When you hold a volcanic rock, the rock in your hand is one of the oldest objects in the world. Volcanic rocks are formed from lava, magma, or ash from volcanic eruptions or flows.. Volcanic rocks have distinctive properties that can help you distinguish them from other rock types, as well as identify the specific type of volcanic rock you have.
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Method 1 of 2: Identifying Volcanic Rock
Step 1. Classify volcanic rocks into two main types:
intrusive igneous rock and extrusive igneous rock. Each of these rock types has certain properties that will help distinguish which type of volcanic rock is yours.
- Magma is molten material that flows beneath the earth's surface. Volcanic rocks are formed from cooled magma.
- The location of the rock formation, as well as how quickly the magma cools will determine the type of volcanic rock.
- Intrusive igneous rock is formed from magma that cools deep within the Earth's surface. Because this occurs within the Earth's surface, magma will cool very slowly.
- Magma will form crystals as it cools.
- Intrusive igneous rocks contain larger crystals which usually coalesce and form rock masses.
- An example of an intrusive igneous rock is granite.
- Magma that flows over the earth's crust is called lava.
- Extrusive igneous rock is formed by the rapid cooling of lava above the earth's surface.
- Extrusive igneous rock contains very small, almost microscopic crystals. These rocks are often referred to as fine-grained igneous rocks. Usually you can't see it with the naked eye.
- The most common type of extrusive rock is basalt.
Step 2. Identify your rock type
There are 7 different texture categories in volcanic rock, each with its own unique features.
- Pegmatite rocks contain very large crystals, with a size of more than 1 cm. This is a type of volcanic rock that cools the longest.
- Remember, the longer the rock cools, the larger the crystal size.
- Phaneritic volcanic rocks consist of interlocking crystals that are smaller than the crystals in pegmatitic rocks, but can still be seen with the naked eye.
- Porphyritic rocks have two different crystal sizes, often with large crystals located in smaller crystal regions.
- Afhanitic rocks have a very fine texture and most of the crystals are too small to be seen with the naked eye. You will need a magnifying glass to observe crystals in aphanitic rocks.
- Volcanic rock that cools too quickly to form crystals has a glassy texture. Obsidian is the only volcanic rock with a glassy texture, and can be recognized by its black color. This rock looks like solid black glass.
- Vesicular rocks, such as pumice, have bubbles and form before gas can escape when lava is petrified. It is also formed in a very rapid cooling process.
- Pyroclastic rocks have a texture consisting of volcanic fragments that range from very fine (ash) to very coarse (tuff and breccia) textures.
Step 3. Look at your rock composition
Composition refers to the percentage of minerals in your rock. You will need a rock guide to determine the minerals present in the rock. There are four main types of composition in volcanic rocks:
- Identifying the composition of a rock can be very difficult if you are not an experienced rock collector or geologist.
- If you have questions about how to identify a rock, contact a collector or geologist at your local college or university.
- Felsic rock has a light color. Its mineral composition consists mostly of feldspar and silicates such as quartz.
- Granite is an example of a felsic rock.
- Felsic rock has a low density and contains 0-15% mafic crystals. The mafic minerals are olivine, pyroxene, amphibole, and biotite.
- Mafic rocks are dark in color and contain mostly magnesium and iron. This rock contains mafic mineral crystals as much as 46-85% and has a high density.
- Basalt is an example of a mafic rock.
- Ultramafic rocks also have a darker color and contain higher amounts of minerals than mafic rocks. This rock has a mafic mineral crystal content of more than 85%.
- Dunite is an example of an ultramafic rock.
- Intermediate rocks contain mafic mineral crystals as much as 15-45%. This rock contains almost the same amount of felsic and mafic minerals and has an intermediate color (a mixture of primary and secondary colors).
- Diorite is an example of an intermediate rock.
Method 2 of 2: Distinguishing between the Main Rock Types
Step 1. Know the differences between the three main rock types
The three main rock types are volcanic rock, metamorphic rock (malih), and sedimentary rock.
- Volcanic rocks are formed from the rapid or slow cooling of magma/lava.
- Metamorphic rocks are formed under the influence of heat, pressure, or chemical activity.
- Sedimentary rocks are basically formed from smaller pieces of rock, fossils, and sediments.
Step 2. Look for markings in the form of layers on your stone
The presence of scattered layers can help identify the main type of stone you have.
- If the rock contains layers, it will have sections of different colors and may or may not contain small crystals or fossils. You need to find it using a magnifying glass.
- In cross section, the layers in the stone will look like stripes of various colors that overlap each other.
- Having a diffuse layer can help identify the main type of stone you have.
- Volcanic rock has no layers. If your rock has layers, it is either metamorphic or sedimentary rock.
- Sedimentary rock has a smooth layer, looks like shale, and is composed of silt, sand, and gravel.
- Sedimentary rocks can also contain crystals. If the layers in your rock consist of crystals of various sizes, your rock is a sedimentary rock.
- Metamorphic rocks have layers consisting of crystals of the same size.
- Layers in metamorphic rocks also have a bent and irregular shape.
Step 3. Inspect your stone for visible signs of grain
You'll need a magnifying glass to do this, as some grains and crystals can be so small that you won't be able to see them with the naked eye. If your stone appears to have grain, proceed to the next step to classify your stone by grain type. If no grain is visible, use the following criteria to classify your stone:
- Volcanic rocks are very dense and hard. These rocks can have a glassy appearance.
- Metamorphic rocks can also look like glass. You can tell them apart from volcanic rocks by the fact that metamorphic rocks tend to be brittle, light, and have an opaque black color.
- Sedimentary rock without grains will look like dry clay or mud.
- Sedimentary rocks without grains also tend to be smooth, because they are usually easily scratched by nails. These rocks also react to hydrochloric acid
Step 4. Classify the type of grain in your stone
Remember, not all stones have visible grains. The grain will look like a collection of sand, fossils, or small crystals.
- Only metamorphic and sedimentary rocks contain fossils. Sedimentary rocks can have fossils that look like intact or decomposed forms of leaves, shells, footprints, etc. Metamorphic rocks contain only fossils that have been fragmented.
- Sedimentary rock contains grains consisting of sand, silt, or gravel. These grains can be round (clastic), or consist of other rocks.
- If the grain in your rock contains crystals, you can use the direction and size of the crystal to identify the stone.
- Volcanic rocks contain crystals in different directions. These rocks can also have large crystals with smaller crystals in the base mass.
- Sedimentary rocks contain crystals that are easily crushed or scratched.
- Metamorphic rocks contain crystals that have a streaky or scaly appearance. These displays are often long and regular in parallel patterns.
Step 5. Look at your stone for additional characteristics
You need to look for metallic look structures or grooved lines.
- Rocks that have a metallic appearance with a scaly or smooth texture are metamorphic rocks.
- Volcanic rocks can have a vesicular texture. This is when the stone appears to have pores with many holes.
- Pumice is an example of a rock that has a texture with many pores.
- Volcanic rocks are very hard. Many types of volcanic rock have a rough grooved structure on their surface.
