How to identify the three major types of rocks By Andrew Alden, ThoughtCo., adapted by Newsela staff on 09.19.19 Word Count 819 Level MAX

Image 1. This is an outcrop of igneous rock in Iceland. Igneous rock is created by volcanic activity. Photo by: Phil Degginger/Science Source

In geology, pictures of rocks can be used to help you best determine which of the three major types a particular rock belongs to: igneous, sedimentary or metamorphic.

By comparing your rock sample with photographic examples, you can identify key characteristics, such as how the rock was formed, what minerals and other materials it contains, and where the rock may have come from.

Sooner or later, you're bound to encounter hard, rock-like substances that aren't rocks. Such items include man-made substances like concrete and bricks, as well as rocks from outer space (such as meteorites) that have dubious origins.

Before beginning the identification process, make sure that your sample has been washed to remove dirt. You'll also want to make sure you have a freshly cut surface so you can identify color, grain structure, stratification, texture and other characteristics.

Igneous Rocks

This article is available at 5 reading levels at https://newsela.com. Igneous rock is created by volcanic activity. These rocks come from magma and lava as they cool and harden. Igneous rock is most often black, gray or white. It often has a baked appearance.

Igneous rock may form crystalline structures as it cools. This gives it a granular appearance; if no crystals form, the result will be natural glass. Examples of common igneous rock include: basalt, granite and obsidian.

Basalt is formed from low-silica lava. Basalt is the most common type of volcanic rock. It has a fine grain structure and is usually black to gray in color. Another type of igneous rock is granite. This rock may range from white to pink to gray, depending on the mix of quartz, feldspar and other minerals it contains. It is among the most abundant type of rock on the planet. Obsidian is a type of igneous rock that is formed when high-silica lava cools rapidly. This forms volcanic glass. It is usually glossy black in color, hard and also brittle.

Sedimentary Rocks

Sedimentary rock, also called stratified rock, is formed over time by wind, rain and glacial formations. These rocks may be formed by erosion, compression or dissolution. Sedimentary rock may range from green to gray, or red to brown, depending on iron content and is usually softer than igneous rock. Examples of common sedimentary rock include: bauxite, limestone and halite.

Bauxite is usually found at or near the earth's surface. This sedimentary rock is used in the production of aluminum. It ranges from red to brown with a large grain structure. Limestone is formed by dissolved calcite. This grainy rock often contains fossils from the ocean because it is formed by layers of dead coral and other marine creatures. It ranges from cream to gray to green in color. Halite is more commonly known as rock salt. This sedimentary rock is formed from dissolved sodium chloride, which forms large crystals.

Metamorphic Rocks

Metamorphic rock formation occurs when sedimentary or igneous rock becomes changed, or metamorphosed, by conditions underground.

The four main agents responsible for metamorphosing rock are heat, pressure, fluids and strain. All of these agents are capable of acting and interacting in an almost infinite variety of ways.

Most of the thousands of rare minerals known to science occur in metamorphic rock. Common examples of metamorphic rock include: marble, phyllite and serpentinite.

This article is available at 5 reading levels at https://newsela.com. Marble is a coarse-grained, metamorphosed limestone. It ranges in color from white to gray to pink. The colored bands (called veins) that give marble its characteristic swirled appearance are caused by mineral impurities. Phyllite is a shiny, colorful metamorphosed slate. It ranges in color from black to green-gray. It is recognizable by the flakes of mica it contains. Serpentinite is a green, scaly rock. It is formed beneath the ocean as sediment is transformed by heat and pressure.

Other Rocks And Rock-Like Objects

Just because a sample looks like a rock doesn't mean it is one, however. For example, meteorites are (usually) small, rock-like formations cast from outer space which survive the trip to earth. Some meteorites contain rocky material in addition to elements such as iron and nickel. Others are comprised solely of mineral compounds.

Concretions resemble smooth, often oblong masses. They are found along riverbeds, seemingly cemented together. These are not rocks, but rather masses formed by dirt, minerals and other water-borne debris.

Fulgurites are hard, jagged, oblong masses formed by soil, rock and sand that have been fused together by a lightning strike.

Geodes are sedimentary or metamorphic rocks containing a hollow, mineral-filled interior such as quartz.

Thundereggs are solid, agate-filled lumps found in volcanic regions. They resemble geodes when opened.

This article is available at 5 reading levels at https://newsela.com. Quiz

1 Read the section "Metamorphic Rocks."

Select the sentence from the article that suggests that a metamorphic rock was once another type of rock.

(A) In geology, pictures of rocks can be used to help you best determine which of the three major types a particular rock belongs to: igneous, sedimentary or metamorphic.

(B) Metamorphic rock formation occurs when sedimentary or igneous rock becomes changed, or metamorphosed, by conditions underground.

(C) Most of the thousands of rare minerals known to science occur in metamorphic rock.

(D) Common examples of metamorphic rock include: marble, phyllite and serpentinite.

2 One conclusion a reader could make after reading the article is that igneous rock forms faster than other rock types.

Which of the following statements accurately paraphrases evidence from the article to support the conclusion?

(A) Igneous rock is formed because of volcanic activity, while sedimentary rock takes time to build up on the surface of the Earth, and metamorphic rock takes time to form because of certain conditions underground.

(B) Igneous rock is a form of crystal, while sedimentary rock is formed by layers building up on the surface of the Earth, and metamorphic rock is formed from intense heat and pressure over time.

(C) Igneous rock is formed because of volcanic activity, while metamorphic rock takes time to build up on the surface of the Earth, and sedimentary rock takes time to form because of certain conditions underground.

(D) Igneous rock is a form of crystal, while metamorphic rock is formed by layers building up on the surface of the Earth, and sedimentary rock is formed from intense heat and pressure over time.

3 Read the sentence from the introduction [paragraphs 1-4].

Such items include man-made substances like concrete and bricks, as well as rocks from outer space (such as meteorites) that have dubious origins.

Which of the following words, if it replaced the word "dubious" in the sentence above, would change the meaning of the sentence?

(A) questionable

(B) unclear

(C) familiar

(D) uncertain

4 Read the following sentence from the section “Sedimentary Rocks.” Then, fill in the blank.

Sedimentary rock, also called stratified rock, is formed over time by wind, rain and glacial formations.

The word "stratified" in the sentence above tells the reader that ____.

(A) sedimentary rock can be found in a variety of colors

(B) sedimentary rock takes a long time to form

(C) sedimentary rock forms in many different climates

(D) sedimentary rock often has a layered appearance

This article is available at 5 reading levels at https://newsela.com.