What are the 3 main rock types and how are they formed?

A group of friends are hiking and enjoying the scenery.

Photo by Evan Sanchez on Unsplash

Selena: “Hey, look at these rocks, they’ve got some unusual textures. There must be hundreds of varieties.”

Amari: “There aren't that many kinds of rocks. I remember from geology class that there are only 3 main rock types."

Blake: "That's really interesting, I don't think many people would know that."

Selena doesn't know much about rocks herself. She decides to do some research so she can start a rock collection.

Rocks are everywhere in all shapes, colors, sizes, and textures — from pebbles on the beach to boulders on hillsides and mountains ranges. 

If you're like Selena, you’re probably thinking there must be thousands, if not millions of rocks, right?

Well, the "rock cycle" makes only 3 main rock types:

• Igneous rock

• Sedimentary rock

• Metamorphic rock

Each type has a specific set of physical characteristics.

Image courtesy of Sheri Amsel via Exploring Nature

Click play on the audio player below to listen to a description of the diagram shown above.

The rock cycle changes rocks from one formation to another by:

Cooling & Crystallization

• lava eruptions quickly crystallize & become solid when cool

• underground magma slowly cools with or without crystalizing

Compression & Burial

• large pieces of rock sink into magma below the earth

• becomes solid when cool as it rises to the surface

Compaction & Cementation

• layers of tiny rocks, sand, mud, & debris

• crushed, squeezed & cemented together

Weathering & Erosion

• ice, snow, wind, rain & heat break down & dissolve rocks

• spreads minerals, remains from animals, plants & sea life

Igneous rock is formed when eruptions from volcanic activity or cracks in the earth's floor cause lava and magma rocks to melt, cool, and crystalize.

Click play on the audio player below to listen to a description of the diagram shown above.

This process produces 2 basic kinds of igneous rocks:

1. Extrusive rocks form when lava above ground quickly cools, becomes solid, and crystalizes. This produces fine (or invisible) quartz crystals.

Common extrusive rocks are andesite, basalt, dacite, obsidian, pumice, rhyolite, scoria, and tuff.

Obsidian

• solid lava glass eruptions

• hard & brittle volcanic rock

• black, smooth glossy appearance

2. Intrusive rocksform deep underground when magma slowly cools under extreme pressure and produces distinctive crystals.

Common intrusive rocks are diabase, diorite, gabbro, granite, pegmatite, and peridotite.

Granite

• mainly quartz crystals

• dark grains of feldspar & mica minerals

• colors of light pink-grey-white

Sedimentary rock is made from built-up deposits of grains, minerals, tiny rock pieces from older rocks, minerals, animals, plants, and sea life.

Extreme weather and erosion cement materials together into solid layers of sedimentary rock.

Click play on the audio player below to listen to a description of the diagram shown above.

This process produces 3 types of sedimentary rocks:

1. Clastic sedimentary rocks are made of pieces of rock, grains, and minerals from old weathered rocks.

Common rocks are breccia, conglomerate, sandstone, siltstone, and shale.

Breccia

• small & large pointed rocks

• pieces cemented together

• color of natural earth tones

2. Chemical sedimentary rocks are made from buildups of natural minerals and calcium deposits from marine life in rivers and oceans.

Common rocks are dolomites, rock salt, flint, iron ore, and chert.

Chert

• cryptocrystalline minerals

• quartz minerals

• grey, waxy appearance

3. Organic sedimentary rocksare made from buildups of deposits from organic minerals like plants, trees, animal bones, sea shells, and other debris. Common examples are chalk, coal, diatomite, some types of dolomite, and limestone.

Metamorphic rocks are igneous and sedimentary rocks that change (meta) or morph (transform) through a process called “metamorphism".

These original rocks physically change, re-crystallize, and become new types of rock when exposed to extreme heat, pressure, and chemical processes.

Click play on the audio player below to listen to a description of the diagram shown above.

This process produces two types of metamorphic rocks:

1. Foliated metamorphic rocks with long, flat, or long-grain minerals stretched and cemented together to form patterns of stripes or layers.

Common rocks are gneiss, phyllite, schist, and slate.

Gneiss

• hard, fine-grained rock

• thin black & white stripes

• clay minerals or mica

2. Non-foliated metamorphic rocks are stronger and more solid in texture without visible mineral layers.

Common examples are hornfels, marble, novaculite, quartzite, and skarn.

Quartzite

• hard, tough, durable solid rock

• grains of mineral quartz

• quartz sandstone

Selena is looking at 3 rocks in her collection:

She's trying to identify their rock types from the following list of descriptions:

Description 1:

Granite rock contains quartz crystals with grains of feldspar minerals.

Description 2:

Gneiss rock has a pattern of thin stripes or bands of minerals arranged in layers

Description 3:

Limestone rock contains from grains of rock salt minerals & calcium deposits

Description 4:

Breccia rock is sealed together with sharp-pointed pieces of rock, soil & minerals

Photo by Jamie Fenn on Unsplash

Now that you've learned about the 3 major rock types, plan a hiking trip or visit a national park, and expand your knowledge: