Crater Lake

For a Google Earth .kmz file to explore Crater Lake, scroll to the very bottom of the page.

Crater Lake is the only National Park in Oregon and is well know as one of Oregon’s natural wonders. The volcano’s current topography was created during the last catastrophic eruption occurring 7700 years ago (http://volcanoes.usgs.gov/volcanoes/crater_lake/crater_lake_geo_hist_135.html). This eruption devastated the surrounding area and created the deepest lake in North America. Crater Lake is situated in south central Oregon and is part of the Cascade Volcanic Arc. This is another composite volcano that has been created from various eruptions and culminating in an extremely explosive eruption forming the caldera seen today.

First things first, the main focus of this volcano is the lake obviously, and that lake has formed in a volcanic feature referred to as a caldera. A caldera is like a volcanic crater but is more specifically created by the collapse of land above a magma chamber after eruption. Generally, calderas are larger than craters and much deeper. The caldera at Crater Lake is a 5 mile by 6 mile (8 x 10 km) and is measured currently at 1,949 ft (594 km) (http://volcanoes.usgs.gov/volcanoes/crater_lake/). This is an unbelievably deep lake. The other thing about the lake that is most notable is the clarity of the lake (see Figure 1). There is a reason for this clarity and that is due to the water that filled the caldera. The filling of the caldera is only done through rainfall, snowfall, and melting of snowpack from the caldera rim. There are no stream inlets into or stream outlets from Crater Lake. This keeps lots of extra sediment from getting into the water and keeps the water somewhat still preventing sediments from being entrained, or basically keeping them from being kicked up into the water column. This lack of sediments keeps the water clear and allows visitors to see at least a 100 ft of depth into the lake (http://www.nps.gov/crla/faqs.htm). So what force was great enough to create a caldera so deep? Well, prior to Crater Lake forming the volcano was called Mt. Mazama and the volcano began forming 420,000 years ago (Klimasauskas et. al., 2002). These first eruptions created Mt. Scott, which can still be seen today to the east-southeast of the current caldera. Overtime, eruptions kept occurring and moved the newer volcanoes further to the west and these various eruptions and lava flows have been exposed in the caldera walls. At its greatest height Mt. Mazama was at least 12,000 ft (3700 m) tall (http://volcanoes.usgs.gov/volcanoes/crater_lake/crater_lake_geo_hist_134.html). To put this in perspective, the current Mt. Hood cone is 11,250 ft (3,429 m), almost 1000 ft shorter than Mazama at its tallest. Explosive eruptions had been occurring at Mt. Mazama for 30,000 years prior to the 7,700 culminating eruption that created some of the main attractions seen around the lake today such as Llao Rock, Redcloud Cliff, and the Pumice Castle (sometimes called the “Fairy Castle”) (Klimasauskas et. al., 2002). Though some of the eruptions were quite explosive, none were nearly as powerful as the eruption 7,700 years ago.

The eruption started from a single vent on the northeastern side of the volcano (Klimasauskas et. al., 2002). The eruption caused a huge plume of ash to enter into the atmosphere to about 30 mi in height. Winds carried this ash a tremendous distance. Much of the ash covered the Pacific Northwest and southern Canada and in fact some even went as far as the Greenland Ice Sheet where it accumulated in a great enough thickness to be used by scientists to verify the date of eruption (Zdanowicz et. al., 1999). So much magma was discharged from the volcano that the top of the cone began to collapse in on itself forming what geologists refer to ring faults, basically arcuate faults that form inside the volcano. As these faults activated and the top of the volcano fell into the magma chamber, the faults started to erupt magma as well causing pyroclastic flows to run downslope and fill surrounding valleys with pumice and ash for up to 300 ft (100 m) (Klimasauskas et. al., 2002). After the eruptions finished the resulting 5 by 6 mi caldera had formed (see Figure 2).

Several eruptions occurred after the eruption that occurred 7,700 years ago. Most eruptions took place over the 750 years after the main eruption and include Wizard Island, Miriam Cone, and the Central Platform (Klimasauskas et. al., 2002). The very last known eruption at Crater Lake 4800 years ago was a small dome that erupted underwater on the eastern flank of Wizard Island (2002, see Figure 3). Since then the volcano has remained quiet and is just accumulating sediments on the floor of the caldera. This doesn’t mean that the volcano will not erupt in the future. Any eruption in the future is assumed to take place in the same area that the most recent eruptions took place but will most likely not be as large as the 7,700 year eruption. This is because the magma in the chamber is not voluminous enough to create such a large eruption. However, if there were to be an eruption, there are several hazards that could occur such as tephra ejections from the caldera due to the interaction of water and magma and larger eruptions could even create pyroclastic surges (2002). There are also several non-eruptive hazards that could occur such as landslides from the caldera walls and movements of faults near the volcano that could create earthquakes. Landslides from the caldera walls could cause waves that flood shoreline areas around the lake but it is unlikely to create a wave that overtops the caldera walls. Local earthquakes could also cause landslides that form large waves. Either way, Crater Lake still has the possibility to create hazards to the surrounding area.