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Igneous Rocks
Niti Mankhemthong
Modified from
http://www.authorstream.com/Presentation/
kcollazo-170754-igneous-rocks-scienceeducation-ppt-powerpoint/
Igneous Rocks
Igneous rocks form when magma
cools and hardens.
Photo used with permission from Mike Jarvis, Naperville Central HS,
Naperville, IL
Igneous from
the Latin
meaning “Fire
formed”, is a
rock type that
forms from
the
solidification
of a molten
mineral
solution.
All igneous rocks have a random
fabric of fused mineral grains.
Igneous Rocks

Intrusive (Plutonic) rocks form inside
the Earth (underground) when
magma cools slowly to create large
crystals.
 (Intrusive

= Inside)
Extrusive (Volcanic) rocks form on
the Earth’s surface (above ground)
when lava cools quickly to create
small crystals.
 (Extrusive
= Exit/outside)
Which is which? How do you know?
Intrusive: Large
crystals
Extrusive:
Small crystals
Igneous Rocks

There are 3 types of igneous rocks
1. Felsic = Light color, high in silica,
viscous (thick and slow), low in iron,
calcium or magnesium
Felsic = Feldspar + Silica
2. Mafic = Dark color, low in silica, not
viscous (thin and watery), high in iron,
calcium and magnesium
Mafic = Magnesium + Iron (Fe)
3. Ultramafic = Very dark in Color
Which is which? How do you know?
Felsic: Light
overall color
Mafic: Dark
overall color
Igneous Rocks


Texture refers to the crystal size.
Textures are divided into five main types:
1. Fine-grained (aphanitic)

crystals are invisible or hard to see with the naked eye

They often have cooling cracks.

They make poor building materials.

Such as basalt
2. Coarse-grained (Phaneritic)

Crystals are easily visible (granite)

Crystals fit together

They are generally the strongest igneous rocks due
to interlocking btw grains

Such as granite
Which is which? How do you know?
Coarse Grained:
Large crystals
Fine Grained:
Small crystals
3. A porphyritic texture



is a rock with large crystals surrounded by fine-grained
crystals.
Large and small crystal are not touching each other. It
is easy to confuse a porphyritic rock with a phaneritic
rock.
Porphyritic rocks are both intrusive and extrusive.

This is a porphyritic andesite similar to the rocks we see on the campus.
Porphyritic Texture:
Rhyolite: Light colored (felsic), fine
grained (volcanic) igneous rock
Igneous Rocks
4. A glassy texture



like broken glass, and you can’t see any grains,
no matter how hard you try (no crystal).
This magma cools so quickly that crystals have
no time to form at all.
Glasses are not very stable, since they are not
crystalline. They are not a good aggregate for
concrete or good building stones, because they
take on water.
Glassy Texture:
Volcanic Glass/Obsidian: is usually mafic. Even when
felsic it appears dark nonetheless because the small %
of mafic minerals in the rock form a homogenous
solution with the felsic minerals tinting the overall color
to dark. Notice the conchoidal fracture.
5. Vesicular:





Vesiclesare holes, so a vesicular rock is a “holey” rock.
The holes are formed when gas and other volatiles
escape the lava before it hardens.
Vesicular rocks are extrusive origin.
The holes make the rock weak, and it is therefore a poor
building material.
This material is often used for landscaping/xeriscaping
in the El Paso area.
Vesicular Texture:
Pumice: Fine grained,
felsic, vesicular
volcanic igneous rock.
Scoria: Fine grained,
mafic, vesicular
volcanic igneous rock.
Igneous Rocks
Rocks can be grouped into families:
1.
Granite family = felsic, light, contains quartz,
feldspar, & muscovite.
(granite, rhyolite, obsidian, pumice)
2.
Gabbro family = mafic, dark, contains feldspar,
olivine, biotite.
(gabbro, basalt, scoria)
3.
Diorite family = intermediate composition and
color.
(diorite, andesite, obsidian)
Warming up!!
Let’s try some examples, tell me if the
following rock samples are:
1. Felsic or Mafic
2. Fine grained or Coarse grained
3. Intrusive or Extrusive
Be sure to tell me how you knew.
Basalt
Mafic = Dark
color
Fine grained=
Small crystals
Extrusive =
Small crystals =
Formed on
surface
Diorite
Felsic = Light
color
Coarse grained=
Large crystals
Intrusive = Large
crystals = Formed
underground
Gabbro
Mafic = Dark
color
Coarse grained=
Large crystals
Intrusive =
Large crystals =
formed
underground
Pumice
Felsic = Light
color
Fine grained=
Small crystals
Extrusive = Small
crystals = Formed
above ground
Scoria
Mafic = Dark
color
Fine grained=
Small crystals
Extrusive =
Small crystals =
Formed above
ground
Remember: Safety First!!!
Intermission
Textures
Mineralogy
Textures