Igneous Rocks

Igneous rocks form when hot molten rock (magma) cools and literally “freezes” into solid rock. Because magma comes from deep inside the Earth, igneous rocks tell geologists about what is happening in the interior of the Earth. This is different from sedimentary rocks which record the history of the earth’s surface.

 

Basic Information About Igneous Rocks

The size of the crystals in an igneous rock tells us how fast the magma cooled.

If magma cools deep underground under many layers of overlying sedimentary rock, the magma cools slowly. It is insulated by the overlying layers which act as a sort of blanket to trap the heat. In this environment, the crystals have a long time to form and grow. They become quite large. The resulting rock is called an intrusive igneous rock. The texture of the rock (the arrangement of the crystals in the rock) is called phaneritic because the crystals are large enough to be seen with the naked eye. The most common phaneritic igneous rock is called granite. Granite forms a large part of the Earth’s continental crust.

Magma that cools at or near the Earth’s surface cools very quickly. Therefore, the crystals don’t have time to grow. The resulting rock is called an extrusive igneous rock or volcanic rock and contains crystals that are too small to see with the naked eye. This texture is called aphanitic and the most common aphanitic rock is called basalt. It forms much of the ocean floor.

 

Igneous Rocks of Devils Tower National Monument

Devils Tower itself is composed of igneous rock. When you visit Devils Tower you can best see the igneous rocks by hiking along the Tower trail in the monument.

Composition and Texture

The igneous rock that forms the Tower is called phonolite porphyry. The word phonolite (fō'nə-līt') refers to the mineral composition of the rock. The word porphyry refers to its texture or crystal size and arrangement.

Phonolite is a type of rock that contains mainly sodium and potassium feldspar minerals. It contains very little silica. A table containing the mineral composition of several rock samples from the Tower can be viewed by clicking here. Phonolite has a characteristic ringing sound when struck with a hammer, hence the name. (link to video of rock being struck with hammer)

An igneous rock with a porphyritic texture contains both large crystals visible to the naked eye—called phenocrysts—and small crystals that are too small to see. The small crystals form the matrix or groundmass around the larger phenocrysts. So, a porphyritic rock is both aphanitic AND phaneritic meaning that it cooled both fast AND slow. How can this be????

The white phenocrysts in the phonolite of the Tower are made of feldspar. Some smaller black phenocrysts are made of augite (pyroxene) which look like black specks. These crystals formed during a period of slow cooling when the magma that formed the Tower was still deep underground.

Before the rest of the magma could crystallize, the magma and the phenocrysts moved upward as a slushy liquid until they were only about a mile below the surface. At these cooler temperatures, the rest of the magma cooled more quickly forming the aphanitic gray rock around the phenocrysts. So, to get two crystal sizes in a rock there had to be two episodes of cooling for the magma—one period of slow cooling, followed by a period of rapid cooling!

 

Hypotheses for how the Tower formed

Geologists disagree on the exact circumstances under which the magma was intruded. There are three hypotheses (ideas) as to how the Tower may have formed. All three assume the rock that forms the Tower cooled and hardened about one mile underground. Erosion slowly exposed the Tower as the softer sedimentary rock around it was stripped away. The three hypotheses disagree about the original shape of the Tower, and whether or not some of the magma actually reached the surface to erupt as a volcano.

Formation of the Tower

There are three hypotheses (ideas) as to how the Tower may have formed. All three assume that the rock that forms the Tower cooled and hardened approximately one mile underground. The Tower was then slowly exposed as erosion removed the softer sedimentary rock from around the harder igneous rock of the Tower. The three hypotheses differ in their interpretation of the original shape of the Tower and whether or not some of the magma actually reached the surface to erupt as a volcano.

Hypothesis #1—the Tower is a Volcanic Neck

In this hypothesis, the Tower represents a hardened plug of magma in the underground plumbing system of an overlying volcano. The volcano eroded away along with the surrounding sedimentary rocks and the plug (the Tower) is all that remains. This is an exciting interpretation! However, there is not much evidence for lava flows or ash falls of this age (50 million years ago) that would have come from the volcano. Perhaps they may have eroded away as well?

Hypothesis #2—the Tower is a Laccolith

A laccolith is a mushroom-shaped igneous rock. The stalk of the mushroom is the feeder pipe to the underlying magma source and the cap of the “mushroom” is the magma above the feeder pipe that squeezed in between the layers of overlying sedimentary rock causing the layers above it to bulge upward into a dome. If this is how the Tower formed, it would have been much wider than it is today and the cap of the “mushroom” may have extended all the way west to the Missouri Buttes. Many of the surrounding peaks in the area, including Sundance Mountain, are laccoliths.

Hypothesis #3—the Tower is a Stock

In this hypothesis, the magma of the Tower merely pushed its way into the overlying sediments and cooled underground as a stock in its present shape and size. This is the simplest explanation for the Tower.

 

Formation of the Tower’s columns

As the phonolite porphyry of the Tower hardened and cooled, the rock began to contract. As it did so, long, vertical fractures (cracks) called columnar joints began to form in the new rock of the Tower.

Some of the columns are 600 feet tall and 15 feet in diameter. Columnar jointing is a common phenomenon. It occurs in homogeneous substances that cool and shrink at just the right speed. The ends of the columns form shapes that are roughly hexagonal (6-sided).

The ends of the columns at Devils Tower are much harder to see because the top of the Tower is covered with soil and grasses.

 

The Geology of the Monument 50 million years ago

The magma at Devils Tower hardened into rock about 50 million years ago during an epoch of time called the Eocene—a part of the Tertiary Period.

At the end of cooling of the Tower (about 50 million years ago), the Tower was still not visible at the surface. It was concealed under layers of softer sedimentary rocks. We know these overlying layers were here because they are preserved today in the Powder River Basin to the west near Gillette, Wyoming! To find out what happened to them at the monument, READ ON!

 

Links

Devils Tower Main Index

Geologic Overview

Sedimentary Rocks

Igneous Rocks (current section)

Erosion

Review

 

Multimedia version

 

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