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TITLE: How Do Rocks Deform?


How a particular rock deforms depends on:

  • its temperature and depth;
  • the amount, rate, and direction of the stress;
  • the rock’s inherent strength;
  • and the quantity of water in the rock.
[Photo: Close-up of phyllite showing angular folds.]

This phyllite deformed while it was cool and less ductile, making its folds angular.


Video #1 Transcript
Meet a scientist who creates artificially deformed rocks in the lab and another who studies deformed rocks in the field.

Scientist 1: “We study artificial rocks…”

Scientist 2: “…to learn about real rocks.”

Scientist 1: “Artificial rocks can be deformed at room temperature.”

Scientist 2: “To see how real rocks deform at extremely high temperatures.”

Scientist 1: “Artificial rocks change shape in minutes, hours or days.”

Scientist 2: “To show how real rocks can change in hundreds or thousands of years.”

Scientist 1: “I’m a geologist who deforms artificial rocks in the lab, but this is a real one. The light colored layer used to be straight, but now it’s just about bent double into a fold. This rock was once ductile. Here in Albany, my students and I do experiments and make up artificial rocks and watch them through a microscope while we squeeze them. These artificial rocks have lower melting points than real ones, so even at room temperature, they act like hot rocks deep in the earth. This is an artificial quartzite. I made it by compressing octochloropropane between two sheets of glass. This deformed the crystals and created the ragged looking boundaries between grains. Very similar looking ragged boundaries are commonly seen in real quartzites. If you look carefully, you’ll see some small new grains that have started to grow and that some of the large grains look mottled.”

Scientist 1: “This 450 million year old quartzite is from Vermont. Like Wynn’s sample, it shows many ragged boundaries between the quartz grains. If you look closer, you can see tiny new grains and mottling in some of the large old grains. All of these features tell us that this rock was deformed ductilly. Wynn’s experiments and movies help us visualize how a quartzite like this one deforms.”

Video #2 Transcript
Discover what can make rocks behave like butter and silly putty.

“I’m a geologist who studies how rocks deform. So what am I doing in my kitchen? I’m in my kitchen because rocks deform in the same way a lot of other things deform.”

“When something changes its shape in response to stress, it’s been deformed. Deformation can be brittle or ductile.”

“Take a breadstick. Bend it. It breaks. That’s brittle deformation.”

“Take a caramel bar. Bend it. It changes shape, but it stays in once piece. That’s ductile deformation.”

“What controls whether something is brittle or ductile? Three things: temperature, composition and something called strain rate.”

“Let’s consider how temperature effects whether something is brittle or ductile.”

“Take a frozen stick of butter. Bend it. It breaks. That’s brittle deformation. Now take butter at room temperature. It flows like the caramel bar. That’s ductile deformation.”

“Now if we take a candle at room temperature. Bend it. It breaks. That’s brittle. But what happens if we warm it up a bit? If the candle’s warmed up a bit, it flows, like the caramel bar. Butter at room temperature and wax at room temperature deform differently because they have different compositions. But at lower temperature, both are brittle and at higher temperature, both are ductile.”

“So what do butter and wax have to do with rocks? Rocks are cold near the earth’s surface. And they get hotter and hotter with depth. Cold rocks near the surface tend to break if they’re stressed. They’re brittle. At about 15 kilometers within the earth, most rocks are hot enough to flow if they’re stressed. They’re ductile.”

“Let’s talk about strain rate, which is how fast deformation occurs. Different strain rates can cause rocks to be either ductile or brittle at the same temperature. Here’s how.”

“Take Silly Putty. If I pull it slowly it flows in a ductile fashion. Pull it fast. It breaks. Brittle.”

“Layers of sedimentary rock are just like these layers of Play-Doh. If I push on them slowly, they fold. Some mountains made of folded rocks look just like this.”


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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits