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Igneous rocks are the most basic type of rock. They are formed from solidified magma derived from the Earth’s mantle. Magma can solidify above the surface or underground, resulting in two subtypes of igneous rock. When magma erupts from a volcano, it forms extrusive or volcanic rocks on the surface. If the magma cools underground, it forms intrusive or plutonic rock.
The ten most common types of rocks found in volcanoes are basalt, andesites, rhyolites, dacites, obsidian, and pumice (volcanic rocks), and gabbro, diorites, pegmatites, and granite (plutonic rocks). They are all forms of igneous rock created by the magma that flows from volcanoes millions of years ago.
The plutonic or intrusive rock contains large crystals that are usually visible without a microscope. Extrusive or volcanic rocks have a fine-grained texture and cool too fast to form large crystals. Sometimes the magma cools so rapidly that no crystals are formed, and this type of rock is called volcanic glass. It is a common component of obsidian. Even though plutonic rock forms underground, it can become exposed through weathering.
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What Types of Rock Are Found In Volcanoes
Volcanoes can be dangerous but a great spot for finding unique and beautiful rocks and minerals too. Let’s take a look and the ten most common rocks you can find in or near volcanoes in the sections below.
The Earth’s crust is mainly basalt, a heavy, dark, grainy rock. Basalt is a hard black igneous rock with low silica content. The Hawaiian islands are examples of basalt eruption. Mount Kilauea is the name of the most active volcano in Hawaii, and its basalt flows have extruded over one cubic mile of lava covering forty-eight square miles of land.
Andesite is an extrusive volcanic rock whose mineral composition is intermediate between basalt and rhyolite. Mounts Shasta, Hood, and Adams have expelled significant quantities of andesitic rock. It corresponds to the plutonic igneous rock, diorite.
Andesite is named after the Andes mountain range in South America and is the most common volcanic rock after basalt.
It is typically found in volcanoes sitting on plate boundaries between continental and oceanic plates. Stratovolcanoes expressing andesite are located in Central America, Washington, Oregon, and Alaska.
Rhyolite tends to be whitish, grey, or pink and contains more than 70% silica. It does not contain magnesium or iron in significant quantities. As the lava cools, gas bubbles form a void called a vug in rhyolite.
If water containing minerals fills a vug, conditions may be right for crystals and gems such as agate, red beryl, topaz, and jasper to form in them. Almandine-spassartine garnets are also found in rhyolite vugs.
Granite magmas produce rhyolite obsidian pumice or tuff. Yellowstone volcano produces granitic magma.
Thundereggs are hollow spherical rocks that form from gas bubbles in rhyolite and other volcanic rock. They can be filled with opal, chalcedony, jasper, and agate crystals, but even if they don’t contain gemstones, they display beautiful patterns when cut open.
They occur widely in Oregon, where a favorite rockhounding site is Whistler Springs. Other popular areas are White Fir Springs in the Ochoco Mountains and Succour Creek Canyon in Oregon.
TIP: The most challenging and time-consuming aspect of working with opals is cutting. Check out the complete guide on cutting and polishing opals in the article below:
How to Cut & Polish Opals: Follow These 9 Simple Steps
Dacite is found in lava flows, sills, dikes, lava domes, and pyroclastic debris. Other minerals that may be found in dacite include augite biotite and hornblende. Volcanic activity at Mount Saint Helens has produced dacite and andesite in volcanic domes and volcanic ash.
Dacite is often produced in explosive eruptions when the magma reaches the surface. Ancient people have used it to make sharp tools and useful objects, but the edges are not as sharp as obsidian.
Obsidian typically forms as an extrusive igneous rock from fast-flowing lava and has a glassy appearance. It occurs as black for the most part but can also be brown, blue, red, tan, orange, and yellow.
In mahogany obsidian, some of these colors may be swirled together in beautiful patterns when the stone is polished.
Rainbow obsidian contains mineral impurities that form an iridescent sheen and is highly valued in the jewelry trade.
Glass Butte in Oregon is a volcano composed mainly of rhyolite flows. It is a favorite rockhounding site where gem-quality double flow, black, pumpkin, rainbow, silver sheen, fire, and gold sheen obsidian can be found.
It can be picked up from the ground’s surface or dug out of it. Obsidian is always seen near recently active volcanoes and rarely lasts for more than a couple of million years. Ancient peoples have used it to make arrowheads, scrapers, knives, and spearpoints.
TIP: Obsidian fakes are hardly spotted with the unaided eye. Time, patience, and additional equipment are needed to differentiate between natural and fake obsidian. Find out more in the article below:
Real vs. Fake Obsidian Stone: Check These 8 Key Differences
Pumice is another extrusive volcanic rock that is very porous and pale in color. It is so light and foamy that it can float on water. It forms when a volcano discharges lava with very high gas and water content and has been described as a froth of felsic volcanic glass.
Pumice is mined throughout the American west, with Oregon producing the most, followed by Arizona, Idaho, Nevada, California, and New Mexico.
Gabbro is a dark-green or black plutonic rock that typically has coarse grains. It contains small amounts of olivine, feldspar in abundance, and augite.
It can be polished to a brilliant black luster and is used to make cemetery markers, floor tiles, kitchen countertops, and other dimensions of stone products. It is often sold under the name “black granite”. Garnets have been found in gabbro.
Diorite is a plutonic igneous rock composed mainly of feldspar, hornblende, and pyroxene. When the same magma erupts to the surface, the rock it forms is called andesite.
Andesite is lighter colored than basalt because it contains less iron and more silica and may have a salt and pepper appearance. Diorite polishes as well and has been cut into cabochons and used as a gemstone occasionally.
In Australia, there is a beautiful form of diorite that contains sizeable pink feldspar crystals. It has been called pink marshmallow stone and is cut into cabochons.
BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
Pegmatites are coarse-grained rocks that are a major source of many kinds of gemstones such as beryl, apatite, fluorite, garnet, rose and smoky quartz, sapphires, topaz, and tourmaline.
They are extreme igneous rocks because they form during the final stage of magma crystallization and contain huge crystals.
Some of the best topaz, tourmaline, and aquamarine deposits in the world have been found in pegmatite. Crabtree pegmatite in western North Carolina has yielded many lovely transparent emeralds and a green variety of beryl. The Harding pegmatite in New Mexico contains spodumene crystals five meters long.
TIP: Not many people know that mineralogically emerald is a variety of beryl group minerals. Find out the main differences between emerald and beryl in the article below:
Beryl vs. Emerald: 7 Key Differences (Are They The Same?)
Granite is a common type of plutonic igneous rock and is coarse-grained in texture. It contains mainly feldspar, quartz, and mica and can be white, grey, or pink, depending on its mineral content.
World-famous natural exposures of granite include Yosemite Valley in California, Stone Mountain in Georgia, Mount Rushmore in South Dakota, and the White Mountains in New Hampshire.
Tourmaline, one of the most popular gemstones in the world, can be found in granite. Other accessory minerals in granite include beryl, topaz, apatite, and zircons.
Tips For Rockhounding In Volcanoes
It is not safe to go rockhounding in the crater of an active volcano. However, many volcanic and plutonic rocks can be found in places that were volcanically active in the past.
The Gila National Forest lies within the Mogollon-Datil volcanic field, which is responsible for the agate, jasper, obsidian, quartz crystals, thundereggs, and chalcedony found there. The Marysvale area in Utah has both volcanic and plutonic rocks.
Rockhound State Park is in the Little Florida Mountains in New Mexico. These mountains were the scene of past volcanic activity and consist of andesite, dacite, and rhyolite.
Northern Arizona’s San Francisco Volcanic Field is a prominent geological landmark, and San Francisco Mountain is an enormous stratovolcano where obsidian can be found. In Colorado, the Creede Caldera area is full of volcanic rocks that erupted around twenty-seven million years ago.
A rock hammer, shovel, hat, safety glasses, and sturdy gloves are essential equipment when going hunting for volcanic and plutonic rocks.
TIP: The right quality equipment is always the basis of success. If you are looking for rockhounding equipment, check out the article below for recommendations on the best rockhounding equipment:
The Complete Guide: All Tools You Need for Rockhounding