What are rocks made of?
Rocks are the building blocks of our planet, and understanding them gives us a peek into the Earth’s story. By learning to identify rocks, we can start to understand where they came from, how they formed, and even how we might use them.
Rocks are generally divided into three main types: igneous, sedimentary, and metamorphic. These categories are based on how the rocks were formed.
This guide is designed to give you a systematic way to identify rocks, focusing on key features and providing easy-to-use identification charts. Whether you’re a student, a hobbyist, or just curious about the world beneath your feet, this guide will help you learn to identify rocks.
Basic Principles of Rock Identification
So you want to identify a rock? Here are the basics:
What to look for:
- Color: Color can give you a hint about what a rock is made of, but it isn’t foolproof.
- Texture (Grain Size): How big are the grains? How are they arranged?
- Hardness: You can use the Mohs scale to get a sense of how hard the rock is.
- Composition: What minerals make up the rock?
What tools to use:
- Hand lens
- Streak plate
- Hardness kit
- Diluted hydrochloric acid
How to tell rock types apart:
First, decide if the rock is igneous, sedimentary, or metamorphic. Then, consider how the rock formed. Did it cool from magma? Did it form from sediments? Or did heat and pressure change it?
Igneous Rocks: Formation and Characteristics
Igneous rocks are formed from cooled and solidified molten rock. This molten rock can be either magma, which is found underground, or lava, which emerges onto the surface. The speed at which the molten rock cools plays a huge role in the rock’s final texture.
How Igneous Rocks Form
Igneous rocks are classified as either intrusive or extrusive:
- Intrusive (plutonic) rocks cool slowly beneath the Earth’s surface. This slow cooling allows for the formation of large, easily visible crystals, resulting in what’s called a phaneritic texture.
- Extrusive (volcanic) rocks cool rapidly on the Earth’s surface. This rapid cooling results in small, microscopic crystals (aphanitic texture) or even a glassy texture.
Classifying Igneous Rocks
Igneous rocks are classified based on their mineral composition and texture:
- Mineral Composition:
- Felsic: Rich in light-colored minerals like quartz and feldspar.
- Mafic: Rich in dark-colored minerals like pyroxene and olivine.
- Intermediate: Contains a mix of light and dark minerals.
- Ultramafic: Composed almost entirely of dark-colored minerals.
- Texture:
- Phaneritic: Coarse-grained, with visible crystals.
- Aphanitic: Fine-grained, with microscopic crystals.
- Porphyritic: Contains large crystals (phenocrysts) in a fine-grained matrix.
- Vesicular: Contains holes (vesicles) formed by gas bubbles.
Igneous Rock Identification Chart
Igneous rocks form when magma or lava cools and solidifies. Intrusive igneous rocks cool slowly beneath the Earth’s surface, resulting in larger crystals (phaneritic texture). Extrusive igneous rocks cool quickly on the Earth’s surface, resulting in smaller crystals or a glassy texture (aphanitic or glassy texture).
Intrusive Igneous Rocks
- Granite: Felsic in composition, granite is a phaneritic rock made up of quartz, feldspar (both orthoclase and plagioclase), and mica (biotite and muscovite). It’s typically light-colored, coming in pink, gray, or white.
- Diorite: With an intermediate composition and phaneritic texture, diorite contains plagioclase feldspar and amphibole. Its appearance is often described as “salt-and-pepper.”
- Gabbro: A mafic rock with a phaneritic texture, gabbro is composed of pyroxene and plagioclase feldspar. It’s typically dark green or black.
- Peridotite: An ultramafic rock with a phaneritic texture, peridotite is mainly olivine, with a bit of pyroxene. This rock is usually greenish.
Extrusive Igneous Rocks
- Basalt: This mafic rock has an aphanitic texture and contains pyroxene and plagioclase feldspar. Its color is usually black or dark gray.
- Rhyolite: A felsic rock with an aphanitic texture, rhyolite contains quartz and feldspar and is typically light-colored (pink, gray, or white).
- Andesite: An intermediate rock with an aphanitic texture, andesite contains plagioclase feldspar and amphibole or pyroxene, giving it a grayish color.
- Obsidian: This felsic rock has a glassy texture because it’s volcanic glass that formed from rapid cooling. It’s almost always black.
- Pumice: Pumice is a felsic rock with a vesicular texture. It’s light-colored and extremely lightweight because it’s full of vesicles (holes).
Sedimentary Rocks: Formation and Identification
Sedimentary rocks are formed through a fascinating process that involves the breakdown of existing rocks and the accumulation of sediments over time.
How sedimentary rocks form
- Weathering and Erosion: Existing rocks are broken down into smaller sediments.
- Transportation: Wind, water, and ice carry sediments away.
- Deposition: Sediments build up in layers.
- Lithification: Compaction and cementation turn loose sediments into solid rock.
Types of sedimentary rocks
- Clastic: These rocks are made of fragments of other rocks. Sandstone, shale, and conglomerate are good examples.
- Chemical: These rocks form when minerals precipitate out of a solution, like water. Limestone and rock salt are chemical sedimentary rocks.
- Organic: These rocks are formed from the remains of plants and animals. Coal is an example of an organic sedimentary rock.
Identifying sedimentary rocks
Here are some clues that can help you identify a sedimentary rock:
- Look for layers (strata) and sedimentary structures like ripple marks.
- Examine the size of the grains and the composition of the rock.
- Test for carbonates by dripping acid on the rock. Limestone will react.
Metamorphic Rocks: Formation and Identification
Metamorphic rocks are rocks that have changed form because of heat, pressure, or exposure to chemically active fluids.
These changes can happen on a grand scale through regional metamorphism, which is usually caused by tectonic forces. Or they can happen on a smaller scale through contact metamorphism, where the heat from magma intrusions transforms the nearby rocks.
Metamorphic rocks are generally divided into two groups:
- Foliated. These rocks have minerals aligned in parallel layers. Examples include slate, schist, and gneiss.
- Non-foliated. These rocks don’t have distinct layering. Examples include marble and quartzite.
To identify a metamorphic rock, look for foliation (layering). Examine its mineral composition and texture, and consider what parent rock (or protolith) it may have formed from.
Wrapping Up
Identifying rocks means looking closely at their color, texture, hardness, and what they’re made of. It also means understanding how igneous, sedimentary, and metamorphic rocks are formed.
This guide is a starting point. To become a real rockhound, keep learning and practicing. Use field guides, online resources, and expert advice to grow your knowledge.