Basic Facts About Igneous
Rocks
0 Igneous rocks- crystalline solids which form directly
from the cooling of magma.
0 This exothermic process involves a phase change
from a liquid state of matter to a solid state of matter.
0 The surface of the earth, where it exposed to the
coldness of space, is made of igneous rock.
Naming Igneous Rocks
0 Igneous rocks are given names based upon two
things: what they are made of (composition) and how
big the crystals are (texture).
Texture
0 How large are the mineral grains in the final solid
rock?
0 The slower the magma cools, the larger the mineral
grains will be.
0 Classifications by texture name igneous rocks to be
either intrusive or extrusive.
Comparisons of Texture
Intrusive
Extrusive
0 Coarsely-grained
0 Finely-grained
0 Mineral grains are
0 Mineral grains are
large enough to see
without a
magnifying glass
0 Also referred to as
phaneritic
only visible with
aid of a magnifying
glass
0 Also referred to as
aphanitic
Texture Classifications
Phaneritic
Aphanitic
0 Granite
0 Obsidian
0 Gabbro
0 Basalt
Composition
0 The composition of igneous magmas is directly
related to where the magma is formed
0 Magmas are generally mafic if they are associated
with crustal spreading
0 Magmas are felsic if they are associated with crustal
compression and subduction
Mafic
0 Basalt is produced if the
magma erupts at the
surface
0 Gabbro is produced if
the magma never makes
it out of the magma
chamber
0 Basalt and gabbro are
compositionally the
same however, they are
two different rocks
based on texture.
Intermediate
0 Intermediate magmas
are associated with
crustal compression and
subduction like the felsic
magmas.
0 Intermediate magmas go
through the same
differentiation process
as felsic magmas as well.
0 Intermediate magmas
produce diorite and
andesite.
0 Diorite is intrusive
whereas andesite is
extrusive.
Felsic
0 Mafic seafloor basalt and
continental sediments
are subducted back into
the crust, where they remelt, thus continues the
differentiation process.
0 Magma enriched in the
lighter elements is the
product.
0 The final result of the
differentiation process,
the felsic magmas, lead
to the formation of
granite or rhyolite.
0 Granite is intrusive
whereas rhyolite is
extrusive.
Mafic-Basalt
IntermediateDiorite
Felsic- Granite
Classifications of Composition Chart
References
0 Google.com/images
0 jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry
13.html