIZ 3

Why do deep-focus earthquakes occur along subduction zones?

Geologists had noted that trenches are associated with two other geologic features: volcanoes and deep-focus earthquakes. Deep-focus are earthquakes that originate more than 300 kilometers (190 miles) below Earth's surface. In areas far from deep-sea trenches, deep-focus earthquakes are rare.

What do carbon and oxygen stable isotope studies tell us about the past?

Oxygen isotope ratios in fossils of marine organisms reflect the temperature of the water in which the organisms lived and also sometimes reflect the presence of continental glaciers. When these ratios are plotted for an interval of geologic time that included a mass extinction, there is usually a rather abrupt shift in the ratio at the time of the extinction. Carbon isotope ratios usually exhibit excursions that parallel oxygen isotope excursions associated with mass extinctions This correspondence strongly suggests that the carbon isotope excursions also resulted from climatic change.

Describe two methods for measuring plate motion.

Radiometric dating shows how rapidly the seafloor has moved away from the ridge. Movement of oceanic lithosphere over a stable hot spot produces a chain of volcanoes that can be dated to give the velocity of the lithospheric plate. The satellite-based Global Positioning System permits geologists to measure present-day plate motions with remarkable precision.

How does the burial of dead plant tissue affect atmospheric CO2 and O2?

The atmospheric reservoir of CO2 shrinks and the atmospheric reservoir of O2 expands.

How do glaciers promote chemical weathering?

The glacier, which has temporarily receded in summer, is grinding up the rock beneath it. This grinding produces greater rock surface area, which accelerates the process of chemical weathering.

Describe the origins of the major plate driving forces.

The idea that continents move horizontally over Earth's surface—an idea labeled continental drift—had been proposed previously, but it had failed to receive general support. Later, plate tectonics, a concept developed from the 1950s through the 1970 became the modern version. The driving force behind plate tectonics is convection in the mantle. Hot material near the Earth's core rises, and colder mantle rock sinks.

How does paleomagnetism demonstrate that continents have moved in the course of time?

t refers to the magnetization of ancient rocks at the time of their formation. When geologists first began to measure rock magnetism, they found that the magnetism of recently magnetized rocks was consistent with Earth's current magnetic field. The magnetism in older rocks, however, had different orientations. As data accumulated, it began to appear that Earth's magnetic north pole had wandered. A plot of the pole's apparent positions, as indicated by rocks of various ages in North America and in Europe, showed that the pole seemed to have moved to its present position from much farther south, in the Pacific Ocean. However, the path obtained from European rocks differed in detail from that obtained from North America. This shows that plates moves instead of poles.

Lithospheric plates move because of all of the following except _______.

the magnetic and gravitational fields of Earth act on the plates to pull them downward

How does the angle of subduction relate to the rate of plate movement?

A change in the angle of subduction usually reflects a change in the rate of plate movement. When the plate that is not being subducted is moving rapidly toward the subduction zone, it overrides and bears down on the upper part of the subducted slab. Migration of the subduction zone causes the subducted slab to dip at a low angle. In contrast, when the plate that is not being subducted is moving too slowly to roll the subduction zone back rapidly, the subducted slab is free to rotate toward a vertical position under the influence of gravity.

How does the burial of dead plant material affect atmospheric CO2 and O2?

Burial of dead plant tissue which contains reduced carbon, upsets the balance of the global photosynthesis respiration cycle. Because it prevents some reduced carbon from decomposing and returning to the atmosphere as CO2 the atmospheric reservoir of CO2 shrinks. At the same time, oxygen that would have been used to decompose the organic matter if it had not been buried remains in the atmosphere, and the atmospheric reservoir of O2 expands.

Explain how a geologist might distinguish an orogen added to the continent primarily by accretion from one added primarily by stabilization.

Continental accretion occurs at the margin of a single large craton. They can result from the suturing of an island arc or microcontinent to a large craton along a marginal subduction zone. It can also result from the compression and metamorphism of sediments that have accumulated along a continental shelf. The latter process is sometimes referred to as orogeni stabilization because it thickens the crust and hardens both unconsolidated sediments and soft sedimentary rocks

What controls the ratio of Mg to Ca in the oceans? How does this affect the mineralogy of limestones?

The primary cause is change in the volume of mid-ocean ridges, which results from changes in the rate of seafloor spreading, as well as change in the total length of mid-ocean ridges present on Earth. When rates of oceanic crust production are high, low Mg2+/Ca2+ ratios produce calcite seas, in which calcite forms ooids and marine cements, and organisms with calcite skeletons become successful reef builders. Aragonite and high-magnesium calcite instead play these roles when the rate of oceanic crust production is low, as it is today, and the Mg2+/Ca2+ ratio of seawater is therefore high.

Describe one positive feedback and one negative feedback process that influence global warming.

We can consider what will result from changes in vegetation at high northern latitudes as humans' burning of fossil fuels produces global warming in the future as a positive feedback. The fertilization effect of CO2 on plants is a negative feedback against the buildup of this gas in the atmosphere.

Explain how continental rifting begins. Describe the sedimentary environments associated with this process.

When rifts develop, they often begin as three-armed grabens at plate boundaries known as triple junctions. When a large continent breaks apart, the jagged line along which it divides often represents a composite structure formed from arms of several three-armed rifts. A three-armed rift usually contributes two of its arms to the composite rift, while the third arm becomes a failed rif or a plate tectonic dead end. More than one kind of plate boundary can meet at a triple junction. Each boundary may consist of a spreading zone, a subduction zone, or a transform fault.