Rocks Tell Stories

Three Rocks and Their Minerals
Each of these rocks—granite, gneiss, and sandstone—is made mostly of these three common mineral types: mica, quartz, and feldspar. How can three rocks made of the same basic ingredients look so different? The minerals came together in varied ways. By studying a rock's minerals, scientists uncover clues to its origins and history. In this exhibit, so can you.

Gneiss. Gneiss is a metamorphic rock—one that was transformed from older rocks by heat and pressure within the Earth. Two continents collided 1.7 billion years ago, burying and heating a sandstone. Its minerals recrystallized and separated into alternating layers. Molten granite invaded this gneiss, forming a vein that was later deformed into a zigzag shape.

Granite. Granite is an igneous rock—one that solidified from a hot, molten state. The crystals in this granite formed when molten rock cooled underground 1.1 billion years ago. They grew until they bumped into neighboring crystals. As the granite slowly cooled, the crystals grew fairly large.

Sandstone. Sandstone is a sedimentary rock—one that formed from sediments deposited at Earth's surface by water, ice, or wind. The sand and pebbles that make up this sandstone are fragments that came from older rocks. The fragments were carried downhill by rivers and streams 300 million years ago, deposited in layers, then bound together by natural cement.

Rock Snapshots
Every rock contains a bit of Earth's history. Rocks provide clues about the nature and timing of the events that formed them. Geologists—arriving on the scene thousands, millions, or even billions of years later—use these clues to reconstruct Earth's history and learn about the processes that have shaped the planet.

Each of these rocks recorded an event that took place within a human time scale—seconds, hours, or years. Each preserved the record of a fleeting event for up to hundreds of millions of years.

Basalt. A volcano spit out this clot of magma, which then twisted, cooled, and solidified during its half-minute descent. Escaping gases produced the spongy texture.

Fulgurite. Lightning hit a volcano, instantly melting the rock to create this fulgurite. The liquid cooled too quickly for crystals to form. Hollow tubes lined with greenish—black glass record the electric current's path through the rock.

Sandstone. A meteorite struck the Earth. Within seconds, the impact sent out a shock wave that created cone-shaped fractures, called shattercones. Their tips originally pointed to the impact site.

Sandstone. A small four-legged animal walked across a sand dune 265 million years ago, leaving behind its foot- and claw-prints. Before the tracks were erased, another sand layer covered and preserved them.

Shale. Raindrops gently splattered pits into fine-grained sediment some 215 million years ago. Before the next storm erased the impressions, more sediment covered and preserved them.

Sandstone. Marine animals—probably trilobites—scratched out furrows in the mud 500 million years ago. Sand filled in the furrows, and both eventually turned to rock.

Time Detectives
Scientists have discovered several methods to determine a rock's age. Geologists can work out the sequence of events recorded in a rock and its age relative to others around it. Paleontologists can look for fossils in sedimentary rocks to figure out the rocks' ages based on the worldwide geological time scale defined by fossils. Geochemists can use dating techniques based on radioactive decay to determine a rock's absolute age.

Relative Time. With this method, geologists work out the sequence of events recorded in a rock and its age relative to others around it. In 1815, William Smith—an observant canal engineer—used fossils in rock exposures like these three to create geologic sections like the one on the right. By the mid-1800s, geologists developed a relative time scale based on fossil correlations of distant rocks. But they could not determine the rocks' actual ages until atomic clocks were discovered in the 20th century.

Fossils. What time is it? If you found one of these three index fossils in a rock, you'd know. Each animal was once widespread but quickly became extinct. Only a small percentage of fossils meet these criteria. Paleontologists look for fossils in sedimentary rocks to figure out the rocks' ages based on the worldwide geological time scale defined by fossils.

Atomic Clocks. In the 20th century, geologists made a timely discovery: Rocks contain atomic clocks. They enable geologists to calculate when a rock formed—its absolute age—by measuring its radioactive elements. At last, geologists were able to attach years to the fossil-based, relative time scale and figure out, for example, exactly when the dinosaurs ruled the Earth.

All Rocks Contain Water
You can't necessarily sip it with a straw, but water is a component of most rocks. Both at and below Earth's surface, water in rocks drives geological processes. Within Earth, water plays a critical role in transforming and melting rocks. Where is the water in rocks? At shallow depths, much of the water is stored in tiny pores between mineral grains. Throughout the crust and mantle, water is held primarily as pairs of hydrogen and oxygen atoms (hydroxyl groups) in the atomic structures of minerals.