IGNEOUS ROCKS AND THE
ROCK CYCLE
INTRUSIVE
IGNEOUS ROCK
 Formed under the surface of the
earth
Formed from cooling magma
Cools slowly
Forms medium mineral grains
Granite is a light-colored igneous rock with grains large enough to be visible with
the unaided eye. Granite is composed mainly of quartz and feldspar with minor
amounts of mica and other minerals. This mineral composition usually gives
granite a red, pink, gray or white color with dark mineral grains visible
throughout the rock. Granite is used to make many objects that we encounter in
daily life. These include counter tops, floor tiles and paving stones.
EXTRUSIVE
IGNEOUS ROCK
 Formed on the surface of the
earth
Formed from cooling lava
Cools quickly
Forms tiny mineral grains
Basalt is a dark-colored, fine-grained, igneous rock. Basalt underlies more of
Earth's surface than any other rock type. Most areas within Earth's ocean basins
are underlain by basalt. Although basalt is much less common on continents, lava
flows and flood basalts underlie several percent of Earth's land surface. Basalt is
also an abundant rock on the Moon. Much of the Moon's surface is underlain by
basaltic lava flows and flood basalts.
Crushed basalt is used for road base, concrete aggregate, asphalt pavement
aggregate, railroad ballast, filter stone in drain fields and may other purposes.
Basalt is also cut into dimension stone. Thin slabs of basalt are cut and sometimes
polished for use as floor tiles, building veneer, monuments and other stone
objects.
VOLCANIC GLASS
IGNEOUS ROCK
 Formed on the surface of the
earth
Formed from cooling lava
Cools so quickly that no mineral
grains form
Pumice is a light-colored, extremely porous igneous rock. The pores are
actually gas bubbles that were trapped in the rock during the rapid cooling
of a gas-rich frothy magma. During explosive volcanic eruptions, a rush of
gas from the vent shreds the magma and blows it out as a molten froth.
The froth rapidly solidifies as it flies through the air and falls back to Earth
as pieces of pumice. This material can range in size from tiny dust particles
to large block of pumice the size of a house. Pumice is use to
make lightweight concrete, as landscaping stones and as an abrasive in
conditioning "stone washed" denim for jeans, an abrasive in bar and liquid
soaps such as "Lava Soap“, and an abrasive in pencil erasers.
Scoria forms when magma containing abundant dissolved gas flows from a
volcano or is blown out during an eruption. As the molten rock emerges
from the Earth the pressure upon it is reduced and the dissolved gas starts
to escape in the form of bubbles. If the molten rock solidifies before the
gas has escaped the bubbles become small rounded or elongated cavities
in the rock. You can find scoria all over North America: The red variety of
scoria is commonly used as landscaping pebbles at Taco Bell. Landscapers
know this rock as lava rock.
Obsidian is found in many locations worldwide. It is confined to areas of
geologically recent volcanic activity. Most obsidians have a composition similar
to granite. Granites can form from the same magma as obsidian and are often
geographically associated with the obsidian. Rarely volcanic glasses are found
with a composition similar to basalt.
Obsidian older than a few million years is rare because the glassy rock is rapidly
destroyed or altered by weathering, heat or other processes. The first use of
obsidian by people probably occurred when a sharp piece of obsdian was used
as a cutting tool. People then discovered how to skillfully break the obsidian to
produce cutting tools in a variety of shapes. Obsidian was used to make knives,
arrow heads, spear points, scrapers and many other weapons and tools.
PLUTONIC
IGNEOUS ROCK
 Formed deep under the surface
of the earth
Formed from cooling magma
Takes millions of years to cool
Large mineral grains form
Usually found after years of
erosion uncovers it.
WHERE CAN WE
FIND THEM IN THE
UNITED STATES?
The Plains of Abraham is a huge expanse covered with ash and pumice, both
light gray in color. Scattered randomly across this pale backdrop are black
basalt boulders, many the size of cantaloupes but some as big a Volkswagen
Beetles. All were deposited in a brief, terrifying rainfall when Mt. St. Helen’s
exploded in Washington in 1980. From the Plains is a view of Mount Rainier,
also located in the Cascade Range in Washington.
In the Grand Canyon, the Ruby Pluton (background) consists mostly of
diorite, an igneous rock with a different composition than granite. The
Ruby Pluton formed as a magma chamber for the volcanoes of an island
arc, prior to their collision with the main continental landmass. Other
igneous rocks, like these of pink granite (running through the rocks in
the foreground) formed during the actual collision between the island
chains and the continent.
Obsidian glass and Paulina Lake
Newberry Volcanic National Monument
Oregon, USA
The geologic landscape of Joshua
Tree National Park in California has
long fascinated visitors to this desert.
How did the rocks take on such
fantastic shapes? What forces
sculpted them? When the rocks
formed, rectangular joint fractures
occurred, As ground water percolated
down through the joint fractures, it
began to transform some hard
mineral grains along its path into soft
clay, while it loosened and freed
grains resistant to dissolving.
Rectangular stones slowly weathered
to spheres of hard rock surrounded
by soft clay containing loose mineral
grains. Over time, flash floods began
washing away the protective ground
surface. As they were exposed, the
huge eroded boulders settled one on
top of another, creating those
impressive rock piles we see today.
The largest
plutons are
batholiths,
such as
these
plutonic
rocks of
Colorado’s
Pike's Peak,
which are
part of a
1,300square-mile
batholith.
DEVIL’S TOWER NATIONAL MONUMENT
Wyoming
Devils Tower is composed
of a grayish rock studded
with feldspar crystals. As
the magma cooled
underground, it formed
hexagonal or six-sided
columns although columns
have from four to seven
sides. The last large
column fell about 10,000
years ago. Devil’s Tower
was the important
geologic feature in the
1977 movie “Close
Encounters of the Third
Kind”.
Colorado's Rocky Mountains boast spectacular views of numerous
plutonic (intrusive) rocks. These rocks were formed long ago as
magma rose from deep sources and solidified before making it all of
the way to the surface. As this magma rose, in many places, it
brought with it precious minerals such as gold, silver, lead, and
molybdenum (used in hardening steel). After millions of years,
erosional processes stripped off the overlying rocks, exposing them
as we see today.