Igneous Rock Classification Lab

1. Igneous Rock Classification Lab What is a rock? rocks are composed of minerals, mineraloid or organic materials Granite (composed of minerals) Opal (composed of mineraloids) Coal (composed of organics) ElementsMineralsRocks

2. Igneous Rock Classification Lab Let’s form an igneous rock • Two major types of igneous rocks • Intrusive– magma solidifies below the Earth’s surface • magma cools very slow • crystals form interlocking “mosaic” textures • very coarse-grained visible minerals • referred to as Plutonic rocks • Extrusive – magma solidifies above the Earth’s surface • magma cools very fast • minerals can not be seen with un-aided eye • very fine-grained texture (no visible minerals • referred to as Volcanic rocks Extrusive rocks Intrusive rocks

3. Igneous Rock Classification Lab Igneous Rock Classification Identification of Igneous rocks is based on two main characteristics Texture – the appearance of the rock due to the rate of magma cooling Composition – the type of minerals found in the rock (mineral composition) Textures of igneous rocks Intrusive rocks(Textural terms) phaneritic texture – crystals are visible and form a mosaic of interlocking mineral aggregates (less than 1 cm) Interlocking crystal grains phaneritic texture

4. Igneous Rock Classification Lab pegmatitic texture – crystals are very large (>1cm) Large orthoclase and plagioclase minerals Extrusive Igneous Rocks(textural terms) aphanitic texture – crystals are too small to see See, you really can not see any mineral crystals

5. Igneous Rock Classification Lab porphyritic texture –crystals can be separated into two distinct visible sizes. There can be small grains or large grains, but crystals appear in 2-distinct sizes Phenocrysts Groundmass (matrix)

6. Igneous Rock Classification Lab Vesicular texture-sponge like appearance, texture contains numerous cavities or holes Vesicles – gas bubbles cavities Pyroclastic texture – textures created by rapidly cooling lava that is “hurled” through the air picking up fragments (tuffaceous texture) rock fragments – pieces of rock “incorporated” into the rock (tuffaceous)

7. Igneous Rock Textures IntrusiveExtrusive Rock cooled slowly Rock cooled rapidly Texture types: phanartitic pegmatitic Texture types: aphanitic porphyritic vesicular pyroclastic tuffaceous

8. Igneous Rock Classification Lab Igneous Rock Composition mineral composition = mineral assemblages= chemistry The mineral is either ferromagnesian (dark colored) or felsic (light colored ferromagnesian (mafic) minerals rich in Fe, Mg – creates a dark colored rocks Pyroxene (Augite) Amphibole (hornblende) Mica - Biotite

9. Igneous Rock Classification Lab Igneous Rock Composition Mineral composition = Mineral assemblages = Chemistry The mineral is either ferromagnesian (dark colored) or felsic (light colored) Felsic – mineral composition is light colored minerals Ca-plagioclase K-feldspar (orthoclase) Mica- muscovite Quartz

10. Igneous Rock Classification Felsic Mafic 80% Origin Texture 40% 20% Rock Descriptions Very coarse crystal grains, usually all grains are approximately the same size w/ respect to each other. Grains> one inch Very Coarse Mafic Pegmatite Granite Pegmatite Intrusive A mosaic of coarse-grains that are easily visible to the unaided eye – grains greater than 1 mm Grano- diorite Phane- ritic Peridotite Gabbro Diorite Granite Fine to very fine grains, usually to small to be distinguished with the unaided eye or even with a hand lens Aphan- itic X Dacite Rhyolite Basalt Andesite Glassy – non-crystalline, non-granular Color varies between black and brown Glassy Obsidian Extrusive Highly vesicular, finely crystalline to glassy Pumice (light), Scoria (dark) Por- ous Pumice Scoria Composed of fragments of all sizes Produced in volcanic environments, Ash Partially re-fused or cemented Basaltic Tuff Rhyolitic Tuff Frag- mental

11. Igneous Rock Classification Lab Let’s use Textureand Composition to identify igneous rocks Steps for identifying igneous rocks • Identify the texture (phaeritic/aphanitic) • If the rock is phaneritic, estimate the % of felsic and mafic minerals • High mafic = dark colored rock • High felsic = light colored rock • 50% felsic, 50% mafic = intermediate color • Use your igneous rock schematic and cross reference the texture and composition (light, intermediate, dark) and name the rock. Add modifying terms (porphyritic, vesicular ect…….)