1. Igneous rocks: Some definitions
Molten rock
in the Earth is magma
Magma is buoyant, rises to surface,
& sometimes breaks through
when magma reaches Earth’s surface it
is called lava
An igneous rock is formed when magma or
lava cools and solidifies

2. Why should we care?
Igneous rocks make up bulk of Earth’s crust
Earth’s mantle is basically one huge igneous rock
Important rocks economically
Striking landscape features

3. Igneous rocks that form
- at the surface are volcanic (extrusive)
Igneous rocks that form
- deep down are plutonic (intrusive)

4. Plutonic rocks
To see them, they must be uplifted to surface
And softer surrounding rock eroded away
Magma body

5. Plutonic rocks
For us to ever see them, they must be uplifted to the surface
And softer surrounding rock eroded away

6. Plutonic igneous
rock example:
El Capitan
Yosemite Nat. Park,
California

7. As magma cools, atoms arrange in an
orderly crystal structure
crystallization

8. Note: different crystal sizes

9. Coarse-grained igneous rock
Close up of crystals
Note the variety of crystal sizes

10. TYPES OF MAGMA
1. granite ------ rhyolite
“coarse”
“fine”
phaneritic
aphanitic
silica (SiO2) rich
feldspar and quartz

11. 2. basalt---------gabbro
“fine”
“coarse”
aphanitic phaneritic
iron – magnesium
(Fe – Mg) rich
ferromagnesian minerals,
plagioclase (Ca, Na
feldspar)

12. Minerals to Rocks:
Major Rock Groups
IGNEOUS
SEDIMENTARY
METAMORPHIC

13. Igneous Rock - Molten rock that has cooled and
crystallized into sold rock.
• Crystal size determined by how fast
the rock cooled. Due to how
crystals grow.
– Long time = large crystals
– short time = small crystals
• Course grain - crystals are easily seen
with naked eye.
• Fine grain - crystals < 1mm, but can
still be seen without magnification.
• Glassy - no crystals can be seen
without magnification.

14. Forms and Structures of
Igneous Rocks
Intrusive
Structures
Extrusive
Structures
Lens-shaped Cone-sheet,
Subjacent
Sheet
Ring Dikes
Plutons
Intrusions Intrusions
Laccolith
Batholith
Phacolith
Stock
Dikes Sills
Lopolith
Diapirs
Lava Plateau
Volcanic Cone
Vesicula
Columnar
r
Jointing
structure
Pillow lava
Amygdaloidal
Block lava
structure
Ropy lava

15. Igneous
Rocks
Intrusives
and
Extrusives

16. Igneous Rock Structures

17. Intrusive
Granite
(large crystals)

18. Intrusive Igneous Rocks
• Also called Plutonic
• Rocks form inside the
earth. Granite is the
most common.
• Cools slowly - Large
crystals (course grain)
• Magma - molten rock
inside the earth.

19. Intrusive Igneous Rocks Exposed
Shiprock California
Granite Outcrop, Yosemite

20. Subjacent Plutons
 Bathoilth
 Large, irregular, major deep seated intrusive
masses of very large size, elongated parallel to
major tectonic regime.They have an outcrop
areas greater than100km2
 Stock
 Is similar to batholiths, with surface area less
than 100 km2. They have steeply plunging
contact with no visible floor. Boss is a term
applied to stocks of circular section
 Diapirs
 are bodies of rock and/or magma that are
buoyant and moves upwards, piercing rocks
above them.They appear as ballon like bodies
intruded and displaced the country.

21. Extrusive
Basalt
(small crystals
+
glass)

22. The Structures of Lavas
 Block Lava
(subareial lava flows, result when volatile in the magma are boiled
off in the vent before eruption
 Ropy Lava
(lava flows with less loss of volatiles show contorted snaky folds)
 Pillow Lava
(Submarine lava flows, exhibit a pillow structure that consists of
isolated pillowed shaped masses piled one upon another
 Vesicular Lava
(Lava flows with gas cavities or vesicles which are trapped during
solidification, eg. Scoria, Pumice,…)
 Amygdaloidal structure
(W
hen vesilcles are later filled with secondary minerals such as
Calcite, chlorite, chalcedony,..)
 Columnar Jointing
(fractures that separate polygonal, pencil-like cooling structures
Several centimeters across.)

23. Extrusive Igneous Rocks
• Also called volcanic
• Rocks form at the
earth’s surface.
Known as Basalt
• Cools rapidly - small
crystals (glassy)
• Lava - molten rock at
the earth’s surface

24. Extrusive Igneous Rocks form near
volcanoes
Mt. Pinatubo
Hawaii

25. Lava flow at
Volcanoes National Park,
Hawaii

26. Fig. 4.16

27. Fig. 4.17

28. Vesicular structure

30. Obsidian - extrusive
Glassy texture (no defined crystals)

31. Fig. 4.12

32. Pumice extrusive with
vesicles

33. Fig. 4.28

35. Fig. 4.30

36. Pillow Structure

37. Fig. 4.29