What Are Igneous Rocks?

“Igneous rocks are formed when molten rock cools in the earth or on the surface of volcanoes.”

All the rocks in the world are classified into three categories: igneous, sedimentary, and metamorphic. Igneous rocks form when magma cools slowly under the earth’s surface or rapidly when thrown out of an active volcano. The mineral composition of the magma, the speed of cooling, and the surrounding environment (such as the presence of water), all affect the kinds of igneous rocks that are formed, creating rocks of different colors and with different sizes of crystals. These rocks are beautiful and fascinating, but they are also important scientifically and commercially, as they can tell us things about the interior of the earth. They can also provide clues to the location of valuable mineral deposits. Read on to learn more about igneous rocks and how you can recognize them when you go rock hunting.

Fun Facts About Igneous Rocks

• Igneous rocks are formed by cooling magma under the earth’s surface or cooling lava near or on the earth’s surface.
• Pumice is an igneous rock that can float on water because of gas pockets in its pores.
• You may have igneous rocks in your house, as it is a common material for construction, interior design, decorative items, and jewelry.
• Obsidian is a kind of volcanic glass that is sharper than a scalpel and was used in ancient times to perform surgery.
• Half Dome, the most famous feature of Yellowstone National Park, is a gigantic igneous rock.
• Color is one of the means of classifying these rocks, but you have to break the rock to see the interior color because weathering can affect the exterior.

How Igneous Rocks Form

Igneous rocks form when magma or lava cools, but this can happen under different conditions that result in different kinds of rocks. The two main types of igneous rocks are intrusive and extrusive.

Intrusive

Most igneous rocks are intrusive. Another term for an intrusive rock is “plutonic.” This type of rock forms when magma cools and solidifies while still in the crust. It is well-insulated by surrounding rock, so it retains heat and cools slowly. This results in coarse-grained rocks with large mineral grains that can be seen with the naked eye. Some of the more common intrusive rocks are granite, gabbro, or diorite. The cores of most mountain ranges are made of intrusive igneous rocks. Large areas of these are sometimes exposed through erosion. A great example of this is Half Dome, an iconic geologic formation of Yosemite National Park made of igneous rock that formed underground in a slowly cooling magma chamber, then was uplifted through plate tectonics and weathered through glaciation and other erosive processes.

Extrusive

Extrusive rock is also sometimes called volcanic rock. It is a type of igneous rock that forms when magma reaches the earth’s surface through fissures or volcanic eruptions and quickly solidifies. Extrusive rocks are subdivided into two categories: those that are produced in lava flows and tephra (pyroclastic material). When magma reaches the surface, it is called lava. Lava forms different kinds of rocks based on factors such as temperature, composition, and crystal content. Suppose lava contains a higher content of dissolved gasses, such as water vapor or carbon dioxide. In that case, a volcanic eruption can be more violent and fling cooling lava high into the air. Extrusive rocks typically have a fine-grained structure that can be observed under a microscope. Examples of these types of rocks are andesite, rhyolite, and many more.

Classifications

Igneous rocks are classified not just as intrusive or extrusive, but in other categories as well. Two other classifications for these rocks are texture and composition.

Texture

Igneous rocks come in six main types of textures:

1. Aphanitic textures are found in rocks such as basalt, andesite, and rhyolite. These crystalize rapidly at or near the surface. They do not have time to form large crystals, so they have a fine-grained structure that cannot be seen with the naked eye.
2. Glassy or vitreous textured rocks like obsidian and pumice have few or no crystals because the heat of the lava is quenched quickly by water, for example.
3. Pegmatitic textures occur during slow cooling. Minerals in the rock may grow so large they can reach sizes of a few inches to several yards. Pegmatites are examples of this kind of texture.
4. Phaneritic texture forms from slow cooling, resulting in crystals that can be observed with the naked eye. Examples include gabbro, diorite, and granite.
5. Porphyritic texture results when conditions change quickly as magma is still cooling. This results in a rock with some large crystals embedded in rock with a finer crystalline structure. This can also be the result when a volcano erupts, flinging out rock that has not yet finished forming large crystals, then rapidly cools at the surface with small crystals.
6. Pyroclastic textures result from explosive eruptions that fling out rocks in glassy fragments, such as ash, lapilli, and volcanic bombs.

Composition

There are three main igneous rock compositions:

1. Felsic (granitic) – These rocks are high in silica and low in iron and magnesium. They come in light colors like white, pink, tan, light brown, or light grey.
2. Intermediate (andesitic) – This classification is for rocks that have proportions of silica, iron, and magnesium between the levels of felsic and mafic rocks. Their colors are medium shades of green, grey, or brown.
3. Mafic (basaltic) – Rocks that are high in iron and magnesium and low in silica. These rocks are dark colors like black or very dark shades of brown, grey, or green.

Commercial Uses

Igneous rocks have a lot of commercial uses:

• Construction: Granite and basalt are examples of igneous rocks used in building and decoration, such as countertops building stones, monuments, and wall cladding. They are also used as aggregate in road construction.
• Industrial: Rocks such as pumice and perlite are used as abrasives and in the manufacture of insulation and ceramics.
• Jewelry: Amethyst and garnet are semi-precious stones used in jewelry.
• Decorative: Obsidian, granite, and volcanic glass are used in sculptures and as decorative stones in interior design and landscaping.
• Industrial raw material: These rocks are used as raw material for the production of cement, glass, ceramics, and other commercially important materials.

Where To Find Igneous Rocks

As one of the basic types of rocks found in the world, igneous rocks can be found all over the planet. Taken together with metamorphic rocks, which are sedimentary or igneous rocks that have been transformed physically or chemically, they make up 90-95% of the top 10 miles of the earth’s crust.

In many places, igneous rocks have been overlaid with thick layers of sedimentary rocks. Great places to find igneous rocks are road cuts and quarries that expose geologic layers. Mountain ranges are also areas where plate tectonics may have pushed up a large number of igneous rocks. You might find igneous rocks in a stream bed that washes down from higher elevations.

Even if you live in the city, igneous rocks are pretty easy to find. Check out the stone used in buildings, monuments, gravestones, landscaping, retaining walls, and water features. Granite, basalt, and igneous rocks are popular for these applications because of their strength, durability, weather resistance, and beauty. Look around your house, workplace, or public buildings. Igneous rock is often used for scratch- and heat-resistant kitchen and bathroom countertops, decorative objects such as sculptures or paperweights, or as focal points in aquariums. Check out your jewelry box or the ring on your finger. Got a diamond? That’s an igneous rock.

How Can You Identify Igneous Rocks?

There are several ways that an amateur can tell if a rock is igneous:

• Texture: Igneous rocks have a coarse, grainy texture. The size of the crystals in the rock can indicate the cooling rate of the magma or lava that formed the rock. Slow cooling results in large crystals, while rapid cooling results in small crystals.
• Color: Igneous rocks can come in a variety of colors, but some common colors include black, gray, white, and pink. Color can change due to weathering and chemical reactions on the exterior of the rock. Breaking the rock with a hammer can let you make a better observation of the true color of the specimen.
• Hardness: Igneous rocks are generally harder than sedimentary rocks and can scratch a steel knife blade.
• Luster: Igneous rocks have a dull or glassy look to them.
• Mineral content: Igneous rocks contain a lot of silicate minerals such as feldspar, quartz, and mica.
• Environment: Igneous rocks are often found in areas where volcanic activity has occurred in the past.

Some rocks can have characteristics of both igneous and metamorphic rocks and can be difficult to classify. If you are unsure, consulting with a geologist or a rock identification guide may be helpful.

Favorite National Parks for Amateur Geologists

Although you can likely easily find small igneous rocks where you live, if you would like to see larger igneous formations or a larger variety of them, you might want to visit one of these national parks.

• Volcanoes National Park, Hawaii: Home to world-famous active volcanoes and enormous fields of lava flows. A geologist’s dream!
• Devils Tower National Monument, Wyoming: A huge igneous rock that is the solidified core of an ancient volcano. Most people will recognize it from the movie “Close Encounters of the Third Kind.”
• Mount Rushmore, South Dakota: This famous monument features enormous busts of Presidents Washington, Jefferson, Theodore Roosevelt, and Abraham Lincoln, all carved out of solid granite.
• Yosemite National Park, California: The most famous site here is Half Dome, a large igneous rock formation popular with climbers.
• Grand Canyon National Park, Arizona: This is the place to go to observe all kinds of rock types and strata dating back millions of years.
• Joshua Tree National Park, California: Here you can see not only iconic Joshua trees but desert and mountain areas with unique and beautiful igneous rock formations.

Remember to follow all the park’s rules regarding touching or removing rocks. These parks receive millions of visitors a year. If everyone took home one small rock, it would be like ants systematically eating a cake until it was all gone. Don’t do it. But go look, take pictures, and admire the beauty of igneous rocks.