Granite: Igneous Rock - Pictures, Definition & More (2024)

What is Granite?

Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth's surface.

Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles, and other minerals. This mineral composition usually gives granite a red, pink, gray, or white color with dark mineral grains visible throughout the rock.

Table of Contents

What Is Granite?
The Best-Known Igneous Rock
Granite in the Continental Crust
Definitions of Granite
Introductory Course Definition
Petrologist's Definition
Commercial Definition
Uses of Granite

The Best-Known Igneous Rock

Granite is the best-known igneous rock. Many people recognize granite because it is the most common igneous rock found at Earth's surface and because granite is used to make many objects that they encounter in daily life. These include countertops, floor tiles, paving stone, curbing, stair treads, building veneer, and cemetery monuments. Granite is used all around us - especially if you live in a large modern city.

Granite is also well known from its many world-famous natural exposures. These include: Stone Mountain, Georgia; Yosemite Valley, California; Mount Rushmore, South Dakota; Pikes Peak, Colorado; and White Mountains, New Hampshire.

Granite in the Continental Crust

Introductory geology textbooks report that granite is the most abundant rock in the continental crust. At the surface, granite is exposed in the cores of many mountain ranges, within large areas known as "batholiths," and in the core areas of continents known as "shields."

The large mineral crystals in granite are evidence that it cooled slowly from molten rock material. That slow cooling had to have occurred beneath Earth's surface and required a long period of time to occur. If these granites are exposed at the surface today, the only way that could have happened is if the granite rocks were uplifted and the overlying rocks were eroded.

Most parts of Earth's continents are covered with sediments or sedimentary rocks. The rocks below are usually granites, metamorphosed granites, or closely related rocks. These deep granites are often referred to as "basement rocks."

Multiple Definitions of Granite

The word "granite" is used in a variety of ways by different people.

A simple definition is used in introductory geology or earth science courses.

A more precise definition is used by petrologists (geologists who specialize in the study of rocks).

And, the definition of granite expands wildly when used in the crushed stone and dimension stone industries.

These multiple definitions of granite can lead to communication problems. However, if you know who is using the word and who they are communicating with, you can interpret the word in its proper context. Three common usages of the word "granite" are explained below.

A) Introductory Course Definition

Granite is a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz; it also contains minor amounts of mica and amphibole minerals (see the accompanying chart titled Generalized Composition Ranges of Common Igneous Rocks). Once students know how to identify the minerals in granite, this simple description enables them to identify the rock based upon a visual inspection.

During that visual inspection, students should use a hand lens to confirm that the minerals of granite are present in the rock. That inspection would involve confirming that each of the minerals expected in granite is physically present in the rock - and present in the proper proportion.

Here is a summary of what you might observe on a broken surface of granite:

Feldspar Minerals

Feldspar minerals are abundant in granite. They are usually white, gray, pink or reddish in color. Many grains will exhibit two directions of cleavage that intersect at right angles. You should be able to observe this cleavage pattern in granite with a hand lens.

Quartz

Quartz will usually be a transparent mineral that is colorless or gray in color. Many grains will exhibit a conchoidal fracture - with a vitreous luster on the conchoidal fracture surfaces.

Mica Minerals

The mica minerals expected in granites include muscovite or biotite. Micas occur in very thin sheets. They will often be in "books" of numerous sheets stacked upon one another. The surfaces of these sheets will have a highly reflective vitreous luster. The edges of a "stack of sheets" will look similar to the edge of a stack of playing cards.

Amphibole Minerals

Amphibole minerals such as hornblende are dark in color and will often have a prismatic habit.

B) Petrologist's Definition

Granite is a plutonic rock in which quartz makes up between 10 and 50 percent of the felsic components. Alkali feldspar accounts for 65 to 90 percent of the total feldspar content. Applying this definition requires the mineral identification and quantification abilities of a competent geologist.

This type of analysis cannot be done precisely by a student in a classroom or a geologist in the field. This is an example of the complexities that can be involved in assigning a formal name to an igneous rock.

Many rocks identified as "granite" using the introductory course definition will not be called "granite" by the petrologist. They might instead be alkali granites, granodiorites, pegmatites, or aplites. These names are for specific types of granite.

These names require a consideration of the grain size and the mineral composition of the rock - beyond determining that the rock is a granite. A petrologist might call these "granitoid rocks" rather than granites. There are many types of granite based upon mineral composition and texture.

The accompanying chart (Generalized Composition Ranges of Common Igneous Rocks) illustrates the range of granite compositions. From the chart you can see that orthoclase feldspar, quartz, plagioclase feldspar, micas, and amphiboles can each have a range of abundances.

C) Commercial Definition

Use of the word "granite" in the dimension stone and crushed stone industries is different from how the word is used by geologists. In these industries, the name "granite" refers to an igneous rock that meets the following criteria:

1) a rock with visible grains that interlock with one another

2) a rock that is harder than marble

Using these criteria, gabbro, basalt, pegmatite, schist, gneiss, diabase, diorite, and many other igneous rocks will be called "granite."

These "granites" are used to make crushed stone that is used for highway construction, concrete, building construction, fill, railroad ballast, and many other purposes. They are used in the dimension stone industry to make countertops, floor tiles, curbing, building veneer, monuments, paving stones, and many other products. These granites might be used with sawn, sheared, or polished surfaces.

When "Granite" Is Inadequate

So, the name "granite" is a name used for igneous rocks that are composed of orthoclase feldspar, quartz, plagioclase feldspar, micas, and amphiboles that are present in crystals large enough to be visible with the unaided eye.

That name isn't specific enough for some purposes and for some people.

Special names are used for granitic rocks based upon their grain (crystal) size. If a granitic rock has especially large grains (over one centimeter across), it is often called "pegmatite." If it is an especially fine-grained rock from crystallizing at a shallow depth, it might be called "aplite."

Granitic rocks that have a mineral composition that borders upon diorite might be called "granodiorite." Those especially rich in plagioclase feldspars, at the expense of alkali feldspars, might be called "monzodiorites."

The accompanying triangular diagram displays a classification method used for granitic rocks based upon the relative abundances of quartz, alkali feldspars, and plagioclase feldspars. This is not a chart for use by the beginning student of igneous rocks. It is a classification used by experts who have the skills and equipment needed to quantify the mineral composition of the rock.

Uses of Granite

Granite is the rock most often quarried as a "dimension stone" (a natural rock material that will be cut into blocks or slabs of specific length, width, and thickness). Granite is hard enough to resist abrasion, strong enough to bear significant weight, inert enough to resist weathering, and it accepts a brilliant polish. These characteristics make it a very desirable and useful dimension stone.

Most of the granite dimension stone produced in the United States comes from high-quality deposits in five states: Massachusetts, Georgia, New Hampshire, South Dakota, and Idaho.

Granite has been used for thousands of years in both interior and exterior applications. Rough-cut and polished granite is used in buildings, bridges, paving, monuments, and many other exterior projects. Indoors, polished granite slabs and tiles are used as countertops, floor tiles, stair treads, and many other practical and decorative features.

High price often reduces the popularity of a construction material. Granite often costs significantly more than man-made materials. However, granite is frequently selected because it is a prestige material, used in projects to produce impressions of elegance, durability, and lasting quality.

Granite is also used as a crushed stone or aggregate. In this form it is used as a base material at construction sites, as an aggregate in road construction, railroad ballast, foundations, and anywhere that a crushed stone is useful as fill.