Saturday, January 10, 2009
Yellowstone's Earthquake Activity
I've been following the news about the earthquakes in Yellowstone. Beginning on December 26 there have been a swarm of earthquakes under Yellowstone Lake. There was a lull in activity for a few days, but they have started back up although the center has moved about 10 miles. While most have been very small quakes, the largest have been up to 3.9 on the Richter Scale. While I first saw the news about the quakes on MyYahoo, I keep up to date with Jim MacDonald's Yellowstone Newspaper. He surfs the web and provides links to news stories, articles and blogs related to Yellowstone National Park.
I first visited Yellowstone as a child. My husband and I have been back a number of times over the years, staying longer each visit. We keep the natural trail guides from each trip, because as an active geothermal area the information as to activity changes over time. It is interesting to see which features are active today vs when we first went. During our early visits to the park, no mention was made of the caldera because the Yellowstone caldera is so large that it took satellite imagery for it to be discovered. According to Wikepedia, a caldera is a cauldron like volcanic feature formed by the collapse of land following a volcanic eruption. A caldera forms after a volcano empties its magma chamber and the chamber's ceiling is no longer strong enough to hold the volcanic rock above it. The first caldera I ever saw was the Valles Caldera in New Mexico. (Remember - at the time we did not know Yellowstone was a caldera.) It is 14 miles in diameter and we drove straight across it before it became a National Preserve. It was an impressive site to be on the flat land between the far away ridges of the rim of the caldera.
The discovery about Yellowstone's explosive volcanic past occurred during the 1980's. Since then a lot of study and research has revealed an amazing history of eruptions - both great and small. Yellowstone is classified as a supervolcano. According to Wikepedia, a supervolcano is a volcanic eruption which is substantially larger than any volcano in historic times (generally accepted to be greater than 200 cubic kilometers). This kind of eruption is typically sufficient to cause a long-lasting change to weather (such as the triggering of an ice age) sufficient to threaten the extinction of species, and cover huge areas with lava and ash. I've seen the BBC documentary,Supervolcano . It gives a very scary presentation as to what could occur if Yellowstone had that kind of eruption today.
Because I do pay attention to news about Yellowstone, I know that a few years ago, they had to close part of Norris Geyser basin because the ground had gotten too hot and there were noxious gases present. I also know that the ground has been rising at a more rapid pace than "normal" the last few years. Since 2004, the park has been rising 3 inches a year more than three times faster than ever measured before. So when the news about these earthquakes that were happening at a much higher than normal frequency, I was intrigued and concerned. While I do not really believe that these are precursors to a volcanic event that will cause extinctions, I believe that something very interesting is happening up there right now.
Here are some things I learned as I looked deeper into Yellowstone's volcanic past.
Geologic time is always amazing because it tends to be measured in millions of years and hundreds of thousands if years orather than decades or hundreds of years. Yellowstone's caldera forming eruptions go extremely far back in time:
2,100,000 years ago - Caldera forming eruption that created Huckleberry Ridge
1,300,000 years ago - Caldera forming eruption that created the Island Park Caldera
640,000 years ago - Caldera forming eruption that created the Lava Creek Tuff
Deposits of ash from these eruptions have been found as far away as Iowa, Louisiana, and California.
If Yellowstone were to erupt at one of these levels all of North America and in some ways the entire Northern Hemisphere would have severe effects.
Fortunately, not all of Yellowstone's eruptions have been that catastrophic. About thirty eruptions of rhyolitic lava flows have amost filled the Yellowstone Caldera since the last major caldera forming eruption 640,000 years ago. These flows may move slowly, but are very destructive. The last major lava flow seems to be about 70,000 years ago.
Earthquakes are common - in fact 1,000 to 3,000 earthquakes are typical in a given year. Another source said the usual number of quakes was 2000 per year. However, with over 900 in roughly 2 weeks, it is easy to see that the current activity is more than the norm. Earthquakes can change the patterns of the geyser eruptions - making some more active and others less so. The 1959 Hebgen earthquake was one of the stronger ones at 7.5 on the Richter Scale. Part of the mountain slid into a campground and dammed the Madison River creating Hebgen Lake. 28 people in the campground were killed. A 1975 quake near Norris Geyser Basin registered 6.5 and was felt throughout the area.
The last kind of eruptive activity are hydrothermal explosions.From Hanksville.org: "Hydrothermal explosions occur when ground water, heated above the boiling point (superheated) expands explosively after a rapid decrease in pressure as the water nears the surface. Ten such hydrothermal explosion craters are found in Yellowstone." 13,800 years ago the largest hydrothermal exposion created a 1.5 mile crater at Mary Bay. A 1989 hydrothermal explosion at the Porchop Geyser in Norris Geyser Basin threw rock debris fifteen feet across the spring. Hydrothermal exposions seem to be independent of volcanic magma flow - none were followed by an eruption of magma.
In 2003, a long linear fissure on a hillside above Nymph Lake north of Norris Geyser basin vented steam and threw rocks.
The geysers, mud volcanoes, and hot spring pools are also effects of the magma heat source under Yellowstone.
This chart summarizes the kinds of activity I've been describing.
Right now Yellowstone National Park is buried underneath a thick layer of snow. It is hard for scientist to get to the surface area near where these earthquakes originate. I wonder if there will be evidences of hydrothermal explosions in the spring when everything thaws out. It is going to be interesting to watch the events unfold in Yellowstone. Most likely, these quakes will die down and become just another footnote in the history of Yellowstone. But, it is always possible that they are the precursor of something larger. Will we be the generation of people that get to witness a new lava flow in Yellowstone? I have to say I would rather not be in the generation of people who survive a cataclysmic caldera forming eruption.
Watch for part two where i will compare Yellowstone's caldera forming earthquakes with other earthquakes around the world.
For more reading, here are some of the sites I used to research this blog:
The charts came from the USGS service site.
US Geological Survey
Yellowstone National Park - Calderas
For most recent earthquake activity at Yellowstone: University of Utah Seismograph Stations
The USGS update as of January 8th also reports 900 earthquakes in Yellowstone between December 26, 2008 and January 8, 2009.
University of Utah's site has a Deseret News article from 1975 listing the 1959 Yellowstone Quake as 7.1. However, the The USGS site reports it as 7.5 and Wikepedia's 1959 Yellowstone Earthquake article lists the magnitude as 7.3-7.5.
My husband found a USGS Animated Earthquake Map for Yellowstone. It shows where last weeks quakes were and where the new ones are in relation. (Granted, the link worked a few minutes ago and isn't working as I post this, hopefully it will work for my readers.)