2. Formation of Igneous Rock
• When lava cools and hardens it forms
igneous rock.
• The rock has two categories and is based on
how the lava cools.
– Intrusive igneous rock
– Extrusive igneous rock
3. Intrusive Igneous
• The cooling of magma deep below the
earth’s surface.
• Called intrusive because the lava that
forms them intrudes other rocks.
• The magma then slowly cools to form
rock.
• The slow cooling allows minerals to
collect and form large crystals.
4. Extrusive Igneous
• Formed when there is a rapid cooling
of lava or melting of rocks on the
earth’s surface.
• The rapid cooling does not give
crystals time to form.
• The main difference between the two
types of igneous rock is the size of
the crystal when the rock is forming.
5. Texture of the Rocks
• The larger the crystals, the more rough the
rock will be.
• When rapid cooling occurs and crystal are
not allowed the opportunity to form, the rock
will appear extremely smooth.
• Crystal formation will be so small that they
are not seen by the unaided eye.
6. Texture of the Rocks
• An igneous rock with both large and small
crystals is called porphyritic.
• The porphyritic texture is created when lava
cools slowly and is then sped up.
7. Texture of Rock
• When thick lava cools rapidly no crystals
form at all.
• Usually thicker lava has an increased
amount of trapped gases and this will cause
the igneous rock to become full of holes.
8. Composition of Rock
• The mineral make of the lava determines the
chemical make-up of the rock.
• There are 3 different types of families:
– Felsic
– Mafic
– Intermediate
