Earth > If Rocks Could Talk > Obsidian

Obsidian: Can we get started already? I don't have all day!

Obsidian: That's right. I erupted from the Medicine Lake Volcano in northeast California. It's one of the largest volcanoes in the Cascade Mountains, a long line of volcanoes that runs from western Canada to northern California.

Obsidian: I exploded from a volcano—what more do you want to know?

Obsidian: Well, I started many miles underground in Earth's mantle. In some places, there is so much heat and pressure down there that the rock can melt to form magma. You might think that this magma just sits around after it forms, but it doesn't.

Obsidian: Since the magma is hotter and lighter than the rock it melted from, it slowly rises up through cracks deep underground in Earth's crust. Eventually, it collects in holding tanks called "magma chambers."

Obsidian: You humans have it easy. Let's put it this way: A magma chamber is usually a few miles below Earth's surface, and lot of pressure can build up down there. But the worst part is the heat. It's about 1,000 degrees Celsius (1,830 degrees Fahrenheit). That's about five times as hot as your oven when you bake a cake!

Obsidian: If there's any crack or weak zone above the magma chamber, that magma is going to blow!

Obsidian: Bingo! When the magma reaches the surface, it becomes lava. The lava flows out, cools, and hardens. Over time, if the volcano keeps erupting, the lava builds up in layers of rocks.

Obsidian: Yup. It also forms spectacular rocks like yours truly.

Obsidian: Oh, just recently: about 900 years ago.

Obsidian: That may seem like a long time to you, but that's very young for a rock. In fact, that was the last time that the Medicine Lake Volcano erupted. So, can we wrap this up already? I'm not getting any younger.

Obsidian: They studied a dead cedar tree that had been caught up in the lava flow where I was found. They figured out how old the tree was, and that told them about when the volcano erupted.

Obsidian: Don't you know? I'm made of volcanic glass!

Obsidian: I don't know if you've ever seen a volcano erupt, but it happens very quickly. The magma or melted rock that comes out of a volcano sometimes cools and hardens very, very quickly too. When you go from a liquid to a solid this fast, you don't have time to form crystals. Instead, you turn into glass.

Obsidian: Those rocks are intrusive. They cool slowly underground, so they have thousands of years to form crystals. With so many crystals, they can't be shiny glass like me.

Obsidian: Well, I do what I can.