Rocks, Pebbles, and Boulders - First Grade Science
The ground beneath our feet, the tall mountains rising into the clouds, and the sandy riverbeds are all made of rocky materials that form the solid outer crust of planet Earth. Geologists are scientists who study rocks, and they know that every rock tells an ancient story about how our planet was formed over millions of years. Rocks come in an astounding variety of colors, textures, patterns, and sizes, ranging from microscopic grains of dust to pebbles you can skip across a pond, all the way to colossal boulders as big as houses.
What is a Rock?
A rock is a naturally occurring solid material made of one or more minerals tightly bound together. Minerals are the pure, non-living building blocks that form rocks, just as individual ingredients combine to bake a cake.
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| ROCKS VS. MINERALS |
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| MINERAL (Single Ingredient) ROCK (A Mixture of Minerals) |
| - Pure chemical crystal compound. - A solid mixture of different |
| - Same composition throughout. minerals bound together. |
| - Examples: Quartz, Feldspar, Mica, - Examples: Granite, Basalt, |
| Gold, Diamond, Calcite. Sandstone, Limestone, Marble. |
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Mineral Properties
Geologists identify minerals by testing their properties. They check the mineral's color, streak (the powder color left behind when scraped on a tile), luster (whether it looks shiny, glassy, metallic, or dull), and hardness using the Mohs hardness scale. Diamond is the hardest natural mineral on Earth, while talc is so soft you can scratch it with your fingernail!
Sorting Rocks by Size
One of the easiest ways to classify rocks is by their physical size. Scientists categorize rock pieces into distinct groups based on diameter.
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| ROCK SIZE CATEGORIES |
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| BOULDER Huge rock larger than 25 centimeters (larger than head) |
| COBBLE Medium rock about the size of an apple or grapefruit. |
| PEBBLE Small smooth stone that easily fits in your palm. |
| GRAVEL Small rough rock fragments about the size of peas. |
| SAND Tiny mineral grains between 0.06 mm and 2 mm. |
| SILT & CLAY Microscopic powdery dust particles. |
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Boulders
Boulders are massive stones that require heavy machinery or several people to move. Many boulders were transported miles from their original mountain homes by slow-moving rivers of ice called glaciers during the Ice Age!
Pebbles and Cobbles
Pebbles are small, lightweight stones. When rocks tumble down tumbling mountain rivers, the rushing water bangs them against one another. This constant friction grinds off jagged edges, polishing the pebbles until they are silky smooth and rounded.
The Three Major Rock Types
Deep inside the Earth and on its surface, geological processes create three primary types of rocks: igneous, sedimentary, and metamorphic.
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| THE THREE ROCK FAMILIES |
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| IGNEOUS ROCKS SEDIMENTARY ROCKS |
| Formed from cooling melted rock Formed from pressed layers of |
| (magma inside, lava on surface). sand, mud, and ancient shells. |
| Examples: Granite, Obsidian, Basalt.Examples: Sandstone, Limestone. |
| |
| METAMORPHIC ROCKS |
| Changed by intense heat and crushing pressure |
| deep within the Earth without melting. |
| Examples: Marble (from limestone), Slate. |
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Igneous Rocks: Born from Fire
When deep underground rock melts from extreme heat, it turns into molten liquid called magma. When a volcano erupts, this molten rock bursts out onto the surface as lava. When lava cools rapidly in cool air or water, it solidifies into igneous rock. Obsidian cools so fast it forms black volcanic glass!
Sedimentary Rocks: Layer upon Layer
Over thousands of years, wind and rain wash sand, mud, and broken shells into lake bottoms and ocean floors. Layer upon layer piles up. The heavy weight presses the bottom layers down tightly. Over time, natural minerals cement the particles together into solid rock. Sedimentary rocks often contain fossils of ancient plants and prehistoric sea animals!
Metamorphic Rocks: Under Pressure
Deep inside the Earth, tectonic forces squash and heat existing rocks. Although it does not melt entirely, the rock transforms, recrystallizing into a harder, denser new rock called metamorphic rock. Soft limestone turns into sparkling marble used in statues and monuments.
Chapter Review Exercises
Question 1: What is the scientific difference between a mineral and a rock?
Question 2: List four rock size categories in order from the largest to the smallest.
Question 3: How does tumbling down a rushing mountain river turn sharp, jagged rocks into smooth pebbles?
Question 4: Name the three major families of rocks and explain how igneous rocks are formed.
Solutions and Explanations
Solution 1: A mineral is a pure, naturally occurring solid substance with a specific crystal structure throughout, whereas a rock is a solid mixture composed of two or more minerals bound together.
Solution 2: Four rock size categories from largest to smallest are boulder, cobble, pebble, and sand.
Solution 3: As rocks tumble down a rushing river, the force of the water bangs them into each other and drags them across the riverbed. This tumbling action grinds away rough points and sharp corners, smoothing and polishing the stones into rounded pebbles.
Solution 4: The three major rock families are igneous, sedimentary, and metamorphic. Igneous rocks are formed when melted liquid rock (magma underground or lava on the surface) cools down and hardens into solid stone.