The Earth is gaining ground, and it’s happening deep below the surface of the oceans.
Stern’s research focuses on the formation of modern and ancient continental crust. With the help of his graduate students and colleagues at UTD and around the world, he is trying to determine the age and composition of continental crust in the region surrounding the Red Sea known as the Arabian-Nubian Shield. His current research projects in Egypt, Arabia and Ethiopia aim to unlock how the fundamental Earth process of subduction generates thickened arc crust, which is proto-continental crust.
“In Africa and Arabia you can see the formation of continental crust during a time span of a few hundred million years. And today in the western Pacific, continental crust is being produced in the island arcs south of Japan.
“In the Mariana Arc System, where Dr. Stern spends time studying the ocean floor on research vessels, there’s a line of islands and seamounts that are basically volcanoes. There’s an entire complex geologic system that exists there,” he said.
This is about plate tectonics. Scientists say the Earth’s surface is defined by rocky plates that are 60 to 100 miles thick. They move very slowly over the Earth’s mantle, as if on a conveyor belt.
“The fastest plates move as fast as your hair grows and the slowest ones move as fast as your fingernails grow. But with a planet as massive as the Earth, that’s a huge amount of new sea floor being created every year at the mid-ocean ridges and the same amount being destroyed at the trenches.
“This is why Africa and South America look like they were once together. They once were joined and then an ocean formed between them. These mid-ocean systems exist in the Indian Ocean and in the Pacific,” Stern said.
Stern, whose love of the outdoors dates back to his childhood and years of scouting and hiking through the Sierras, says that wherever new oceanic crust is being produced, it has to be destroyed. And it is being destroyed at the trenches in subduction zones.
“At a subduction zone, you’ve got one plate on top of another plate and this plate is going down and the place where it begins to subduct creates the trench. But on this upper plate there are varied responses to the material going down. There are volcanoes, earthquakes, and if you are on continental crust, you’ll have mountains like the Andes and the Rockies,” Stern said.
When did plate tectonics begin?
That’s why Dr. Stern is studying the western Pacific. He wants to know whether subduction processes there are producing new continental crust today.
“Actually, my most exciting research now is about how subduction zones form and determining when plate tectonics began on Earth.
“Earth is the only planet with plate tectonics. That means it’s special in space, and it’s probably special in time, too. There must have been a time when the Earth didn’t have plate tectonics. The Earth had a very different tectonic, geologic style. There were no mid-ocean ridges with continents moving apart. There were no subduction zones where oceanic crust would have been going down,” Stern explained.
Earth is estimated to be 4.5 billion years old. There is evidence of plate tectonics only for the last billion years, Stern said. So when did plate tectonics really begin on Earth?
“We are actually having a conference in Wyoming about this subject in June.
Using a wide range of analytical techniques, including major and trace element analyses, radiogenic isotopic compositions of strontium, neodymium, and lead, rubidium-strontium and uranium-lead zircon geochronology, and remote sensing, Stern and his team conduct tests that help answer the question.
“We used an ion probe at Stanford University. It is capable of generating compositional and even isotopic information for spots as small as 30 microns. So what we can do is take very small grains – uranium-rich grains called zircons – which we work on all summer to gather enough to study, and then we will date them there.
“In Precambrian times, we have found no useful fossils for breaking out times. In sedimentary rocks younger than 540 million years, there are fossils and paleontologists who can look at them and tell you within a few million years when these rocks were deposited. But that only works for the last 540 million years.
“So rocks that are Cambrian in age or younger, that is, less than 540 million years old, can be dated more easily. But Precambrian time encompasses 4 billion years – about 90% of Earth’s history. You’ve got to use radiometric geochronology in order to understand when something happened” Stern said.
While carbon dating relies on Carbon 14, which has a half life of a few tens of thousands of years, isotopic dating uses both Uranium 235, with a half life of 500 million years, and Uranium 238, with a half life of 4.5 billion years, Stern explained.
“I think plate tectonics began on this planet somewhere between a billion and 600 million years ago. And that ties into Snowball Earth that UT Dallas graduate students Kamal Ali and Sumit Mukherjee are doing because that, I think, was a climatic response of the beginning of plate tectonics,” said Stern, who headed UT Dallas' geosciences department for eight years.
Snowball Earth, a theory attempting to explain a number of phenomena noted in the geological record, proposes an ice age that took place that was so severe that the Earth's oceans froze over completely, with only heat from the Earth's core causing some liquid water to persist.
“The Earth went through a tremendous geologic revolution and it is not surprising that it had a tremendous affect on Earth’s climate. But mine is a minority opinion. Most geoscientists believe that plate tectonics began much earlier, or that Earth has always had plate tectonics.
“Before, I think it was probably something like what’s happening on Venus today. Venus is a very similar planet to the Earth, but it doesn’t have plate tectonics. So I think it was something like that, where there is no subduction and there are no mid-ocean ridges, yet we still have igneous rock and volcanic activity,” Stern said.
That’s the geosciences for you. One second you’re looking to Venus for clues about the origins of plate tectonics, and in the next you might be plunging the depths of a trench in the western Pacific.
Journey to the Center of the Earth
“There is no place on the planet where subduction zones are forming today. We have to go into the geologic record and try to reconstruct that. Subduction zones are where the oceanic crust goes back into the mantle and perhaps keeps sinking down into the core,” Stern said.
Deep Earth describes the layers that make up our planet. Beginning with the crust, the layers, moving inward, are the mantle, the outer core, which is believed to be made of molten iron, and the inner core, which is believed to be solid iron.
But what happens to the material once it gets down there? The core is so hot; they would have to change, right?
“We argue about this,” Stern said. “Some scientists say when it gets to the core it melts again and generates volcanoes in the middle of the plates, like Hawaii, Yellowstone or other kinds of volcanoes. However, other people deny that and it is an area of vigorous scientific debate.
“The deeper you go in the Earth, the harder it is to know what is really occurring. Geophysicists give us images of the deep Earth, and we can get indirect samples from basalts, igneous rocks, and then interpret what’s going on. But I’m a geologist; I’m much more interested in things where they are on the surface. So the processes that can be studied – where you can go down and collect a sample and take it back to the lab and work on it – well, those are my kinds of problems,” Stern said.
To study the Mariana System, in 1997 Stern used Shinkai 6500, a Japanese manned submersible. Submersibles are tiny submarines.
“It was amazing how much room there is in something that’s only a meter across. There were three of us in there – two pilots and a scientist. It’s very peaceful and quiet going down. And there’s nothing to see, so they just turn the lights off. And they turn on music and you sort of doze.
“There are panels with instruments everywhere, and you can see how far you are off the bottom. And then as you get close, they turn on the lights and everybody peers out and waits to be able to see something,” Stern said.
He compared it to being in a window seat when landing in an airplane, looking through the clouds hoping to see something.
“When you finally start to see something, you think, ‘I wonder what that is?’ You get closer to it and try to sort out what it is you are seeing.“I was looking at exposed mantle. The crust lies on top of the mantle and there was a place in the Mariana Trough where the mantle was exposed, so I was able to see it, and I’ve written a couple of papers about this.
“It’s made of a certain type of mantle, which is very, very dark green and composed mostly of a mineral known as olivine, which gemologists call peridot. Olivine makes up most of the upper mantle. It’s very distinctive as opposed to crust, which is made up of igneous rocks. Like many of the rocks out there,” he said pointing to the geosciences department’s rock garden outside his office window.
Returning to a Classroom Near You
When not off on a special faculty development assignment or sailing in ships or traveling to Africa, Stern teaches a number of geosciences courses, including an undergraduate course in Historical Geology, and graduate courses Tectonics, and Isotope Geochemistry.“UT Dallas is a leader in isotope geochemistry,” Stern said.
Dr. William Manton, a geosciences professor and colleague of Stern’s, is a pioneer in studies of lead poisoning in humans, and how lead affects people.
“He really runs the isotopic geochemistry lab. I’m physically challenged, so I basically take advantage of his expertise and my students are trained by him now. When I first came here, I was an equal partner in the lab, but I can’t do that anymore,” Stern said.
Stern, who walks with a cane, suffers from a neurologic disease.
“I was diagnosed when I was 30, but before that I had no problems. I could enjoy running and all kinds of outdoor sports. In a way, I am lucky that I had my physical health during the best part of my life and I was able to enjoy that. As you get older, you are not as physical anyway, so it’s sort of OK now,” he said.
Like most students, Stern said he didn’t know anything about geology when he started college.
“Geology is rarely any freshman's intended major. It is something that most of us discover accidentally.”
- Updated: October 9, 2012