What can exploded stars teach us about the history of our planet and how its surface changes? Read on to learn about a link between supernovas (exploded stars— click the link to learn more about them) and rocks here on Earth.
Geologist Greg Hoke, who teaches at Syracuse University’s College of Arts & Sciences, explains that supernovas release high-energy particles, often called cosmic rays. Some of these subatomic particles (mostly protons) cross the Earth’s magnetic field and collide with atoms in our atmosphere. This causes nuclear reactions, which set off secondary rays. Some of those rays reach Earth’s surface, where they smash into minerals, creating rare isotopes.
Many of these rare isotopes, or cosmogenic nuclides, are radioactive: they decay over time. Because we know how quickly they decay, they can help scientists like Hoke measure how long erosion takes and the ages of landforms above (mountains!) and below Earth’s surface.
How does this work? Once sediment (sand, clay, and other small particles of rock) is shielded from cosmic rays by being buried or ending up in a network of caves, the cosmogenic nuclides become what Hoke calls “a simple decay clock,” also known as a geochronometer. In other words, at a certain point, the cosmic rays can’t reach the sediment, because it’s at least two or three meters below Earth’s surface, so they do not produce new cosmogenic nuclides. We know how quickly the existing nuclides decay, so we can figure out how long it has been since they were closer to Earth’s surface.
When glaciers and ice sheets entrain (pick up and drag) rock from below Earth’s surface, that rock can be exposed to cosmic rays. Measuring its cosmogenic nuclides — isotopes including carbon 14 (C-14), beryllium 10 (Be-10), and aluminum 26 (Al-26) — helps geologists understand how long the rock was exposed at Earth’s surface. Then they are better able to identify the effects of glaciers’ movements and speeds and their effects on the landscape.
There were five large glaciations in the last 750,000 years. Normally, a glacier’s movement across Earth’s surface wipes out the evidence of older glaciers in the same area. This makes it difficult to see the effects of repeated glaciation. Burial dating lets scientists like Hoke look back further in time, seeing past the changes caused by recent glaciers.
When Math4Science spoke with him, Hoke was using burial dating to explore the effects of alpine glaciers and ice sheets on mountains in the south of Poland by collecting sediments deposited in ancient cave networks. Glacial erosion lowers river valleys, which causes the water table to drop. It also causes cave networks to drop. He and his research group have also used burial dating to explore the history of rivers such as China’s Yangtze.
“We can take a bucket of sand in a subterranean (underground) river and isolate the quartz,” which is the main carrier mineral for rare isotopes. That helps Hoke and his colleagues “determine the age of the cave and the long-term erosion rate of the river and of the entire catchment prior to [when the sand was] washed into the cave system.” (A catchment is a large area of land over which water flows.)
Hoke enjoys the “strong outdoor component” of his work. “Your laboratory is not only the lab but out in the field. You go out and collect data and samples.” This can involve challenging hikes along steep mountains, exciting off-road drives in four-wheel drive vehicles, and even travel by horse.
As a child growing up in Maine, Greg “spent a lot of time outside. It’s hard to be in Maine and not be in nature.” “The landscape I grew up in is really rich.” Walking along one mile of beach, he would see very different rocks.
He also had windows into far-away landscapes and times, because his parents subscribed to Discover Magazine and to National Geographic. “I flipped through [National Geographic] cover to cover every month. The pictures and the articles brought home this whole other world.”
Greg also enjoyed building things out of wood and electronics. “I liked doing mechanical, tactile things.” His parents encouraged him to choose a career in engineering or applied sciences, but “I liked the idea of doing basic science rather than applied science…. I decided I wanted to do Earth science or geology because it’s such an integrator. You use chemistry; you use physics; you use math every day and you integrate those things in solving interesting problems.”
The geologist credits his math teachers in Maine with his ability to use numbers professionally, despite the fact that “I was never the strongest math student in my classes.” He arrived at college with the number sense he needed to succeed in the sciences but found that math wasn’t incorporated enough into his classes.
“I wish we used math more heavily at the undergraduate level.” Focusing on qualitative information in college can mean that students are not well prepared for graduate school, when the science they study becomes more quantitative.
Hoke helps his own students avoid this trap. He makes sure they use math, for instance, to understand the shapes of mountains and smaller hills. Different types of functions (and their linear and curving graphs) can describe patterns in Earth’s surface.
“Most hill slopes are not straight: they’re convex.” “The rate of the creep of stuff down” the sides of hills creates their shapes. “The math itself is really simple” and usually “comes down to some algebra and maybe an integration or a derivation but it’s not even that complicated, usually.”
Exponential decay also plays a key role in the investigation of the history of Earth’s surface. Hoke and his students make good use of the cosmogenic nuclides you read about earlier in this article. They use exponential functions and logarithms and the half-lives of each isotope to analyze the quantities of star-struck particles they find after pulling buckets of sediment out of a river or gathering sand from a cave. “There will be math in this class,” the geologist promises his students, knowing that they will better understand the way our planet changes over time if there is.