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Rocks and Geology for Kids: Layers, Rock Types and Experiments
Geology is the study of the Earth: what it is made of and how it changes. It is a great topic for kids because they can collect real samples in their own backyard.
The layers of the Earth
- Crust: the thin, rocky outer layer where we live. It is thinnest under the oceans.
- Mantle: a thick layer of hot rock that moves very slowly, like thick putty.
- Outer core: a layer of liquid metal, mostly iron and nickel.
- Inner core: a solid ball of metal at the center, extremely hot but kept solid by enormous pressure.
A peach is a handy model: skin is the crust, fruit is the mantle, and the pit is the core.
Rocks versus minerals
A mineral is a naturally occurring solid with a specific chemical makeup, such as quartz or mica. A rock is a natural material made of one or more minerals. Granite, for example, is a rock made of quartz, feldspar and mica.
The three types of rock
| Type | How it forms | Examples |
|---|---|---|
| Igneous | Melted rock (magma or lava) cools and hardens. | Granite, basalt, obsidian, pumice |
| Sedimentary | Sand, mud and shell pieces settle in layers and are pressed and cemented over a long time. | Sandstone, limestone, shale, conglomerate |
| Metamorphic | Existing rock is changed by heat and pressure without melting. | Marble, slate, quartzite, gneiss |
Fossils are found mostly in sedimentary rock, because the gentle burial of a plant or animal in sediment can preserve its shape.
The rock cycle
Rocks are always changing, very slowly. Wind and water weather rock into sediment, erosion moves it, and deposition lays it down. Burial and pressure make sedimentary rock. Heat and pressure make metamorphic rock. Melting and cooling makes igneous rock, and then the cycle can begin again. Any rock type can become any other.
Plate tectonics, earthquakes and volcanoes
Earth's crust is broken into huge slabs called tectonic plates that move a few centimeters a year, about as fast as fingernails grow. Where plates collide, mountains such as the Himalayas can form. Where they slide past each other, stress builds up and releases as an earthquake. Many volcanoes form where one plate slides below another, or where magma rises from deep within the mantle, such as the Hawaiian Islands. The "Ring of Fire" around the Pacific Ocean has many of the world's volcanoes and earthquakes.
Hardness: the Mohs scale
Scientists compare mineral hardness using the Mohs scale from 1 (talc, softest) to 10 (diamond, hardest). A fingernail is about 2.5, a copper coin about 3, and a steel knife about 5.5. A harder mineral can scratch a softer one.
Experiments and activities
- Rock hunt: collect 10 rocks and sort them by color, texture and weight. Use a hand lens to look for crystals and layers.
- Scratch test: try to scratch each rock with a fingernail, a coin and a nail. Record which rocks are harder.
- Vinegar test: a drop of vinegar on limestone or marble bubbles because they contain calcium carbonate. Do this outdoors and rinse afterwards.
- Crayon rock cycle (not edible!): use crayon shavings to model sedimentary (press), metamorphic (warm and squeeze) and igneous (melt and cool). Adult supervision needed for any heat.
- Layers in a jar: shake soil, sand and gravel with water in a jar and watch the layers settle.
Practice quiz
- Name the four layers of the Earth in order from the outside in.
- What are the three types of rock?
- Which type of rock usually contains fossils?
- Is granite igneous, sedimentary or metamorphic?
- What process changes limestone into marble?
- What are tectonic plates?
- Which is harder: quartz or talc?
Answer key
1) Crust, mantle, outer core, inner core 2) Igneous, sedimentary, metamorphic 3) Sedimentary 4) Igneous 5) Heat and pressure (metamorphism) 6) Giant slabs of Earth's crust that slowly move 7) Quartz (talc is the softest mineral)
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