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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: Im 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 its 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, youll 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 Wynns 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. Wynns 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.
Im a geologist who studies how rocks deform. So what am I doing in my kitchen?
Im in my kitchen because rocks deform in the same way a lot of other things
When something changes its shape in response to stress, its
been deformed. Deformation can be brittle or ductile.
Take a breadstick. Bend it. It breaks. Thats brittle deformation.
Take a caramel bar. Bend it. It changes shape, but it stays in once piece.
Thats ductile deformation.
What controls whether something is brittle or ductile? Three things:
temperature, composition and something called strain rate.
Lets consider how temperature effects whether
something is brittle or ductile.
Take a frozen stick of butter. Bend it. It breaks. Thats brittle
deformation. Now take butter at room temperature. It
flows like the caramel bar. Thats ductile deformation.
Now if we take a candle at room temperature. Bend it. It breaks.
Thats brittle. But what happens if we warm it up a bit? If the candles
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
earths surface. And they get hotter and hotter with
depth. Cold rocks near the surface tend to break if theyre stressed.
Theyre brittle. At about 15 kilometers within the earth, most rocks
are hot enough to flow if theyre stressed. Theyre ductile.
Lets 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. Heres 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.