Igneous Rocks: Formation, Classification, and Types, Summaries of Geology

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Igneous Rocks

Hot rocks/Fire Rocks

Igneous Rock

• Igneous rocks

form when molten

rock cools and

solidifies. Molten

rock is called

magma when it is

below the Earth’s

surface and lava

when it is above.

Igneous Rock

classification

• Igneous rocks are classified two

different ways:

• Where they were formed
• What they are made from (mineral

composition)

Part 1

Classifying igneous rocks by where

they are formed.

when molten rock cools and solidifies

Intrusive

magma

slowly

large

Crystal size

(below) Magma

Lava (above)

Intrusive Igneous Rock

• Most intrusive rocks have large, well formed crystals. The mineral crystals within them are large enough to see without a microscope.
• The more slowly molten rock cools within the Earth, the larger the igneous rock crystals will be.
• Examples of intrusive igneous rocks are granite, gabbro, and diorite

GraniteGabbroDiorite

Extrusive Igneous Rocks

• Extrusive igneous rocks form

when magma makes its way to

Earth's surface. The molten

rock erupts or flows above

the surface as lava, and then

cools forming rock.

• Most extrusive (volcanic)

rocks have small crystals.

Examples include pumice,

obsidian, and basalt.

when molten rock cools and solidifies

Intrusive

magma

slowly

large

Granite, gabbro, diorite

Extrusive

Lava

Quickly

Small or not visible Pumice, obsidian, basalt

Crystal size

(below) Magma

Lava (above)

Glassy Igneous Rocks

• Pumice (left)
• Scoria (bottom left)
• Obsidian (bottom)
• Note gasses in the lava can cause fine holes called vesicles as seen in the pumice and scoria.

Glassy Igneous Rocks cool so rapidly, that atoms don’t

have enough time to get together, bond and form

crystals. To cool this quickly the rocks MUST be

extrusive.

Part 2

Classifying by mineral composition

when molten rock cools and solidifies

Intrusive

magma

slowly

large

Granite, gabbro, diorite

Extrusive

Lava

Quickly

Small or not visible

Pumice, obsidian, basalt

Crystal size

Physical & chemical properties

felsic

intermediate

mafic

*SiO 2 = Silica Fe = Iron Mg = Magnesium

(below) Magma

Lava (above)

Mafic Igneous Rocks

• Mafic igneous rocks are dense, dark-colored

rocks.

• They form from magma that is rich in iron and

magnesium and poor in silica, which is the

compound SiO 2.

• The presence of iron and magnesium in

minerals in basalt gives basalt its dark color.

• Mafic lava is fluid and flows freely from

volcanoes in Hawaii.

• Basalt is the most common rock type in the

Earth's crust (the outer 10 to 50 km). Most

of the ocean floor is made of basalt

Felsic Rocks

• Felsic igneous rocks are light-colored rocks

of lower density than mafic rocks.

• Felsic rocks are coarse-grained

• Felsic magma is thick and stiff and contains

lots of silica but lesser amounts of iron and

magnesium.

• It is the most common rock type on the

continental land masses. Yosemite Valley in

the Sierra Nevada and Mt. Rushmore are

two notable examples of felsic rocks

when molten rock cools and solidifies

Intrusive

magma

slowly

large

Granite, gabbro, diorite

Extrusive

Lava

Quickly

Small or not visible

Pumice, obsidian, basalt

Crystal size

Physical & chemical properties

felsic

mafic

Rich in Fe & Mg poor in SiO 2 Dense & dark colored

Ocean floor & Hawaii

High SiO 2 Low Fe & Mg Light colored, less dense On the continents

(below) Magma

Lava (above)

intermediate

*SiO 2 = Silica Fe = Iron Mg = Magnesium