Igneous Rocks

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Igneous Rocks : 

Igneous Rocks

Igneous Rocks : 

Igneous Rocks A rock is a group of minerals bound together. The three types of rocks are igneous, sedimentary, and metamorphic.

Igneous Rocks : 

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.

Slide 4: 

All igneous rocks have a random fabric of fused mineral grains.

Igneous Rocks : 

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)

Intrusive: Large crystals : 

Intrusive: Large crystals Extrusive: Small crystals Which is which? How do you know?

Igneous Rocks : 

Igneous Rocks There are two types of magma Felsic = Light color, high in silica, viscous (thick and slow), low in iron, calcium or magnesium Felsic = Feldspar + Silica Mafic = Dark color, low in silica, not viscous (thin and watery), high in iron, calcium and magnesium Mafic = Magnesium + Iron (Fe)

Felsic: Light overall color : 

Felsic: Light overall color Mafic: Dark overall color Which is which? How do you know?

Igneous Rocks : 

Igneous Rocks Texture refers to the crystal size. Textures are divided into two main types: Fine-grained = crystals are too small to see (basalt) Coarse-grained = crystals are easily visible (granite)

Coarse Grained: Large crystals : 

Coarse Grained: Large crystals Fine Grained: Small crystals Which is which? How do you know?

Igneous Rocks : 

Igneous Rocks Other types of textures: A glassy texture occurs when rock freezes instantly. There are no crystals (obsidian). A porphyritic texture is a rock with large crystals surrounded by fine-grained crystals (ryolite). A vesicular texture is a rock that has many holes in it (pumice).

Slide 12: 

Volcanic Glass/Obsidian: may be felsic, but 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. Glassy Texture:

Porphyritic Texture: : 

Porphyritic Texture: Rhyolite: Light colored (felsic), fine grained (volcanic) igneous rock

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

Pumice: Fine grained, felsic, vesicular volcanic igneous rock. Scoria: Fine grained, mafic, vesicular volcanic igneous rock. Vesicular Texture:

Igneous Rocks : 

Igneous Rocks Rocks can be grouped into families: Granite family = felsic, light, contains quartz, feldspar, & muscovite. (granite, rhyolite, obsidian, pumice) Gabbro family = mafic, dark, contains feldspar, olivine, biotite. (gabbro, basalt, scoria) Diorite family = intermediate composition and color. (diorite, andesite, obsidian)

Igneous Rocks : 

Igneous Rocks 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 : 

Basalt Mafic = Dark color Fine grained= Small crystals Extrusive = Small crystals = Formed on surface

Diorite : 

Diorite Felsic = Light color Coarse grained= Large crystals Intrusive = Large crystals = Formed underground

Gabbro : 

Gabbro Mafic = Dark color Coarse grained= Large crystals Intrusive = Large crystals = formed underground

Pumice : 

Pumice Felsic = Light color Fine grained= Small crystals Extrusive = Small crystals = Formed above ground

Scoria : 

Scoria Mafic = Dark color Fine grained= Small crystals Extrusive = Small crystals = Formed above ground

Slide 22: 

Remember: Safety First!!! Intermission