Raven spreads a printed page on the Sky Deck. "The survey's guide, Collecting Rocks. Every group, how it forms, what it looks like."
Rocket reads fast. "Slow cooling, big crystals. Fast cooling, tiny crystals. So obsidian cooled fast?"
"It says obsidian is volcanic glass," Raven says. "What do you notice about glass? No crystals at all."
Nova hovers over the page. "The textbook adds times," she says. "Centuries to millions of years for slow cooling. Seconds to years for fast."
"And twenty million years per step around the cycle," Rocket says. "That is a slow loop."
Raven nods. "Rows first, then conclusions." Nova hums. "Read every row."
The United States Geological Survey publishes a plain-language guide called Collecting Rocks. The open textbook Physical Geology adds timings.
Here are their descriptions, shortened into rows. Read every row before you answer.
| Group | Where it forms | Cooling or pressure clue | Texture | Examples |
|---|---|---|---|---|
| Igneous, intrusive | deep in the crust, hundreds of meters to tens of kilometers down | cools slowly over centuries to millions of years | coarse grained, visible crystals | granite |
| Igneous, extrusive | at or near the surface | cools quickly in seconds to years | fine grained, tiny crystals, or glassy | basalt, obsidian |
| Sedimentary | at the surface, in water or on land | low temperature and pressure; buried hundreds of meters to harden | layers; grains of sand, gravel or mud; shells | sandstone, conglomerate, shale |
| Metamorphic | deep in the crust | intense heat or pressure, no melting | denser, more compact; often banded or foliated | gneiss, schist, slate |
| Statement | True or false? |
|---|---|
| Granite is intrusive igneous rock with visible crystals. | ? |
| Metamorphic rock forms by melting. | ? |
| Shale is a sedimentary rock. | ? |
| Extrusive igneous rock cools over millions of years. | ? |
Careful reading. Tomorrow you find the rock cycle in mountains, beaches and your own neighborhood.