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Dr. Universe: How come some rocks are easy to break and some are hard? - Natalie, 10, Russellville, Kentucky

Dear Natalie,

If you draw with a pencil, you can tell how soft the graphite inside is. Pieces of graphite break off to leave the pencil mark. But can you imagine drawing with a diamond? Diamonds and graphite are both what you might call rocks. How come they’re so different?

To find out, I talked to my friend Katie Cooper, a geologist and associate professor in the Washington State University School of the Environment. Geologists often study how different types of rocks and minerals form—and that’s the secret to whether they’re easy or hard to break.

Rocks are made of minerals, and minerals are made of elements, which are substances made of a single type of atom. You can get to know the Earth’s elements by looking at a Periodic Table.

Some minerals are made of a single element, like diamond and graphite, which are both made of carbon. Others are a mix, like limestone, which is made of calcium and carbon.

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What is a sinkhole? What causes one? - Kathrine, 12, Calgary, Canada

Dear Kathrine,

Sinkholes can be scary to think about. They don’t happen too often, but when they do, they can take people by surprise. The solid ground disappears, and a hole suddenly appears.

It might seem like sinkholes appear out of nowhere. But they actually need specific conditions to form.

To have a sinkhole, you first must have a cave.

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Dr. Universe: How do earthquakes happen? -Aescli E., 10, Utah

We’ve had a lot of earthquakes on our planet this year. Maybe you’ve learned about them from the news or felt one shaking up your own neighborhood. First, it is important to know a bit about the Earth’s outer layer, or crust. The crust is made of seven big pieces called “plates.” They are about 60 miles thick and sort of float on the molten rock beneath them. That’s what I found out from my friend Sean Long, a geology professor at Washington State University who knows a lot about earthquakes. Read More ...