Monday
Monday: Three Kinds of Rock
Rocket tips a paper bag onto the Sky Deck step. Seven rocks roll out. "From the path by the yard," he says. "All different."
Raven lines them up by size. "What do you notice? This one is striped. This one sparkles. This one is sandy and crumbles."
"They all came from the same path," Rocket says. "So why are they different?"
Nova hovers low and shines her light across the row. "Geologists sort rocks by how they formed," she says. "Three ways."
"Melted, layered, squeezed," Raven says slowly, reading the sparkle, the sand and the stripes.
Rocket grabs the striped one. "Squeezed! I knew it." Nova hums. "Maybe. Learn the three groups first. Then sort."
Rocks are continually changing
The United States Geological Survey says rocks are continually changing. Wind and water wear them down and carry bits away.
Those tiny particles collect in a lake or ocean and harden into rock again.
The oldest rock ever found is more than 3.9 billion years old. Earth itself is at least 4.5 billion years old.
Rocks from the very beginning have changed so much that they have become new kinds of rock.
Three groups, by how they formed
Geologists classify rocks in three groups, according to the major Earth processes that formed them.
Igneous rocks formed from melted rock that cooled and solidified. Granite, basalt and obsidian are igneous.
Sedimentary rocks are layered accumulations of sediments: fragments of rocks, minerals, or animal or plant material. Sandstone and shale are sedimentary.
Metamorphic rocks were sedimentary or igneous rocks changed by pressure so intense or heat so high they became new rock. Gneiss and schist are metamorphic.
| Group | How it formed | A clue you can see | Examples from the survey |
|---|---|---|---|
| Igneous | melted rock cooled and hardened | crystals, or glassy with no grains | granite, basalt, obsidian |
| Sedimentary | sediments settled in layers, then pressed and cemented | layers, sandy grains, bits of shell | sandstone, conglomerate, shale |
| Metamorphic | heat or pressure changed an older rock without melting it | stripes or bands, flat flaky sheets | gneiss, schist |
- Read the question.
- Tap your answer.
| Statement | True or false? |
|---|---|
| Rocks are continually changing. | ? |
| Geologists sort rocks into three groups by how they formed. | ? |
| Metamorphic rock forms when rock melts completely. | ? |
| Sandstone is an igneous rock. | ? |
Look at each with the sunlight behind you. Do you see grains, layers, crystals or stripes? Write one word for each rock.
Wash your hands when you are done. Nothing from outside goes near your mouth.
Rocks are never collected from parks, garden beds or other people's property. Hands are washed after handling rocks, sand or soil.
The facts this week come from the United States Geological Survey at pubs.usgs.gov and the open textbook Physical Geology on geo.libretexts.org.
Good start, geologist. Tomorrow you follow the whole rock cycle, from melt to layers to squeezed rock and back.