Rocks and Soil

All About Rocks#

Rocks are everywhere! They are in mountains, rivers, beaches, and even under your feet. Rocks make up most of the solid Earth beneath us.

What Is a Rock?#

A rock is a naturally occurring solid made of one or more minerals. Minerals are the building blocks of rocks. Each mineral has its own color, hardness, and shine.

Rocks come in an amazing variety — big boulders on mountaintops, smooth pebbles in streams, tiny grains of sand on a beach, and sparkling crystals in caves.

Describing Rocks#

Scientists observe rocks and describe them using several properties:

• Color — Rocks can be white, black, grey, red, brown, green, or many colors at once.
• Texture — Some rocks feel smooth and glassy. Others feel rough and grainy. Some have visible crystals.
• Shape — Rocks in rivers are often round and smooth because water has worn down the rough edges. Rocks on mountaintops may be jagged and sharp.
• Size — From tiny grains of sand to enormous boulders.
• Hardness — Some rocks scratch easily. Others can scratch glass.
• Luster — How shiny or dull a rock looks.

Three Types of Rocks#

Geologists (rock scientists) group rocks into three families:

1. Igneous rocks — formed from cooled lava or magma. Examples: granite (speckled), obsidian (shiny black glass), basalt.
2. Sedimentary rocks — formed from layers of sand, mud, and shells pressed together over time. Examples: sandstone, limestone, shale.
3. Metamorphic rocks — formed when existing rocks are changed by heat and pressure deep in the Earth. Examples: marble, slate, quartzite.

The Rock Cycle#

Rocks change over millions of years! Heat, pressure, and erosion slowly transform one type of rock into another. This ongoing process is called the rock cycle.