Mt. Hood

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

Mt Hood is the highest peak in Oregon and perhaps the most recognizable volcano in Oregon due to its proximity to Portland, Oregon’s largest city. Mt Hood is 11,250ft in height and lies approximately 50 miles from Portland. The volcano is part of the current Cascade Volcanic Arc, also known as the High Cascades. The creation of this volcano is due to the subduction of the Juan de Fuca oceanic plate beneath the North American continental plate. The result of subduction brings water-rich minerals into the mantle and when the sediments get deep enough the water present in the minerals will be driven off because these water-rich minerals are no longer stable. This is referred to as metamorphic dewatering (wordy, I know), and that loose water

works its way up into the overlying mantle that is already hot. The added water lowers the minerals melting point in the mantle and allows for the mantle to melt, this is called flux melting (see Figure 1). The magma created then goes towards the surface due to high pressure and the melt’s lower density. So, when that magma gets to the surface and erupts we get a volcano.

Most of the volcanoes in the Cascade Volcanic Arc (notice I didn’t say mountains), are composite volcanoes, or stratovolcanoes. These volcanoes are composed of many different lava flows that tend to be a higher viscosity. So we could say that they are made from a composite of different lava flows. Also, because these lava flows tend to be higher viscosity, the lava will mound up and create a steeper volcano than seen with shield volcanoes. Composite volcanoes are known for their fairly violent eruptions, like the one seen from Mt. St. Helens in 1980. That was the most violent eruption in the Untied States since Novarupta, Alaska 1912. However, Mt. Hood does not show this same catastrophic, explosive eruptive history. Instead, Mt. Hood tends to show more effusive, high viscosity eruptions that rarely go further than 6 to 8 miles from their source (Gardner et al. 2000). These lava flows tend to mound up into lava domes and can reach heights of several hundred feet. These growing lava domes many times exceed their vertical strength and gravity forces them to collapse. These collapses can result in fast moving pyroclastic flows and debris avalanches, which, in turn, can initiate volcanic mudflows, known as lahars. These lahars can travel extremely far, having even traveled down Hood River valley, across the Columbia River and up the White Salmon River of Washington State, at least 30 miles away at their farthest. So, unlike Mt. St Helens, Mt. Hood may not be explosive in its eruptive nature but it still has the potential to create some very serious volcanic hazards.

The most recent volcanic activity from Mt. Hood occurred in the mid-1800’s when early settlers reported small steam explosions from the Crater Rock Area on the southern flanks of the volcano (http://volcanoes.usgs.gov/volcanoes/mount_hood/mount_hood_geo_hist_94.html). Unfortunately, there are no deposits that have been found that can be tied to these small scale eruptions and so it is unclear how much validity these particular observations hold. The youngest eruptions that have associated verifiable volcanic deposits are those related to what is called the Old Maid eruptive period that occurred around the 1790s. This period of eruption followed 1000 years of dormancy and lasted for about a decade (http://volcanoes.usgs.gov/volcanoes/mount_hood/mount_hood_geo_hist_94.html). The eruptions during this time consist of the growth of a lava dome and multiple collapses near the current location of Crater Rock. The collapse of these domes created lahars that flowed down both the Sandy and White River valleys to devastating effects. The Sandy River became so choked with sediments from these repeated lahars that it buried the valley between Sandy and Troutdale in over 65ft of debris (Scott et al., 1997). There was so much sediment deposited in the Sandy River that when Lewis and Clarke visited the river in 1805 and 1806 the called it Quicksand River for the muddy remnants that the lahars had left behind (2000). If you go to the Sandy River now it mainly has a more gravelly substrate, meaning much of the sediments have moved on to the Columbia River and perhaps even as far as the Pacific Ocean.

Another significant lahar was created 1500 years ago that was created during the Timberline eruptive period (http://volcanoes.usgs.gov/volcanoes/mount_hood/mount_hood_geo_hist_94.html). This eruptive period was characterized by lava dome growth,

pyroclastic flows, and lahars, but also very large debris avalanches. One particular debris avalanche scarp can still be seen as an amphitheater around Crater Rock (Scott et al., 1997). This debris avalanche initiated a lahar that was so powerful that it carried boulders as large as 8ft in diameter and deposited them 30 ft above the present level of the Sandy River in the towns of Wemme and Wildwood over 10 miles away from Mt. Hood and traveled 55 miles from its source (Gardner et al., 2000, Scott et al., 1997; see Figure 3).

It is safe to say that Mt. Hood does pose dangers to the surrounding areas, especially in regards to lahars in the surrounding river valleys. Though Mt. Hood shows no signs of imminent eruption, it is being monitored for any signs of activity. Mt. Hood is monitored by several global positioning system (GPS) receivers, seismic stations, temperature gauging stations, and several webcams (http://volcanoes.usgs.gov/volcanoes/mount_hood/mount_hood_monitoring_93.html). If there is any indication of activity geologists will be ready to warn the public. There is one caveat to volcanic hazards on Mt. Hood (and any other volcano with snow and glacial ice on its flanks), lahars can also be formed outside of an eruption during times of great rainfall or even outburst flooding. Once such event occurred on Christmas Day in 1980 (Gardner et al., 2000). This lahar event was triggered by intense, warm rainfall that caused rapid snow melt. The lahar that was triggered travelled down Pollalie Creek at approximately 25 to 35 miles per hour, killing one camper near the mouth of the creek and temporarily damming the East Fork Hood River (Scott et al., 1997). The resulting dam break created a flood that washed out 5 miles of Highway 35, 3 bridges, and one state park and resulted in an estimated cost of $13 million (Gardner et al., 2000, Scott et al., 1997). Though lahars created during non-eruptive periods, like the one occurring in 1980, are only a fractions of the size of those created during an actual eruption, they are nonetheless dangerous and can cause significant damage to areas surrounding the lahar’s chosen river valley.