Rocks and Minerals

What are the three types of rocks? featuring Geologist Ben Andrews

Thanks for staying curious. You're watching "Smithsonian Science How: Behind the Scenes" with Smithsonian geologist, Ben Andrews.

Hi, my name is Ben Andrews and I'm a scientist within the Smithsonian Institution's National Museum of Natural History. I'm here today to talk with you about the three types of rocks that cover the Earth's surface.

Those types of rocks are: igneous, sedimentary, and metamorphic.

We're going to begin here by looking at a few igneous rocks. All igneous rocks begin as molten rock, or magma, and sometimes they don't crystalize at all. When there's no crystals formed we get a rock like an obsidian, right here, which at this time is just a big piece of glass.

Now if we let bubbles form in that rock we could get a pumice, like this white rock here. This pumice is entirely glass, but it also has about 75 percent bubbles in it.

Now if we let a rock crystalize all the way, we can get a rock like a granite, right here. The difference between this granite, this pumice, and this obsidian has nothing to do with composition, but has everything to do with whether there are crystals, and whether there are bubbles. If a magma crystalizes below the surface as an intrusive rock, we get a granite. Whereas if it erupts as a lava, we can get an obsidian, or if it explodes, we can get a pumice.

Now the other type of igneous rocks we have up here are basalt lava, or pumice. We see that right here with the pahoehoe lava or right here with a couple of pieces of basalt pumice, or basalt scoria.

Our next type of rock is a sedimentary rock. We have that right here with this beautiful laminated sandstone. Sedimentary rocks can ultimately be made in a couple different ways. One way is through the layer-by-layer deposition of pieces of other rocks, and that's what we have here. There's about forty or fifty different layers of sand in this sandstone and each one of those is made up of little tiny pieces of sand or other rock.

The other way we can form sedimentary rocks, is through the chemical precipitation of things like a limestone. And often in sedimentary rocks we also find fossils.

Our last type of rock today are metamorphic rocks. On the end of the table here, we have a gneiss. That's spelled G-N-E-I-S-S. And this is a rock that's been totally recrystallized. So it did not melt, but it was taken to such extreme pressures and temperatures that all the individual minerals and bits and pieces of this rock reformed into new mineral grains.

And we can also see that this rock has some amazing folding in it. That's because most metamorphic rocks are being squished and smushed as well as cooked, and that's why we have that amazing series of folds and wiggly-squiggly lines there.

Thank you very much!
 

Virtually explore the Janet Annenberg Hooker Hall of Geology, Gems, and Minerals.

We will not be covering volcanoes, earthquakes, or plate tectonics during our digital school program, but NMNH has several related resources to support your teaching. 

• Video: Volcanoes and Pyroclastic Flows – Join Geologist Dr. Ben Andrews to explore volcanoes and pyroclastic flows — the super-heated gas and ash that is erupted during explosive volcanic eruptions. Ben describes how these eruptions happen, what causes ash and other materials to explode, and how scientists like him study active volcanoes and pyroclastic flows.
• Video: How Volcanic Eruptions Send Materials Up and Out – This video features Dr. Ben Andrews, a geologist at the National Museum of Natural History. He studies volcanoes to get a better understanding of how eruptions happen. What causes ash and other materials during violent "pyroclastic" eruptions to spew out the way they do? Explore the physics of explosive eruptions. Reflect on the hazards scientists face in studying active volcanoes. Visit Ben's volcanology lab to see how he uses his simulator — a giant tank, talcum powder, lasers, and cameras — to model how erupted materials travel. Understand why research on volcanoes is important for our safety.
• Video: Plate Tectonics – See how mapping the location of volcanic eruptions and earthquakes can tell us where tectonic plate boundaries are. Dr. Elizabeth Cottrell explains how the plates interact and how volcanic eruptions actually cause earthquakes.
• Video: Volcano Geochemistry: Windows to Earth's Interior – This video features Dr. Elizabeth Cottrell, a geologist at the National Museum of Natural History. Explore volcanoes as windows into the Earth's interior. Understand how you are connected to the interior of the Earth. Uncover the evidence about geologic events contained in volcanic glass. Think about how plate tectonics have gotten us to where we are today.

Featured Collection: All Sorts of Quartz – Quartz is one of the most abundant minerals in the Earth’s crust and is found on all continents. It exists in a variety of forms and colors, and has many uses. In this collection of museum specimens, you’ll explore the diversity of quartz.