Volcanic activity in the western United States today is almost non-existent. However, around 25 to 40 million years ago, an event termed the Mid-Tertiary Ignimbrite Flare-Up produced a massive amount of volcanic ash and lava. Many questions arise from this event, such as where did the volcanic rock come from, what was the journey like to the surface, and what forces within the earth were important to allow this event to happen.
By looking at the chemical structure of the volcanic rocks, earth scientists can estimate the composition of the original "parent" material. The parent material for the flare-up event probably originated at depths of a few to several tens of miles below the surface. Extra heat from an unknown source may have caused the rock to melt a bit, and this liquid rock called magma began to rise toward the surface. Another way to make rock melt is to bring it closer to the surface. As the liquid rock rose toward the surface, the liquid interacted with the rock it was passing through, acquiring and releasing elements. Before the liquid rock reached the surface, the liquid would be held in large subsurface chambers where solid crystals had time to form from the liquid. At the end of the journey, the liquid rock was erupted onto the surface through a large volcano and quickly chilled into solid rock. The San Juan volcanic field in southwestern Colorado was made by several of these volcanoes. Some liquid rock flowed out of the volcano in the form of lava flows. Other high temperature material was pulverized as it blew out of the volcano and then became welded together upon cooling - this is an ignmibrite rock.
Forces similar to the ones that move the earth's surface around today may have played a role in the flare-up event. Prior to the flare-up, a mountain-building event had just occurred in the western United States and volcanoes had been active along the West Coast. The rocks that were melted to produce the volcanoes along the West Coast may have also produced the volcanoes in the flare-up event. Understanding the forces in the earth responsible for the flare-up event is still a challenge.
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