Transcript Igneous Rock Classification Lab

Igneous Rock Classification Lab
What is a rock?
rocks are composed of minerals, mineraloid or organic
materials
Granite (composed of minerals)
Opal (composed of mineraloids)
Coal (composed of organics)
Elements
Minerals
Rocks
Igneous Rock Classification Lab
Let’s form an igneous rock
• Two major types of igneous rocks
Intrusive – magma solidifies below the Earth’s
surface
•magma cools very slow
•crystals form interlocking “mosaic” textures
•very coarse-grained visible minerals
•referred to as Plutonic rocks
Extrusive – magma solidifies above the Earth’s surface
•magma cools very fast
•minerals can not be seen with un-aided eye
•very fine-grained texture (no visible minerals
•referred to as Volcanic rocks
Extrusive rocks
Intrusive rocks
Igneous Rock Classification Lab
Igneous Rock Classification
Identification of Igneous rocks is based on two main
characteristics
Texture – the appearance of the rock due to the rate of
magma cooling
Composition – the type of minerals found in the rock
(mineral composition)
Textures of igneous rocks
Intrusive rocks (Textural terms)
phaneritic texture – crystals are visible and form a
mosaic of interlocking mineral
aggregates (less than 1 cm)
Interlocking crystal grains
phaneritic texture
Igneous Rock Classification Lab
pegmatitic texture –
crystals are very large (>1cm)
Large orthoclase and plagioclase minerals
Extrusive Igneous Rocks (textural terms)
aphanitic texture – crystals are too small to see
See, you really can not see any mineral crystals
Igneous Rock Classification Lab
porphyritic texture – crystals can be separated into two
distinct visible sizes. There can be small grains or
large grains, but crystals appear in 2-distinct sizes
Phenocrysts
Groundmass (matrix)
Igneous Rock Classification Lab
Vesicular texture-
sponge like appearance, texture contains
numerous cavities or holes
Vesicles – gas bubbles cavities
Pyroclastic texture – textures created by rapidly cooling lava that is
“hurled” through the air picking up fragments
(tuffaceous texture)
rock fragments – pieces of rock “incorporated” into the
rock (tuffaceous)
Igneous Rock Textures
Intrusive
Rock cooled slowly
Texture types:
phanartitic
pegmatitic
Extrusive
Rock cooled rapidly
Texture types:
aphanitic
porphyritic
vesicular
pyroclastic
tuffaceous
Igneous Rock Classification Lab
Igneous Rock Composition
mineral composition = mineral assemblages= chemistry
The mineral is either ferromagnesian (dark colored) or
felsic (light colored
ferromagnesian (mafic)
minerals rich in Fe, Mg – creates a dark colored rocks
Pyroxene (Augite)
Amphibole (hornblende)
Mica - Biotite
Igneous Rock Classification Lab
Igneous Rock Composition
Mineral composition = Mineral assemblages = Chemistry
The mineral is either ferromagnesian (dark colored)
or felsic (light colored)
Felsic – mineral composition is light colored minerals
Ca-plagioclase
K-feldspar (orthoclase)
Mica- muscovite
Quartz
Felsic
40%
Glassy – non-crystalline, non-granular
Color varies between black and brown
Diorite
Gabbro
Peridotite
Andesite
Basalt
Composed of fragments of all sizes
Produced in volcanic environments, Ash
Partially re-fused or cemented
Granodiorite
Highly vesicular, finely crystalline to glassy
Pumice (light), Scoria (dark)
Fine to very fine grains, usually to small
to be distinguished with the unaided
eye or even with a hand lens
Dacite
A mosaic of coarse-grains that are
easily visible to the unaided eye – grains
greater than 1 mm
Mafic
Pegmatite
Granite
Pegmatite
Granite
Very coarse crystal grains, usually all
grains are approximately the same size
w/ respect to each other. Grains> one inch
Rhyolite
Very
Coarse
20%
Glassy Aphan- Phaneitic
ritic
Rock Descriptions
Porous
Texture
Mafic
80%
Fragmental
Extrusive
Intrusive
Origin
Igneous Rock Classification
X
Obsidian
Pumice
Rhyolitic
Tuff
Scoria
Basaltic
Tuff
Igneous Rock Classification Lab
Let’s use Texture and Composition to
identify igneous rocks
Steps for identifying igneous rocks
1.
2.
Identify the texture (phaeritic/aphanitic)
If the rock is phaneritic, estimate the % of felsic and mafic
minerals
1. High mafic = dark colored rock
2. High felsic = light colored rock
3. 50% felsic, 50% mafic = intermediate color
3.
Use your igneous rock schematic and cross reference the
texture and composition (light, intermediate, dark) and name
the rock. Add modifying terms (porphyritic, vesicular ect…….)