Igneous RocksIgneous Rocks

Igneous RocksIgneous Rocks

Igneous RocksIgneous RocksIgneous rocks form when magma Igneous rocks form when magma

cools and hardens.cools and hardens.

Photo used with permission from Mike Jarvis, Naperville Central HS,

Naperville, IL

Igneous Igneous from the from the Latin Latin meaning meaning “Fire “Fire formed”, is formed”, is a rock type a rock type that forms that forms from the from the solidificatiosolidification of a n of a molten molten mineral mineral solution.solution.

All igneous rocks have a All igneous rocks have a random fabricrandom fabric of fused mineral of fused mineral

grains.grains.

Igneous RocksIgneous Rocks Intrusive (Plutonic)Intrusive (Plutonic) rocks form inside rocks form inside

the Earth (underground) when magma the Earth (underground) when magma cools cools slowlyslowly to create to create largelarge crystals. crystals.

((InIntrusive = trusive = InInside)side)

Extrusive (Volcanic)Extrusive (Volcanic) rocks form on the rocks form on the Earth’s surface (above ground) when Earth’s surface (above ground) when lava cools lava cools quicklyquickly to create to create smallsmall crystals.crystals.

((ExExtrusive = trusive = ExExit/outside) it/outside)

Types of Intrusive MagmaTypes of Intrusive Magma

Intrusive: Intrusive: Large crystalsLarge crystals

Extrusive: Small Extrusive: Small crystalscrystals

Which is which? How do you Which is which? How do you know?know?

Igneous RocksIgneous Rocks Can form from two types of magmaCan form from two types of magma

1.1. FelsicFelsic = = Light colorLight color, high in silica, viscous , high in silica, viscous (thick and slow), low in iron, calcium or (thick and slow), low in iron, calcium or magnesiummagnesium

FelsicFelsic == FelFeldspardspar + + SiSilicalicaLOW DENSITY!LOW DENSITY! MaficMafic = = Dark colorDark color, low in silica, not , low in silica, not

viscous (thin and watery), high in iron, viscous (thin and watery), high in iron, calcium and magnesiumcalcium and magnesium

MaficMafic == MaMagnesium + Iron (gnesium + Iron (FFe)e)HIGH DENSITY!HIGH DENSITY!

Felsic: Light Felsic: Light overall coloroverall color

Mafic: Dark Mafic: Dark overall coloroverall color

Which is which? How do you Which is which? How do you know?know?

Igneous RocksIgneous Rocks Texture refers to the crystal Texture refers to the crystal

size.size.

Textures are divided into two Textures are divided into two mainmain types: types:

1.1. Fine-grainedFine-grained = crystals are too = crystals are too small to see (basalt)small to see (basalt)

2.2. Coarse-grained Coarse-grained = crystals are = crystals are easily visible (granite)easily visible (granite)

Coarse Coarse Grained: Large Grained: Large

crystalscrystals

Fine Grained: Fine Grained: Small crystalsSmall crystals

Which is which? How do you Which is which? How do you know?know?

TYPES OF IGNEOUS ROCKSTYPES OF IGNEOUS ROCKS

3 Ways Magma Can Form3 Ways Magma Can Form

Igneous RocksIgneous Rocks Other types of textures:Other types of textures:

AA glassyglassy texture occurs when rock texture occurs when rock freezes instantly. There are no freezes instantly. There are no crystals (obsidian).crystals (obsidian).

AA porphyriticporphyritic texturetexture is a rock with is a rock with large crystals surrounded by fine-large crystals surrounded by fine-grained crystals (ryolite).grained crystals (ryolite).

A A vesicularvesicular texture is a rock that has texture is a rock that has many holes in it (pumice).many holes in it (pumice).

Volcanic Glass/ObsidianVolcanic Glass/Obsidian: may be felsic, but : may be felsic, but is usually mafic. Even when felsic it is usually mafic. Even when felsic it appears dark nonetheless because the small appears dark nonetheless because the small % of mafic minerals in the rock form a % of mafic minerals in the rock form a homogenous solution with the felsic homogenous solution with the felsic minerals tinting the overall color to dark. minerals tinting the overall color to dark. Notice the conchoidal fracture.Notice the conchoidal fracture.

Glassy Texture:Glassy Texture:

Porphyritic Texture:Porphyritic Texture:

RhyoliteRhyolite: Light colored (felsic), fine : Light colored (felsic), fine grained (volcanic) igneous rockgrained (volcanic) igneous rock

PumicePumice: Fine : Fine grained, felsic, grained, felsic, vesicular volcanic vesicular volcanic igneous rock.igneous rock.

ScoriaScoria: Fine grained, : Fine grained, mafic, vesicular mafic, vesicular

volcanic igneous rock.volcanic igneous rock.

Vesicular Vesicular TextureTexture::

Igneous RocksIgneous Rocks

Rocks can be grouped into families:Rocks can be grouped into families:1.1. Granite familyGranite family = felsic, light, contains = felsic, light, contains

quartz, feldspar, & muscovite. quartz, feldspar, & muscovite. (granite, rhyolite, obsidian, pumice)(granite, rhyolite, obsidian, pumice)

2.2. Gabbro familyGabbro family = mafic, dark, contains = mafic, dark, contains feldspar, olivine, biotite. feldspar, olivine, biotite.

(gabbro, basalt, scoria)(gabbro, basalt, scoria)

3.3. Diorite familyDiorite family = intermediate composition = intermediate composition and color. and color.

(diorite, andesite, obsidian)(diorite, andesite, obsidian)

Igneous RocksIgneous Rocks

Let’s try some examples, tell me if Let’s try some examples, tell me if the following rock samples are:the following rock samples are:

1. Felsic or Mafic1. Felsic or Mafic2. Fine grained or Coarse grained2. Fine grained or Coarse grained3. Intrusive or Extrusive3. Intrusive or Extrusive

Be sure to tell me how you knew.Be sure to tell me how you knew.

BasaltBasaltMaficMafic = = Dark Dark color color

Fine grainedFine grained= = Small crystalsSmall crystals

ExtrusiveExtrusive == Small crystals Small crystals = Formed on = Formed on surfacesurface

DioriteDiorite

FelsicFelsic = = Light Light color color

Coarse grainedCoarse grained= = Large crystalsLarge crystals

IntrusiveIntrusive = = Large crystals = Large crystals = Formed Formed undergroundunderground

GabbroGabbroMaficMafic = = Dark Dark color color

Coarse Coarse grainedgrained= = Large Large crystalscrystals

IntrusiveIntrusive == Large crystals Large crystals = formed = formed undergroundunderground

PumicePumiceFelsicFelsic = = Light Light color color

Fine grainedFine grained= = Small crystalsSmall crystals

ExtrusiveExtrusive == Small crystals = Small crystals = Formed above Formed above groundground

ScoriaScoria MaficMafic = = Dark Dark color color

Fine grainedFine grained= = Small crystalsSmall crystals

ExtrusiveExtrusive == Small crystals Small crystals = Formed = Formed above groundabove ground

Remember: Safety First!!!Remember: Safety First!!!

IntermissionIntermission