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Answers to Chapter 4 Review Questions
The evidence used by Wegener to support his idea of moving continents included matching continental shapes and geological features on either side of the Atlantic; common terrestrial fossils in South America, Africa, Australia, and India; and data on the rate of separation between Greenland and Europe.
The primary weakness of Wegener’s theory was that he had no realistic mechanism for making continents move.
Contractionists believed that mountains formed because the crust wrinkled into mountains as Earth cooled and contracted. Permanentists believed that mountains formed by the geosynclinal process.
In the late 19th century the trans-Atlantic paleontological matchups were explained by assuming that there must have been land bridges between the continents at some time in the past.
Prior to 1920, ocean depths were measured by dropping a weighted line over the side of ship. Echo sounding techniques were developed at around that time and greatly facilitated the measurement of ocean depths.
Paleomagnetic studies showed that old rocks on the continents indicated different locations for magnetic north than the position of magnetic north today. They also showed that the difference in pole position from data on different continents increased progressively for older and older rocks. This implied that either Earth had more than one magnetic pole moving around, or that the continents had moved.
Trenches associated with subduction zones are the deepest parts of the oceans.
The ocean ridge areas are the youngest parts of the sea floor and thus there hasn’t been time for much sediment to accumulate.
It was (and still is) thought that high heat flow exists where mantle convection cells are moving hot rock from the lower mantle toward the surface, and that low heat flow exists where there is downward movement of mantle rock.
Earthquakes are consistently shallow and relatively small at ocean ridges. At ocean trenches earthquakes become increasingly deep in the direction that the subducting plate is moving.
In the Hess model new crust was formed at ocean ridges. Crust was recycled back into the mantle at the trenches.
Hess’s model did not include the concept of tectonic plates.
A mantle plume is a column of hot rock (not magma) that ascends toward the surface from the lower mantle. Mantle plumes last tens of millions of years to hundreds of millions of years.
(a) Between the ridge segments there is movement in opposite directions along a transform fault. (b) Outside of the ridge segments the two plates are moving in the same direction and likely at about the same rate. These regions are known as fracture zones.
Tectonic plates are made up of crust and the lithospheric (rigid) part of the underlying mantle. The mantle part ensures that the very different oceanic and continental crust sections of a plate can act as one unit.
A mantle plume beneath a continent can cause the crust to form a dome that might eventually split open. Several mantle plumes along a line within a continent could lead to rifting.
Subduction does not take place at a continent-continent convergent zone because neither plate is dense enough to sink into the mantle.
Continental rifting is taking place along the East Africa Rift, and sea floor has recently been created in the Red Sea and also in the Gulf of California.
The accumulation of sediment at a passive ocean-continent boundary will lead to the depression of the lithosphere and could eventually result in the separation of the oceanic and continental parts of the plate and the beginning of subduction.