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TITLE: 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.



[Illustration: Volcano spitting out clot of magma.] [Photo: 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.

[Illustration: Lightning striking a volcano.] [Photo: 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.

[Illustration: Meteorite impact sending out shock waves.] [Photo: 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.

[Illustration: Small four-legged animal walking across a sand dune.] [Photo: 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.

[Illustration: Raindrops splattering pits into sediment.] [Photo: 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.

[Illustration: Marine trilobite scratching out furrows in the mud.] [Photo: 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.

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits