Rocks, Minerals and Cycles

Rocks may seem unchanging, but the ground under your feet is part of a slow, never-ending recycling system called the rock cycle. Every rock you can pick up has a story — it was once melted, buried, squeezed, washed down a river, or pressed deep below a mountain. Geologists sort rocks into three big families based on HOW they formed: igneous, sedimentary, and metamorphic.

Minerals — the Building Blocks

Before talking about rocks, you have to know what they are made of. A mineral is a natural, solid substance with a fixed chemical recipe (for example, quartz is SiO2). Minerals are the ingredients. A rock is a mixture of one or more minerals, just like a cake is a mixture of flour, eggs, and sugar. Granite, for instance, is a single rock built from three minerals: quartz, feldspar, and mica.

• Mineral = one pure substance with a fixed recipe (quartz, calcite, feldspar).
• Rock = a natural mix of minerals (granite, sandstone, marble).
• Hardness, color, and luster help geologists identify minerals.
• The Mohs hardness scale ranks minerals from 1 (talc) to 10 (diamond).

The Three Rock Families

Each family is named after the way its rocks were born. Igneous rocks come from fire (Latin "ignis" = fire). Sedimentary rocks form from settling pieces. Metamorphic rocks are old rocks that have been changed.

1. Igneous — formed when hot, melted rock (magma or lava) cools and hardens. Examples: granite, basalt, pumice, obsidian.
2. Sedimentary — formed when small bits (sand, mud, shells) pile up in layers and get pressed and cemented together. Examples: sandstone, shale, limestone, conglomerate.
3. Metamorphic — formed when an existing rock is squeezed and heated deep underground until its minerals rearrange. Examples: marble (from limestone), slate (from shale), gneiss (from granite), schist.

The Rock Cycle

Rocks do not stay one type forever. Over millions of years, any rock can be turned into any other type. The cycle never stops — and the same atoms have been recycled through countless rocks since Earth formed.

1. Magma deep underground cools and hardens.
2. An igneous rock forms.
3. Wind, water, and ice slowly weather and erode it into tiny pieces.
4. Sediments are deposited in layers (often at the bottom of rivers or seas).
5. Compaction and cementing turn the layers into a sedimentary rock.
6. Heat and pressure deep underground change it into a metamorphic rock.
7. If it gets hot enough, it melts back into magma — and the cycle repeats.

How to Tell Them Apart

• Igneous: often has tiny mineral crystals (slow cooling) or glassy / bubbly texture (fast cooling).
• Sedimentary: usually shows visible LAYERS or fossils, and may feel grainy.
• Metamorphic: often has bands or stripes of squashed minerals, and feels harder than the rock it came from.