How do we know?
↻ One of the pages with a moving picture
Crystals are clues.
- Igneous rock forms when hot melted rock cools and hardens.
- Wind, water and ice break rock into bits called sediment.
- Sedimentary rock: bits settle in layers, get buried and glued together.
- Metamorphic rock: heat and squeezing change rock without melting it.
Crystals are clues! Melted rock that cools slowly underground grows big crystals, like granite. Lava that cools fast can turn glassy, like obsidian.
That is how you can sort rocks: sparkly crystals hint at igneous, sandy layers at sedimentary, and wavy bands at metamorphic.
Some rocks are very old. The Acasta Gneiss in Canada is about 4 billion years old, some of the oldest rock pieces on Earth. The oldest rocks of the Grand Canyon are about 1.8 billion years old.
Wow number
4 billion years old: some of the oldest rock pieces on Earth, the Acasta Gneiss in Canada.
Myth or fact?
Diamonds are made from squeezed coal. Myth. Diamonds grow deep in the mantle, far below coal.
Some rock can float on water. Fact. Bubbly pumice is light enough to float.
Think about it
What kind of rock might your school, or your sidewalk, be made of?
Where in the world?
Grand Canyon, Arizona, USA where the oldest rocks are about 1.8 billion years old. The Acasta River in Canada is where the 4-billion-year-old rock was found — 36.1069° N, 112.1129° W.
Where these facts come from
Every number on page 23, and who measured it
This page explains rocks made from cooled melted rock.
What are igneous rocks? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/what-are-igneous-rocks
We checked it on 31 October 2026
This page explains rocks made from layers of bits pressed together.
What are sedimentary rocks? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/what-are-sedimentary-rocks
We checked it on 31 October 2026
This page explains rocks changed by heat and squeezing.
What are metamorphic rocks? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/faqs/what-are-metamorphic-rocks
We checked it on 31 October 2026
This page tells us about one of the oldest rocks on Earth.
The Smithsonian's National Museum of Natural History
naturalhistory.si.edu/explore/collections/geogallery/10026559
We checked it on 31 October 2026
This page also tells us how old the Acasta rock is.
The American Museum of Natural History in New York
amnh.org/exhibitions/permanent/planet-earth/how-has-the-earth-evolved/the-earths-crust/the-oldest-rocks-and-minerals-on-earth/the-oldest-known-rock
We checked it on 31 October 2026
This page tells us about a trip to find the oldest rocks in northern Canada.
Searching for Earth's Oldest Rocks: Follow a Smithsonian Expedition to the Remote Northwest Territories — Smithsonian magazine, from the Smithsonian museums
smithsonianmag.com/blogs/national-museum-of-natural-history/2024/09/12/searching-for-earths-oldest-rocks-follow-a-smithsonian-expedition-to-the-remote-northwest-territories
We checked it on 31 October 2026
This page tells us how old the Grand Canyon's oldest rocks are.
Geology — The U.S. National Park Service
nps.gov/grca/learn/nature/grca-geology.htm
We checked it on 31 October 2026
This page tells us diamonds grow deep in the Earth.
Diamonds from the Deep: How Do Diamonds Form in the Deep Earth? — The Gemological Institute of America, gem scientists
gia.edu/gems-gemology/winter-2018-how-do-diamonds-form-in-the-deep-earth
We checked it on 31 October 2026
This page tells us diamonds don't come from coal.
How Do Diamonds Form? — Geology.com, a website about rocks and minerals
geology.com/articles/diamonds-from-coal
We checked it on 31 October 2026
This page tells us pumice holds many trapped gas bubbles, and that because of them it commonly floats on water.
Volcanoes of the United States [USGS] — The U.S. Geological Survey, the government's Earth scientists
pubs.usgs.gov/gip/volcus/page42.html
We checked it on 5 November 2026
This page also explains pumice.
USGS: Volcano Hazards Program Glossary - Pumice — The U.S. Geological Survey, the government's Earth scientists
volcanoes.usgs.gov/vsc/glossary/pumice.html
We checked it on 31 October 2026
Has this changed?
The three kinds of rock are not going to change.
The record for the oldest rock might: scientists still go on expeditions to look for Earth’s oldest rocks.
Words from this page
- igneous
- A kind of rock made when hot melted rock cools and hardens.
- sedimentary
- A kind of rock made from bits of other rock that settle in layers and get glued together.
- metamorphic
- A kind of rock changed by heat and squeezing, without melting.
Try it!
Be a rock detective.
1. Collect 5 small rocks from a yard or schoolyard. 2. Look with a magnifying glass. Do you see crystals, layers or stripes? 3. Sort them into groups. Wash hands after.
Clues: sparkly crystals hint at igneous, sandy layers at sedimentary, wavy bands at metamorphic.
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