Of the many unsolved mysteries of the universe, there is one that is particularly close at hand – or rather, underfoot. The ground below us formed over 2 billion years ago and is part of a very thin veneer, much like the skin of an apple, that encapsulates the planet’s immense heat and allows for life on the surface. The continents and shallow seabeds off of the coasts make up what is known as the continental crust, whose origin is largely a mystery to scientists.
“We know it’s derived from the mantle and most geologists agree that the volcanism that produced the continental crust probably happened at subduction zones, but the processes behind this volcanism are a mystery,” said Janine Andrys, a Ph.D. candidate studying geochemistry at the University of Rhode Island’s Graduate School of Oceanography (GSO) during the Bay Informed lecture in October.
Andrys discussed how the internal processes of the Earth shape the surface of the planet through volcanism and how scientists are studying volcanoes to better understand the origins of the Earth’s surface. This thin layer of rock that makes up both the continental crust and the oceanic crust, which is found in the deep sea, accounts for only 3.5 percent of the entire planet.
“The rest lies beneath our feet in the mantle and the core,” said Andrys, pointing to a diagram of the Earth’s interior that looks like layers within a jawbreaker.
She compared the Earth to a tea kettle where hot material rises and cools in a cyclical motion, allowing the crust to shift and glide over the mantle in several pieces known as plate tectonics. Some of these plates may slide past each other along fault lines, others may collide and are either pushed up to form mountain chains or pushed down to form deep trenches in the ocean. Other plates may spread apart, causing the mantle to rise and create new crust – this is how oceanic crust is formed.
“Since we have creation of crust, we have to have recycling of crust somewhere else because the Earth isn’t getting any bigger,” said Andrys, referring to divergent zones where plates are spreading apart and subduction zones where a denser, heavier oceanic plate collides with a lighter continental plate and is pushed under and recycled back into the mantle. “We’re constantly creating and destroying crust on Earth.”
Volcanoes are typically found at these sites of crust creation and recycling, both on land and underwater, and behave very differently. Those found in areas where the plates are spreading apart have effusive eruptions where magma from the mantle slowly flows out like maple syrup. Volcanoes tend to be more explosive and violent where plates collide.
“The style of volcanism and the way the magma is produced to drive that volcanism is fundamentally different at the two different plate boundaries. Because of this, we have two types of crust on Earth, oceanic and continental,” said Andrys.
Oceanic crust is made up of heavier minerals and is only about 4 miles thick because it’s constantly regenerated at divergent plate boundaries and recycled back into the mantle when it collides with the lighter continental crust. “Although the ocean crust is usually deep underneath the sea surface, we know the formation of it relatively well, partially due to the fact that ocean crust is constantly being made,” she said.
Continental crust, on the other hand, she said, is created in a fundamentally different way since it’s made of much lighter minerals and is substantially thicker, ranging from 18 miles to 27 miles on average with some areas, such as the Himalayas, reaching more than 40 miles.
“The combination of this composition and the difference in thickness allows the continental crust to sit higher on the mantle than the ocean crust,” said Andrys, adding that the thickness of continental crust that also makes it so hard to study.
“The deepest we’ve ever drilled was [7.5 miles] in Russia, which started in the ’70s and ended in 1995,” said Andrys. “It’s really deep but it doesn’t come close to getting past the continental crust … we don’t have many samples from the lower portion, which we think is compositionally different.”
Since acquiring direct samples is not possible, scientists have turned to volcanic rocks, where what was once magma is trapped inside the minerals of erupted rocks. As magma cools, minerals form that can trap a piece of the magma, allowing it to freeze into glass that can be studied.
These samples act as time capsules for the composition of that magma at the exact time the mineral was growing. As magma cools, it forms different minerals so scientists can track its evolution and compare it to see if it has a similar chemistry to continental crust.
“We can look at their chemistry and can compare them to that of continental crust. If we think that they’re similar, that their evolution paths look similar, we can [infer] that this magma body could be a candidate for continental crust,” said Andrys.
By using the formation conditions, such as temperature, pressure, and how much water was in the magma, scientists like Andrys can grow small samples of continental crust in the lab. “If successful, using these natural samples as a starting point, then we can hopefully say something about the origin of the continental crust.”
The Bay Informed Discussion Series is sponsored by Rhode Island Sea Grant in partnership with GSO. This series is held every third Thursday of the month at 7 p.m. at the URI Graduate School of Oceanography Bay campus in Narragansett. These events are designed for the community to get involved and learn more about research at GSO.
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– Meredith Haas and Jake Rousseau | Rhode Island Sea Grant