Download Rocks - Igneous

Physical Geography
Igneous Rocks
Igneous Rocks
•  Igneous rock forms when magma/lava cools and hardens.
•  Rocks that result when lava solidifies are classified as extrusive igneous
rocks.
•  Rocks that result when magma solidifies beneath the earth’s surface are
classified as intrusive igneous rocks.
•  The rate of cooling strongly influences the crystallization process, in
particular the size of the crystals.
Igneous Rocks
Slow cooling ! Formation of large crystals
Rapid cooling ! Formation of small crystals
Instant cooling ! Formation of glass
•  In addition to the rate of cooling, the mineral composition of the magma
and the amount of volatile material also influence the crystallization
process.
•  Consequently, igneous rocks are most often classified by their texture
and mineral composition.
Igneous Rock Textures
•  The term texture is used to describe the overall appearance of the
rock based on the size and arrangement of its interlocking crystals.
•  Texture is an important characteristic because it reveals about the
environment in which the rock formed.
•  The most important factor affecting the texture of a rock is the rate at
which the magma/lava cooled.
Igneous Rock Textures
•  Igneous rock textures include the following:
" Phaneritic Texture
" Aphanitic Texture
" Porphyritic Texture
" Glassy Texture
" Vesicular Texture
" Pyroclastic Texture
Phaneritic Texture
•  Phaneritic textured rocks are
comprised of large crystals that are
clearly visible to the eye with or
without a hand lens or binocular
microscope. This texture forms by
slow cooling of magma deep
underground
Aphaneritic Texture
•  Aphanitic texture consists
of small crystals that
cannot be seen by the
eye with or hand lens.
Aphanitic rocks are
characterized by textures
in which the mineral
grains are not visible to
the eye so they generally
look rather like a blank
slate.
Porphritic Texture
•  T. Porphyritic rocks are composed of at
least two minerals having a conspicuous
(large) difference in grain size. The larger
grains are termed phenocrysts and the
finer grains either matrix or groundmass.
Porphyritic rocks are thought to have
undergone two stages of cooling; one at
depth where the larger phenocrysts
formed and a second at or near the
surface where the matrix grains
crystallized.
Glassy Texture
•  Glassy textured igneous rocks are noncrystalline meaning the rock contains no
mineral grains. Glass results from cooling
that is so fast that minerals do not have a
chance to crystallize. This may happen
when magma or lava comes into quick
contact with much cooler materials near
the Earth's surface. Pure volcanic glass is
known as obsidian
Vesicular Texture
•  This term refers to vesicles (holes,
pores, or cavities) within the igneous
rock. Vesicles are the result of gas
expansion (bubbles), which often occurs
during volcanic eruptions. Pumice and
scoria are common types of vesicular
rocks. The image to the left shows a
basalt with vesicles, hence the name
“vesicular basalt”.
Pyroclastic Texture
•  Pyroclastic rocks are blown out into the
atmosphere during violent volcanic eruptions.
These rocks are collectively termed
fragmental. If you examine a fragmental
volcanic rock closely you can see why. You
will note that it is comprised of numerous
grains or fragments that have been welded
together by the heat of volcanic eruption. If
you run your fingers over the rock it will often
feel grainy like sandpaper or a sedimentary
rock. You might also spot shards of glass
embedded in the rock.
Minerals of Igneous Rock
Plagioclase
•  Most common mineral in igneous rock.
•  Appears frosty white to grey-white in colour.
Quartz
•  A very common mineral in igneous rock.
•  Grains appear dark grey and glassy.
Potassium Feldspar
•  Has slightly pinkish grains.
Minerals of Igneous Rock
Muscovite
•  Not a common mineral.
•  Appears as small shiny grains.
•  Can be flaked off using a nail.
Biotite
•  Appears as small black shiny grains.
•  Crystals are hexagonal.
Amphibole
•  Occurs as slender needle-like crystals that have a blackish colour
•  Commonly found in diorite.
Minerals of Igneous Rock
Pyroxene
•  Commonly found in mafic igneous rock.
•  Occurs as small, dark green crystals.
Olivine
•  Commonly found in ultramafic igneous rock.
•  Occurs as small, light green, glassy crystals.
Classification of Igneous Rock
Classification of Igneous Rock