Igneous Rocks Texture and Composition

The Magma Process to form Minerals (Unit 4) Molten (liquid) rock in a magma chamber At, near or under earth’s surface, RISES The magma begins to COOL. The atoms, ions and molecules combine to form various mineral compounds. The molecules arrange into an orderly repeating pattern to form CRYSTALS. quartz

The RATE at which the molten magma cools determines the crystal size. If the magma cools s-l-o-w-l-y (under the surface taking hundreds of years) then the crystals will be… granite well-formed

Crystallization of Salol Lab Slow cooling made large, well-formed crystals Quick cooling made small crystals Slow cooling allowed for larger, well-defined crystals

Each mineral melts / crystallizes at its own temperature Different minerals in granite crystalize at different temperatures. The last to cool has limited space to form crystals – so smaller not well-formed crystals 500°C

Group/Sort Sort the igneous rocks into 3 groups by their crystal size: large small None: glassy or gas holes

Intrusive igneous Rocks

Intrusive igneous rocks slowly cool and crystalize inside the Earth forming plutonic rocks with large crystals

Violent pyroclastic eruption Ocean floor LAVA Fissure eruption

If the magma reaches the surface, erupts from a volcano as LAVA flows on the surface or erupts underwater, it cools quickly, and the crystals will be small / microscopic / not well-formed basalt If the magma erupts explosively from a volcano and cools at a very fast rate, quickly, then the crystals will…. None obsidian a volcanic rock, cools instantly. so it has no crystal structure, but a glass-like structure EXTRUSIVE

Large, well-formed crystals AND small, fine crystals

Porphyritic Texture http://www.classzone.com/books/earth_science/terc/content/investigations/es0603/es0603page05.cfm Then it erupted out of a volcano to the surface and the rest of the magma cooled quickly forming small, fine crystals The magma started to cool deep in the magma chamber and form large crystals

Igneous rock terms to have in your notes: Magma - Lava Intrusive – Extrusive Plutonic – Volcanic Course texture - Fine texture Porphyritic - Pyroclastic Mafic - Felsic High viscosity – Low viscosity

Classification of Igneous rocks

Grouping by Composition Sort the igneous rocks into 3 groups by their mineral colors: Dark: black, gray, green Light: clear, white, pink Intermediate: half dark/light

Silicates are divided by color Ferromagnesium Iron or magnesium metal Non-ferromagnesium Does NOT contain iron or magnesium

Felsic magma Ma”fe”c* magma Page 121 Mafic Felsic magma Ma”fe”c* magma Feldspar, silica magnesium and iron

The silicon-oxygen tetrahedron (SiO4)- ion Combines with positive +ions (K, Na, Ca, Mg, Fe, Al) to form many different silicate minerals. Show the GeoDe CD-ROM lesson on mineral groups

Igneous Rocks form from molten (melted) rock Magma located   Lava located   Cooling rate is: Crystals are: Cooling rate: Crystals are: Porphyritic Intrusive igneous rocks Where: Extrusive igneous rocks Where: Very coarse grains 10 mm or larger Coarse grains 1 mm -10 mm Fine-grained Less than 1 mm Glassy or gassy (no crystals grains) Example rocks: Example rocks: Example rocks: Example rocks:

FELSIC (light) INTERMEDIATE MAFIC (dark)   FELSIC (light) INTERMEDIATE MAFIC (dark) Color Minerals Name Silica percent % Thick or Thin High or Low VISCOSITY Fast or Slow moving Hotter or colder temp.? Examples: Names of the rocks

Mafic (dark) Dark colored minerals: Magnesium and iron Fe Olivine, pyroxene, Hornblende, augite, biotite Less than 50% silica

Felsic (light) Light colored minerals: Quartz, orthoclase Feldspar More than 60% silica

Which has a HIGH viscosity? Resistance to flow honey water

Is this igneous rock intrusive or extrusive Is this igneous rock intrusive or extrusive? Is it mafic or felsic in composition? Name this rock: Rhyolite! Felsic: light-colored High silica content fine crystals extrusive,

Is this igneous rock intrusive or extrusive Is this igneous rock intrusive or extrusive? Is it mafic or felsic in composition? Name this rock – granite course crystals, intrusive Felsic: light-colored High silica and feldspar content

Is this igneous rock intrusive or extrusive Is this igneous rock intrusive or extrusive? Is it mafic or felsic in composition? Name this rock – obsidian NO crystals, extrusive Felsic: (High silica Content)

Is this igneous rock intrusive or extrusive? BOTH! It has Course AND Fine crystals, So it’s Intrusive AND Extrusive???? http://www.classzone.com/books/earth_science/terc/content/investigations/es0603/es0603page05.cfm

ES0603 basalt diorite granite rhyolite basalt Porphyritic What?? http://www.classzone.com/books/earth_science/terc/content/investigations/es0603/es0603page04.cfm basalt Porphyritic rhyolite diorite

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Edible Igneous Textures Food/candy name Texture Fine, coarse-grained, glassy, gassy, porphyritic Which igneous rock could this represent?

Match the food with the igneous rock texture Next. Use to chart on page 124 to match and igneous rock to the food. 1.Course grained 2. Fine grained 3.Glassy 4.Gassy/porous 5.porphrytic

Edible Igneous Textures Food/candy name Texture Fine, coarse-grained, glassy, gassy, porphyritic Color: Mafic, felsic. intermediate Which igneous rock could this represent? Chocolate Fine-grained mafic Basalt Granola bar Course-grained Granite Root beer barrel glassy looks mafic (but it’s really just clear candy with brown food color, so it’s FELSIC) obsidian Marshmallow gassy felsic pumice Chocolate chip cookie Porphyritic: (fine-gained, with big crystals) Felsic (background) Porphyritic (rhyolite)

Rock Poems I am fine-grained And so light stained. My color ranges from light gray to pink, which probably makes you think… rhyolite

This rock is rough and oh so tough. This rock is not light, but dark as the night. It’s mafic but not fantastic. It’s found in the ocean and happens to be extrusive. basalt

gabbro It may be coarse, it may be grainy. It’s so dark, it makes you zany. It contains pyroxene, augite and olivine. The sweet look of it is so divine. What rock is this? You don’t know? Take a GUESS, it’s…. gabbro

Smooth as glass - didn’t have much time. Got no crystals I am so fine Yes, I even shine. Smooth as glass - didn’t have much time. Got no crystals Shot out of a volcano like a pistol I’m felsic, but I don’t look it Started to cool as soon as I reached the top Here’s a hint, I’m an extrusive rock obsidian