The World's Foundations; or, Geology for Beginners · Ch. IV · Unit 1: Part I: How to read the record
4. Fire-built rocks
Goal: Your child can name the book's kinds of fire-made rock (volcanic, plutonic, metamorphic) and explain why rocks that cool slowly have bigger crystals.
You'll need
- The book, open to this chapter
- A magnifying glass
- Any rocks you have collected
- If you can: a granite countertop, gravestone or building stone to look at
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter IV describes unstratified or igneous rocks: volcanic rocks like lava, plutonic rocks like granite, and metamorphic rocks changed by heat, like some marble. It compares Fire and Water to two workmen rebuilding a house, lists great eruptions and earthquakes, and weighs two theories about the heat inside the earth.
The lesson, step by step
A question to start
SaySome rocks were once so hot they were liquid, like melted butter. Where could rock get that hot?
Let your child guess (volcanoes, deep underground), then read.
Read the chapter together
Read Chapter IV (pages 32 to 41). The key definitions:
Secondly — Plutonic Rocks, such as granite. These have been cooled deep down in the earth, under heavy pressure.
The word ' metamorphic ' simply means 'transformed.'
Three kinds, in plain words
Volcanic rock (like lava) cooled quickly at or near the surface. Plutonic rock (like granite) cooled slowly, deep down. Metamorphic rock was once a different rock, then was changed by great heat and pressure; marble was once limestone.
Aqueous means made by water (Latin aqua); igneous means made by fire (Latin ignis).
Look closely
With the magnifying glass, look at granite if you can find some (kitchen counters, gravestones and old buildings are often granite). You will see separate crystals of different colors fitted together.
Explain: granite cooled slowly underground, so its crystals had time to grow big. Lava cools fast, so its crystals are tiny or it is glassy, sometimes full of gas bubbles.
Sort your rocks
Spread out the rocks you have. Make three piles: "I can see crystals", "I can see layers or grains of sand", and "not sure". Don't worry about being right; this is how a geologist starts. Record the piles in the rock journal.
Fire and Water
Giberne compares the two great workers to workmen rebuilding a house: Water works on the upper floors, Fire works in the cellars, and both work at once. Her surprising calculation: one great Chilean earthquake lifted as much land in a moment as the River Ganges carries to the sea in about four hundred years. Ask: Which worker is fast? Which is slow?
Tell it back
Ask your child to explain the three kinds of fire-made rock as if teaching a younger child. Then: Why does granite have bigger crystals than lava?
What we know now
Giberne describes two ideas: a melted inside under a thin crust, or a solid earth with lakes of melted rock here and there. We now know the mantle under the crust is solid rock, but so hot that it flows very slowly over millions of years. Melted rock (magma) forms in particular places, mostly where the earth's plates are pulling apart or where one plate is sliding down beneath another.
Afterwards
Notice it this week: look at stone used in buildings, steps, walls and countertops. Which ones show crystals? Which show layers?
Answers to the book's problems
Volcanic rocks cooled at or near the surface (lava); plutonic rocks cooled deep underground under pressure (granite); metamorphic rocks were changed by heat (some marble). Granite cooled slowly, so its crystals had time to grow; lava cooled quickly, so its crystals are tiny.
Worked out for this site from the scan. If a number in your copy differs, trust the book.