1. Igneous Rocks and Processes
Overview of rock types
The hot Earth
Magma
Bowen’s Reaction Series
Igneous rocks

2. Rocks are naturally formed solid aggregates usually (but not always) composed of minerals.

3. Rocks are classified by their mode of formation
Rocks are classified by their mode of formation. There are three major rock forming processes on Earth, producing three kinds of rocks.
Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks
Formed when magma (molten rocks) solidifies
Environment: Hot enough to melt rock, pressure varies
All non-igneous rocks formed by processes acting on the surface of the Earth
Environment: normal for Earth’s surface
Formed by chemically and physically altering rocks under heat and pressure deep within the Earth’s crust.
Environment: High pressure, not hot enough to melt rock

4. The Rock Cycle
Sedimentary Rocks
Igneous Rocks
Metamorphic Rocks

5. Igneous Rocks
Igneous rocks form from magma (a complex mix of molten rock with dissolved gases and other materials)
Magmas that cool slowly inside the Earth form plutonic igneous rocks. The mineral crystals in these rocks are usually large because they had lots of time to grow.
Magmas that erupt onto Earth’s surface are called lavas and very quickly. As they cool, volcanic igneous rocks form. If mineral crystals form, they tend to be small because they had little time to form. If cooling is fast enough, no minerals form and the lava solidifies to form a solid glass, with no crystalline structure.

6. Dissolved gasses - mostly H2O, SO2, CO2
When the temperature is high enough (and the pressure* is low enough), some solid Earth materials will begin to melt, forming magma.
Magma - complex mixture of high temperature solid, liquid, and gaseous material.
Dissolved gasses - mostly H2O, SO2, CO2
*If the pressure is too high, melting will not occur – which is why magma is more commonly formed in the crust than the mantle, even though mantle temperatures are much higher than crust temperatures.

7. Bowen’s Reaction Series
felsic

8. olivine pyroxene amphibole biotite K-feldspar quartz muscovite
1200oC
temperature
K-feldspar
quartz
muscovite
600oC

9. Mafic igneous rocks contain minerals with high melting points
Intermediate igneous rocks contain minerals with intermediate melting points
felsic
Felsic igneous rocks contain minerals with low melting points
Bowen’s Reaction Series

10. felsic

11. Crystalline Igneous Rock Texture
Texture - refers to the size of mineral crystals. The more slowly a magma cools, the more time crystals have to grow.
Magma that cooled slowly produces large crystals, and the resulting rock is coarse-grained (phaneritic).
In magmas that cooled quickly only small crystals form, and the resulting rock is fine-grained (aphanitic).

12. Crystalline Igneous Rock Texture
Texture - refers to the size of mineral crystals. The more slowly a magma cools, the more time crystals have to grow.
Porphyritic Texture
Sometimes a magma will have a complex cooling history, and may stay at a mineral’s crystallization temperature for a long time, allowing those mineral crystals to grow large.
If the magma is later cooled more quickly, only smaller crystals of the other minerals will form.

13. Igneous Rocks Basalt Basalt Gabbro
Different magma bodies have different chemical compositions. These chemical compositions are usually described by the total percentage of silica tetrahedra in the magma.
Igneous
Rocks
Mafic Igneous Rocks - Silica poor (<52%)
Iron, calcium and magnesium rich
Contain minerals formed at relatively high temperatures. They are high density and tend to be dark colored.
Common minerals: olivine, augite, calcium-rich plagioclase feldspar
Basalt
University of Hawaii
Basalt
Gabbro
Devil’s Tower National Monument
University of Montpellier

14. Basalt - fine grained mafic rock
T/P
color
Si %
Basalt - fine grained mafic rock
low
light
high
Basalt Columnar Jointing
University of Hawaii
Mafic
felsic
Devil’s Tower National Monument

15. Gabbro - coarse-grained mafic rock
low
light
high
T/P
color
Si %
University of Montpellier
Mafic
felsic

16. Intermediate Igneous Rocks - Silica rich (52% - 69%)
Different magma bodies have different chemical compositions. These chemical compositions are usually described by the total percentage of silica tetrahedra in the magma.
Igneous
Rocks
Intermediate Igneous Rocks - Silica rich (52% - 69%)
Intermediate elemental composition
Contain minerals formed at intermediate temperatures. They are intermediate density and tend to contain an equal combination of dark and light colored minerals.
Common minerals: plagioclase feldspar, mica, hornblende
Diorite
Diorite
Andesite

17. low
light
high
T/P
color
Si %
Intermediate
felsic
Diorite - coarse-grained intermediate rock. Approximately half and half dark and light minerals, no quartz.

18. Andesite - fine-grained intermediate rock.
low
light
high
T/P
color
Si %
Intermediate
felsic

19. Felsic Igneous Rocks - Silica rich (>69%)
Different magma bodies have different chemical compositions. These chemical compositions are usually described by the total percentage of silica tetrahedra in the magma.
Igneous
Rocks
Felsic Igneous Rocks - Silica rich (>69%)
Iron, calcium and magnesium poor
Contain minerals formed at relatively low temperatures. They are low density and tend to be light colored.
Common minerals: K-feldspar, mica, quartz
Granite
Granite
Rhyolite
Rhyolite

20. Rhyolite - fine grain felsic rock
low
light
high
T/P
color
Si %
Felsic

21. Granite - coarse grained felsic rock
low
light
high
T/P
color
Si %
Granite - coarse grained felsic rock
Felsic

22. Felsic

24. Volcanic Igneous Rocks with no Minerals
Some magmas cooled too fast for minerals to form. The rocks are made of glass and contain no minerals, so are NOT described using the felsic-intermediate-mafic system.
obsidian
scoria
glassy – obsidian and other volcanic glass
vesicular – full of holes formed by bubbles in the magma.
fragmental or pyroclastic – formed from debris spewed out by the volcano.
bombs!

25. Volcanic Igneous Rocks
Plutonic Igneous Rocks