July 19, 2014
Researchers Provide New View Of Mount Rainier’s Volcanic Plumbing
April Flowers for redOrbit.com - Your Universe Online
A new, detailed image of the volcanic plumbing beneath Mt. Rainier has been created by a group of researchers from the University of Utah, Woods Hole Oceanographic Institution, the College of New Jersey and the University of Bergen, Norway. The study, published in Nature, measured how fast the Earth conducts electricity and seismic waves.
The researchers were surprised to find that at least part of Mt. Rainier's partially molten magma reservoir is located 6-10 miles northwest of the volcano. Mt. Rainier is 14,410-foot high, just 30 to 45 miles southeast of the Seattle-Tacoma area. The seeming displacement of this reservoir could be due to the placement of the study's sensors, which were placed in a 190-mile long, west to east line approximately 12 miles north of the mountain.
The image reveals that the top of the magma reservoir starts at about five miles under the surface and "appears to be 5 to 10 miles thick, and 5 to 10 miles wide in east-west extent," Wannamaker said. "We can't really describe the north-south extent because it's a slice view."
The image has limitations. It does not include the plumbing that ties Mt. Rainier to the magma chamber five miles below it. What it does show is water, partly molten rock and molten rock that are generated 50 miles below the Earth's surface where one of the seafloor tectonic crustal plates is “subducting.” The plate is being pushed eastward and downward, under the North American plate. The image also reveals how and where those melts rise to join the magma chamber.
The team used both seismic imaging and magnetotelluric measurements to create the most detailed cross-section view of a Cascades range volcanic system. These methods create images of how electrical and magnetic fields in the ground can vary. The variations are a result of how differently underground rock and fluids conduct or resist electricity. Previous images merely indicated water and partly molten rock on top of the subducting plate. The new image shows melting rock from the top edge of the plate to the upper crust, where Wannamaker says magma accumulates before erupting.
Mt. Rainier is the tallest of the Cascade Mountains, and is expected to erupt again someday. The current mountain sits on top of ancestral lava flows that are 36 million years old. Most of the modern Rainier was created during the past 500,000 years, and during the last 11,000 years the volcano has erupted dozens of times. Approximately 5,600 years ago, an eruption formed the large crater that has replaced Rainier's summit.
The most recent eruption was 2,200 years ago. Hot rock and ash flowed 1,100 years ago, and the last large mudflow was 500 years ago.
The image shows yellow, orange, and red areas that represent higher electrical conductivity in places where fluids and melts are located. It also illuminates where water and molten rock accumulate atop the descending slab (the part of the seafloor plate that is being driven under the North American plate), as well as the route they take to the magma chamber.
The seismic imaging data was collected from 2008-2009 for other studies. The magnetotelluric data was collected by the current study team during 2009- 2010. The team used 80 sensors: 60 that made one-day measurements and looked as deep as 30 miles beneath the surface, and 20 more that took measurements for a month, looking even deeper.