It is not clear whether all this volcanism can be attributed to a single mechanism. If mantle plumes are responsible for this volcanism, how many are required? Why does the volcanism in Africa have such a variety of forms? The western United States is another area of extensive volcanism. The distribution of recent surface volcanic rocks (with ages of less than 7 million years) is given in Figure 1–54. Because the San Andreas fault in California is recognized as a major plate boundary between the Pacific and North
American plates, the volcanism of this area may be classified as being platemargin-related; however, the volcanism extends more than 1500 km from the plate margin. Yellowstone National Park in the northwest corner of Wyoming is the center of extensive recent volcanism that occurs at the end of a track of volcanism that extends along the Snake River plain. For this reason the Yellowstone area is classified as a hotspot (see Figure 1–14), and it is thus a possible site of a mantle plume. The ages of surface volcanic rocks on the Snake River plain are given as a function of the distance from Yellowstone. The ages of the oldest volcanic rocks tend to increase with distance from Yellowstone; however, young volcanic rocks occur along much of the length of the Snake River plain. It is difficult to associate these young volcanics, which extend over a distance of some 500 km, with a single melting 80 Plate Tectonics anomaly beneath Yellowstone. Also, it is clear from Figure 1–54 that very young volcanics extend throughout the western United States. Some of these volcanics form the volcanic line associated with subduction of the Juan de Fuca plate beneath Washington, Oregon, and northern California. But much volcanism remains unexplained. Small amounts of volcanism appear to be associated with the tensional tectonics of the Rio Grande rift in New Mexico and the Basin and Range province in Nevada and Arizona. Further discussion of this is given in the next section. Variations in surface heat flow can also be correlated with the distribution of plates. On a worldwide basis the average surface heat flow is about 87 mW m−2. For rocks with an average thermal conductivity this corresponds to an increase in temperature with depth of 25 K km−1. The heat being lost to the surface of the Earth can be attributed to the heat produced by the decay of the radioactive isotopes and to the cooling of the Earth. Plate margins and other areas where volcanism occurs are generally characterized by surface heat flows much higher than the average value just given. The high heat flows may be the result of a thin lithosphere or, in some cases, the migration of magma through a relatively thick lithosphere.
The cooling of the oceanic lithosphere as it spreads from an oceanic ridge leads to a systematic decrease with age in the heat flux to the seafloor.
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