Animation of Newberry Magma Chamber
This story is about scientists looking for a magma chamber beneath Newberry volcano, only 20 miles to the southeast of Bend, Oregon.
People go fishing, kayaking or skiing around Paulina lake, which is one of two lakes found inside the caldera of Newberry volcano. The caldera was formed when the volcanic mountain collapsed several tens of thousands of years ago. It turns out that Newberry is one of the 160 volcanoes that the USGS monitors for volcano hazards. Although the current status of Newberry is “normal”, meaning that all is quiet and the volcano seems inactive, we wanted to know how much magma is currently beneath Newberry volcano and where it is located.
In this study we used seismic waves to “see” what is under the ground at Newberry. Seismic waves travel deep through the earth much as waves or ripples travel on the surface of a pond. When there are obstructions the waves are slowed down or broken up. In this study we used the distortion of seismic waves to figure out where the magma chamber is beneath Newberry volcano and what it’s size is.
Newberry is a large sheld volcano, located 40 miles east of the Cascade mountains. Because it is a shield volcano it has gently sloping sides and covers a large area: it is 20 miles across with a volume of about 80 cubic miles. At the center there is a large oval-shaped caldera that is 4 by 5 miles in diameter. The caldera formed when a large explosive eruption emptied the magma chamber, which then collapsed.
Inside Newberry’s caldera are Paulina Lake and East Lake and also many volcanic cones and lava flows. The visitor’s center is close to where Paulina Creek cuts through the walls of the caldera and drains through a narrow gorge to the west.
Several features suggest that there is still magma beneath Newberry volcano.
First there are hot springs around both Paulina Lake and East Lake. At times the hot springs are too hot to bathe in and must be mixed with the cold lake water to be comfortable.
Secondly, drilling at the center of the caldera found temperatures of 540°F at about 3,000 feet below the caldera floor – the highest temperature recorded at a Cascade volcano.
Finally, Newberry volcano had an eruptive episode 1400 years ago, so in about 480 A.D. This Big Obsidian Eruptive Episode occurred over a period of about 200 years. It started with an eruption plume that spread volcanic ash to the east of the volcano. The final phase of the eruption produced the Big Obsidian Flow, which covers about 1 square mile in the southern part of the caldera.
The high temperatures, hot water, and recent eruptions all suggest that there may still be magma beneath the volcano.
Prior to this study it was thought that a magma chamber about as wide as the caldera might be found about 2 ½ miles beneath the caldera. This was based on work done by scientists at the USGS in the early 1980s. But, they did not know how much molten rock is stored by this magma body.
This is a ripple tank – it makes plane waves. For these plane waves the wavefront is a straight line. When the waves encounter an obstacle, the waves are disrupted. Here a block is placed in the ripple tank. The waves behind the obstacle are broken up and reflections off the obstacle generate secondary waves.
Magma beneath volcanoes is hard to see because the magma chambers are often small. Typically scientists search for magma chambers by looking for seismic waves that are slowed down, which is what happens when they pass through molten rock instead of through solid rock. We can do a much better job when we also look at how the magma chamber disrupts the seismic waves.
The team together with a large group of volunteers dug holes, installed seismometers and hooked up the electronics. The instruments were spaced unusually close together to detect the magma chamber.
To do this work the team used an explosion to make a sound wave that travelled to the depth where they thought the magma chamber was located. The sound wave moves through the rocks of the earth in the same way as ripples travel away when a pebble is dropped in a pond.
Here you see a simulation of the seismic wavefield traveling away from the explosion and through the ground under Newberry volcano. You are seeing a slice through the earth and the seismic sensors sit at the surface. We focus on the pink compressional waves – they are like sound waves.
Where the magma body is located, the wavefront becomes disrupted. Because of the presence of molten rock the seismic waves slow down – this causes an indentation in the wavefront. Once the wave leaves the magma body this indentation actually slowly recovers or “heals”. By the time the first wave arrives at the sensors on the surface not much of the indentation is left. This is called “wavefront healing” and is the reason why finding small magma bodies beneath volcanoes is so difficult.
In addition to this first wave, some of the energy is focused within the magma body and then directed back up to the surface at a different angle. This generates a second wave that is diagnostic of the existence of a magma body beneath Newberry. Based on the timing and the instruments on which the second wave is seen, scientists can determine the location and depth of Newberry’s magma body.
Emilie’s team found that the magma chamber stretches beneath most of the caldera. The magma chamber is down just over two miles below the surface and it contains between 0.2 and 1.2 cubic miles of magma.
Emilie’s team collected the raw data from the sensors and loaded it into applications that interpret the very basic numbers the sensors were recording. The team plotted the seismic data and studied the vertical motions that the sensors measured. In a software program used to invert the arrival times of the first waves the team painted a basic picture of the magma chamber beneath the Newberry caldera. Then using the second wave the team discovered a small shallow magma chamber under Newberry and wrote a paper about these findings that appeared in a scientific journal.
Emilie wanted a better visualization of her computations, so she contacted two students studying digital arts at the University of Oregon and asked the students to help. The students loaded the data from the computations into their 3D animation software and began to build a better looking model of the magma chamber. Now we can more accurately visualize the size and shape of the magma body beneath the surface of the Newberry caldera.
In this work Emilie’s team used a new method to better detect small magma bodies. They looked, not just at how fast the wave travelled, but also at the disruptions that generate new waves. There is not very much magma, but it is relatively shallow.
Our findings can be useful for scientists and decision makers who need to forecast the likelihood of an eruption and the potential volume. For example if earthquakes start happening beneath Newberry caldera, knowing where the magma chamber is, and how large it is, will help decide what the earthquakes mean for the potential of an eruption and how large that eruption might be.