Geological History - Yellowstone Treasures - page 368

Among the many questions still to be answered about what exactly has happened and is hap-
pening even today in Yellowstone, two stand out.
I. What is the cause of the Yellowstone hot spot?
Geoscientists are studying two main possibilities for explaining how and why the Yellowstone
hot spot formed and then migrated across the Snake River Plain to Yellowstone. Evidence for
both theories is mainly based on interpretation of the chemical composition of the volcan-
ic rocks at Yellowstone and on interpretation of magnetic data, gravity data, and earthquake
waves that travel through the rocks below Yellowstone and the Snake River Plain.
M ANTLE P LUMES. In this older theory, plumes are thought to come from hot areas at the top of
Earth's metallic core, 1,800 miles (2,900 km) below the surface. The heated rocks are less dense
than their neighbors and so rise as a column to the surface of the earth. Some of the rock melts
on its way up. The idea is that as the North American plate traveled southwest over a station-
ary mantle plume, it left a trail of calderas that form the Snake River Plain. A possible Yellow-
stone hot spot plume is shown in Figure 5B ( page 305 ) . The figure shows the plume today at
its current location under Yellowstone. About 17 million years ago, when present-day north-
central Nevada was above the plume, large amounts of volcanics spilled out on the ground.
Then, starting about 14 million years ago, the first of the calderas formed when southwest
Idaho was above the plume. A deep mantle plume that stayed in one place while the North
American Plate moved over it explains two things: how the trail of calderas formed, and how
chemical elements from deep inside the earth reached the surface. How the plume punched
its way through the subducting plate is not understood.
There are several difficulties with this idea. Analysis of earthquake waves identifies a plume
of hot and partially melted rock below Yellowstone, but only in the uppermost 300 miles (500
km), not the full 1,800 miles (2,900 km) to Earth's core. The chemistry of the volcanics indic-
ates that they come from very deep, although some geoscientists disagree about this. Another
difficulty is that there is another track of calderas, called the Newberry volcanic chain, starting
at the same place and time as the Yellowstone track but progressing northwesterly into Ore-
gon. This direction contradicts the northeasterly trend of the Yellowstone track. The North
American Plate cannot have been traveling in two very different directions at the same time.
A F RAGMENTING S UBDUCTING P LATE. A newer theory is that the subducting oceanic plate tore
apart and became fragmented. The edge of a sliver of the fragmented plate is illustrated in
Figure 5C. In this theory, the subducting oceanic plate, instead of diving steeply deep within
the earth, descended at a very shallow angle. As it did so it broke into several pieces. Magma
formed at the leading edge of the largest sliver, creating the hot spot. This orphaned segment
of the oceanic plate continued to move northeast under the overriding southeastward-mov-