1. IGNEOUS ROCKS

2. Igneous Rocks Originate in Lava and Magma
Extrusive igneous rocks can be in the form of:
lava flows
ash
Intrusive rocks can be:
batholiths
dykes
sills
laccoliths

3. Classification of Rocks

4. Classification of Rocks
Rocks are aggregates of minerals. Geologists divide rocks into three groups: igneous, metamorphic, and sedimentary.
Igneous rocks crystallize from magma.
Metamorphic rocks form by the deformation and/or recrystallization of pre-existing rock by changes in temperature, pressure, and/or chemistry.
Sedimentary rocks form by weathering and erosion of preexisting rock to make sediment, which is lithified into rock.

5. ROCK CYCLE

6. Classifying Igneous Rocks
If the magma erupts at the surface, it is called an extrusive igneous, or volcanic rock
Volcanic rocks are classified by the size, abundance, and type of crystals. If no crystals are visible, geologists call the rock aphanitic. If crystals are abundant, the rock is called porphyritic.

7. CLASSIFYING IGNEOUS ROCKS
Intrusive igneous rocks are coarse- grained because they cooled slowly inside the Earth.
Extrusive (volcanic) igneous rocks are fine- grained because they cooled quickly at the Earth's surface.

8. Igneous Rocks Larger Grains (Cooled slowly) Small Grains (cooled fast)
Rhyolite
Granite
Granite and rhyolite contain quartz, orthoclase and plagioclase feldspar,
(light colored minerals) and minor amounts of ferromagnesian (dark
colored) minerals.

9. GRANITE, GRANITIC ROCKS
Granite comes from silicic magma which cooled slowly underground forming large grains.

10. BASALT Basalt is made from mafic magma that cooled rapidly above
ground and so is fine grained.

11. Igneous Rocks Larger Grains, Slow Cooled Small Grains, Rapid Cooling
Basalt
Gabbro
Gabbro and basalt contain abundant ferromagnesian minerals (dark
colored) and lesser amounts of plagioclase feldspar. Note the basalt
has gas bubbles (called vesicles by geologists).

12. Basaltic Columns

13. OBSIDIAN

14. Igneous Rocks
Obsidian is a black or dark-colored volcanic glass, usually of rhyolite composition, characterized by conchoidal fracture. It is sometimes banded.
Obsidian is made from lava that was quenched, often in water.