By Matt Johnston
Eastern Illinois University
John Wesley Hillman was a young prospector in search of a lost mine. Little did he know that while in search of his mine, he would stumble upon one of the most majestic geologic phenomena in North America, and possibly the world. Wesley was following his mule, and all at once the mule stopped at the edge of a cliff, and as Wesley reached the top, he stood in amazement of what is now called, Crater Lake. He originally called the lake "Deep Blue Lake", but in 1869 a group of people from Jacksonville, Oregon decided to call the lake "Crater Lake" (Harris,1977). However, this isn't a very ideal name due to the fact that Crater Lake is actually a caldera.
A caldera is formed from the largest and most explosive volcanoes. These volcanoes eject tens to hundreds of cubic kilometers of lavas, and pyroclastics out onto the earth's surface. With such a large volume being ejected, this creates a massive void space within the interior of the earth. Which in turn causes a collapse of the overlying surface forming a caldera. The Crater Lake caldera resides in what is left of Mount Mazama. Before the eruption occurred, Mount Mazama had an elevation of around 12,000 feet. The caldera is around 5 miles across from rim to rim (Harris,1977). It formed during a catastrophic eruption of more than 50 cubic kilometers of magma (Wood & Kienle, 1990). In addition to this massive amount of magma, huge volumes of ash, dust, and other pyroclastic materials were shot into the air. These materials swept down the slopes in huge avalanches of ash. The event happened around 6,600 years ago. The official depth of Crater Lake is 1,932 feet, which makes it the deepest lake in the United States (Harris, 1977). Another interesting fact about the lake is that it has no inlet or outlet. This means that all of the water that occupies the lake is due only to precipitation and surface runoff from rain and snow. Up until 60 years ago, there was no aquatic life. Since then the lake has been stocked. The level of the lake has remained fairly constant from year to year (Harris 1977).
The Geologic history of Mount Mazama goes well beyond the formation of Crater Lake. The history has been well documented in the 1977 book Geology of National Parks by Ann G. Harris. I have included a brief summary of this history. The mountain's history is similar to that of its sister mountains of the Cascades. These include Mount Rainier, Shasta, Hood, and some others. These mountains make up the High Cascades. Mazama did most of its growing during the Pleistocene Epoch. Just to the west of the High Cascades, is a belt composed of older lavas and pyroclastics that form the Western Cascades. These are much older than the High Cascades, and were deeply eroded before the Cascade volcanoes began to erupt. Both of these ranges are the result of subduction of oceanic crust along the western coast of the United States. At the end of the Miocene, the High Cascades had nearly risen to there present day elevation. Pliocene basalts and andesites were beginning to build up. Mount Mazama was continuing to be built up by lava flows and pyroclastic flows. Mount Mazama was somewhat set apart from its sisters because it had many parasitic cones and other eruptions that occurred along its flanks. At this point, the area was also subject to extreme glaciation. Once at its maximum height, eruptions tended to be more siliceous in content. Eruptions were becoming increasingly violent, and with these violent eruptions, came large amounts of dacitic pyroclastics. The eruptions began to shift off of the central vent forming the Northern Arc of Vents. It is thought that these formed as a result of a giant fissure extending into the magma chamber. This arc of vents would later form the northern wall of the caldera. From this arc, came massive amounts of andesitic flows followed by dacitic flows. After a lapse in volcanic activity, magma once again began to rise. Once again this was followed by a series of violent eruptions of dacitic pyroclastics. The pyroclastics were followed by several lava flows sweeping down the northeast side of the volcano. Eventually leaving behind a welded tuff. Once again, following this event was a period of inter-volcanism. Activity in the mountain began again with steam emissions. Next, light ash falls, and other pyroclastics began to occur. These events started off as fairly calm but greatly increased in intensity. Up to 20 feet of ash had accumulated at the base of the mountain. These ash deposits can be seen south as far as California and Nevada, as far east as Montana, and north up into Canada. Radiometric dates of this ash dates it at about 6,600 years old. As mentioned before, these ash flows were followed by huge avalanches of lava. Anything that was combustible would have been incinerated. The final avalanches are thought to have come from very deep in the magma chamber, due to the presence of dark scoria. With all of this material being ejected from the mountain, this created a huge void space. The interior of the mountain was no longer able to support the overlying material, and collapsed into a massive caldera. To aid in the understanding of the eruptive history, I have included a stratigraphic column of the area (figure 1 - click here).
At this point, the volcanism still did not cease. For some time afterward, small spurts of volcanism have built up another small cinder cone within the caldera. This cinder cone is known as Wizard Island. It stands 2,600 feet high, but only 774 feet is exposed over the water line (Harris 1977). The youth of this volcano leads many people to believe that we are yet to see a lot of activity at Crater Lake.
Harris, A.G. (1975). Geology of National Parks. Kendall/Hunt Publishing Company.
Wood & Kienie. (1990). Volcanoes of North America: United States and Canada. Cambridge University Press, 345,193-195.
Photo is courtesy of W.E.Scott and the USGS