Download Igneous Rocks

Igneous Rocks
Standards
Classify matter in a variety of ways
Describe the composition and structure of
Earth’s materials, including:
the major rock types (i.e., sedimentary,
igneous, metamorphic) and their formation
Igneous Rocks
• Rocks formed from magma
• Make up 95% of Earth’s crust, much of
which is covered by thin layer of
sedimentary rocks
Formation of Igneous Rocks
• Form from the solidification of magma
(molten rock)
Hot temperatures in the aesthenosphere
(600 - 1400 C) cause rock to melt
As rock heats up, it becomes less dense
and rises
When it cools and solidifies, either
underground or above ground, it becomes
igneous rock
Classification of Igneous Rocks
• Classification is based on:
1. Where rock is formed
2. Texture (which is directly affected by
where rock is formed)
3. Mineral content (what minerals are in
rock)
Environment where Rock is Formed
• Two main groups of igneous rocks:
1. Intrusive (plutonic)
2. Extrusive (volcanic)
Intrusive (Plutonic)
• Named after Pluto, god of the underworld
• Forms from solidification of magma
underground
• Forms 1 kilometer or deeper below
surface of Earth
• Ex: granite
Granite
Photo: http://hsc.unm.edu/medicine/integrative_med/brian_shelley_biosketch.shtml
Extrusive (Volcanic)
• Named after Vulcan, god of fire
• Forms from eruption and solidification of
magma (lava) on surface of Earth
• Forms less than 1 kilometer deep
• Ex: basalt
Textures
• Rock texture refers to the size, shape and
arrangement of mineral grains
Intrusive Texture
• Intrusive magmas cool slowly, over
hundreds of thousands of years, due to
insulating properties of surrounding rocks.
• This allows time for crystals to grow =
larger crystals.
• These rocks are medium to large grained,
called Phaneritic
• Ex: granite – most abundant rock in
Earth’s crust
Intrusive Texture
Extrusive Texture
• Extrusive magmas (lavas) cool rapidly.
• There is little time for crystal growth = very
small or micro crystals.
• These rocks are fine grained, called
aphanitic
• Ex: basalt
Extrusive Texture
Extrusive Texture
• Some extrusive lavas cool so rapidly there
is no time for crystal growth
• In this case they have a glassy texture
• A glassy texture can be either massive
(like obsidian [volcanic glass]) or frothy
(like pumice)
Obsidian
Pumice
Extrusive Texture
• Some extrusive rocks can contain holes
that are called vesicles.
• Vesicles form when gases (air bubbles)
escape from the cooling rock leaving
behind a cavity.
Vesicular Basalt
ExtrusiveTexture
• Some extrusive rocks form a porphyry (or
porphyritic texture) – large crystals, called
phenocrysts, are present in a fine-grained
groundmass.
• Chocolate chip cookie analogy: chocolate
chips are like the phenocrysts and cookie
dough is like the groundmass.
Porphyritic Texture
Color Classification
• Based on mineralogy
• Felsic rocks – igneous rocks that are light
colored. They are rich in minerals that
contain lots of silica.
• Mafic rocks – dark colored igneous rocks.
They are rich in iron and magnesium
• Intermediate – rocks midway between
felsic and mafic.
• Ultramafic – very dark rocks, containing
very little silica.
Igneous Rock Pairs
• Igneous rocks are classified in pairs, in
which members of the pairs contain
identical minerals, but have different
textures (intrusive vs. extrusive)
Color
Intrusive
Extrusive
Felsic
Granite
Rhyolite
Intermediate Diorite
Andesite
Mafic
Gabbro
Basalt
UltramaficPeridotite
Felsic
Granite
Rhyolite
Intermediate
Diorite
Andesite
Mafic
Gabbro
Basalt
Ultramafic
Peridotite