Lood B Asalts

The Indian Deccan traps occur over an area of at least 1 million km2. (Deccan is Sanskrit for southern; traps is Swedish for staircase, which the edge of a giant sequence of nearly horizontal lava flows sometimes resembles.) In places they are 2 km thick. Their total volume exceeds 1 million cm3, more than the outpourings of all the Ring of Fire volcanoes put together. Scientists believe that the Dec-can eruptions produced 30 trillion tons of carbon dioxide, 6 trillion tons of sulfur, and 60 billion tons of halogens, gases that enhance the greenhouse effect.

To reflect their vastness and mode of eruption, geologists call these enormous outpourings flood basalts.11 They occur in the geologic record from the Precambrian to the Tertiary and on nearly every continent. Examples are the Columbia River country of Oregon and Washington, the Parana basin of Brazil, the South African Karroo, and the Siberian traps. Before the advent of plate tectonics, their origin was a mystery, but Tuzo Wilson, tolerated but unheeded in my graduate school seminar, provided the key insight.

Wilson was a master at providing an innovative interpretation of facts that had stumped others, in this case, that the Hawaiian Islands lie along a straight line. Wilson noted, as had many, that the islands to the southeast, where Kilauea and the other great volcanoes are active today, are youngest and that they grow steadily older in a line to the northwest. Beyond the most northwesterly island lies a linear chain of submarine seamounts that become progressively deeper and older, also to the northwest. Unlike the many others who knew these facts, Wilson deduced their meaning. Deep in the earth's mantle, beneath the present active volcanoes, lies a hot spot, a zone that melts periodically, sending jets of less-dense magma up to be extruded onto the surface. But plate tectonics tells us that the rigid, uppermost surface layer of the earth moves horizontally over the fluid mantle, so that as time passes different sections of the crust lie over a given spot in the mantle deep below. Now we can begin to see what Wilson envisioned: A fixed hot spot deep beneath the central Pacific crust episodically spurts magma toward the surface, but by the time each spurt arrives, the crust has moved laterally so that the new eruption occurs at a different point on the surface, producing a new volcano. The former volcanic sites move progressively further and further away from the point directly above the hot spot, becoming older, eroding, and finally disappearing beneath the sea. From this, Wilson deduced that the crust in the mid-Pacific has been moving steadily to the northwest, over the fixed position of the deep hot spot. Since the ages of the Hawaiian volcanoes and some of the seamounts are known precisely, we can calculate the speed at which the crust there is moving; the rate checks exactly with results obtained using the magnetic reversal time scale.

Jason Morgan of Princeton extended Wilson's idea to explain the origin of flood basalts. When he reconstructed the past positions of tectonic plates, he noticed that the basalt provinces lie directly over present-day active volcanoes. For example, the hot spot that is now under Yellowstone National Park once produced the Columbia River basalts. Morgan thought that hot spots at the base of the earth's mantle sometimes produce huge, bulbous masses of hot, low-density, low-viscosity basaltic magma, that then "float" to the top of the mantle, like a hot air balloon rising through colder, denser air. There these giant mushrooms of magma rapidly decompress and flow out on the surface. Some think all this happens in only a few million years. Thus hot-spot eruptions can produce not only midoceanic islands but, perhaps, giant floods of basalt.

In the late 1980s, the Ocean Drilling Program explored the western Indian Ocean and found a chain of seamounts extending from southwest India to the island of Reunion. The ages of the seamounts ranged from 2 million years near Reunion to around 55 million to 60 million years just south of India. A reconstruction of movement of the plates in the Indian Ocean area shows that at the end of the Cretaceous, the Deccan trap province resided just over the hot spot that is now feeding Reunion.

Was this article helpful?

0 0

Post a comment