4/1/2023 0 Comments Depths of the earth quickpress![]() "If there was no transformation happening in my sample, it wouldn't break," Burnley said. And right at the transition from olivine to ringwoodite, under enough pressure, the mineral could actually break instead of bending. In some conditions, for example, olivine can skip the wadsleyite phase and head straight to ringwoodite. In experiments in the 1980s, the pair found that olivine mineral phases were not so neat and clean. Burnley and her doctoral advisor, mineralogist Harry Green, were the ones to come up with a potential explanation. Geologists were puzzled by earthquakes in the upper mantle until the 1980s, and still don't all agree on why they occur there. ![]() As olivine transforms into its higher-pressure phrases, it becomes more likely to bend and less likely to break in a way that triggers earthquakes. And both behave very differently: Graphite is soft, gray and slippery, while diamonds are extremely hard and clear. Graphite is the form that's stable at Earth's surface, while diamonds are the form that's stable deep in the mantle. Both are made of carbon, but in different arrangements. It's similar to graphite and diamonds, said Burnley. What's important about these mineral phases is not their names, but that each behaves differently. That last transition marks the end of the upper mantle and the beginning of the lower mantle. And because seismic waves move differently through different mineral phases, geophysicists can see signs of these changes by looking at vibrations caused by large earthquakes. Geoscientists can't probe that far into the Earth directly, of course, but they can use lab equipment to recreate extreme pressures and create these changes at the surface. Finally, around 423 miles (680 km) deep into the mantle, ringwoodite breaks down into two minerals, bridgmanite and periclase. Another 62 miles (100 km) deeper, wadsleyite rearranges again into ringwoodite. Around 249 miles down, the pressures caused olivine's atoms to rearrange into a different structure, a blue-ish mineral called wadsleyite. Much of the planet's mantle is made up of a mineral called olivine, which is shiny and green. ![]() The problem with earthquakes deeper than around 249 miles has to do with the ways the minerals behave under pressure. "At that depth, we think all of the water should be driven off, and we're definitely far, far away from where we would see classic brittle behavior," she said. The pores in the rocks that hold water have been squeezed shut, so fluids are no longer a trigger. Those quakes have long been mysterious, Burnley said. But even before the 2015 Bonin aftershock, quakes have been observed in the lower mantle, down to about 420 miles (670 km). These kinds of dynamics can explain quakes as far down as 249 miles (400 km), which is still in the upper mantle. Under these conditions, rocks are also prone to brittle breakage, Burnley said. ![]() But at this depth, earthquakes can happen when high pressures push on fluid-filled pores in the rocks, forcing the fluids out. Deeper in the crust and lower mantle, the rocks are hotter and under higher pressures, which makes them less prone to break. When these rocks undergo stress, Burnley said, they can only bend a little before breaking, releasing energy like a coiled spring. In the crust, which extends down only about 12 miles (20 km) on average, the rocks are cold and brittle. The vast majority of earthquakes are shallow, originating within the Earth's crust and upper mantle within the first 62 miles (100 km) under the surface. This makes the quake something of a head-scratcher. (Image credit: Shutterstock) (opens in new tab) The deepest earthquake ever, which occurred off Japan in 2015, reached into Earth's lower mantle. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |