Igneous Rocks

Igneous Rocks

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• Molten rock cools  mass of intergrown crystals and/or glass! – Crystalline/glassy textures!

Magma! • Below the earth’s surface molten rock = magma – Molten rock, dissolved gasses, growing crystals (as the magma cools)

• Magma under pressure

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– Less dense than surrounding rocks – Rises/squeezes into cooler crust *Magma pushing itself into crust = Intrusion  Cools to form intrusive igneous rock (visible crystals, coarse

Magma  Lava • Magma approaches earth’s surface • Decrease in pressure • Bubbles of carbon dioxide – dissolved gasses separate from magma  Lava • Erupts onto Earth’s surface  Extrusive Igneous Rock – Fine-grained, tiny crystals/glass

Intrusions • Batholiths – massive, 100km^2 +, small bodies amalgamate • Sills – sheet-like, in between layers of bedrock • Laccoliths – smaller pools • Pipes – Feed volcanoes! • Dikes – cut across layers of bedrock in sheets – Ring dikes – look circular from above – Radial dikes – come off of pipes

Textures of Igneous Rocks • Classify and infer origin • Mineral size  cooling rates, availability of chemicals for crystals – Large crystals  long time  slow cooling • phaneritic (1-10mm), pegmatitic (>1cm)

– Tiny crystals  short time  quick cooling • Aphanitic (<1mm)

– Volcanic glass (no crystals)  instantaneous!

Phaneritic

Pegmatitic

Aphanitic

Other Textures • Vesicular Texture – bubble trapped in cooling lava • Pyroclastic Texture – volcanic ash to blocks • Porphyritic – Phenocrysts and matrix –Can indicate slow to fast cooling –Changes in magma composition/viscosity

Minerals in Igneous Rocks • Type and abundance of minerals • 8 Rock-Forming Minerals – Quartz, potassium feldspar (K-spar), plagioclase feldspar, muscovite mica, biotite mica, amphibole, pyroxene and olivine. – Light colored  felsic (feldspars, silica) – Dark colored  mafic (magnesium, iron) • Aka ferromagnesian

Identification Process • Steps 1 and 2: Color and Mineral Composition – Estimate the % of mafic minerals in the rock – Identify the minerals and estimate their percent by volume within the whole rock – Can use GeoTools

• Step 3: Texture – Intrusive? Extrustive? Phaneritic? Glassy?

More on the color index…. • Color Index (CI) – estimates the proportion of mafic and felsic mineral crystals in igneous rock – Requires both CI and specific mineral IDs

• Felsic – CI 0-15 or 0-15% mafic • Intermediate – CI 16-45 or 16-45% mafic • Mafic – CI 46-85 or 46-85% mafic minerals

Rhyolite (felsic)

Pumice (felsic)

Phonolite (Intermediate)

Diorite (Intermediate)

Pyroxene (Mafic)

Chromite (ultramafic)

Bowen’s Reaction Series • Modeling how different kinds of igneous rocks can differentiate from a single body of magma as it cools • Periodite  Melt rocks at 1200-1400 deg C

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– Allow it to cool – Note the crystals forming at each temperature – Sudden cooling

• Continuous crystallization series and a discontinuous

Bowen’s Series • Discontinuous series – When a mafic mineral forms at high temperature, it reacts with the magma at a lower temperature to produce a different mineral! – Olivine  replaced by pyroxene  etc.

• Continuous series – Calcium-rich gets replaced by sodiumrich

• Lowest temperatures – Last crystallization

Composite Volcanoes (Stratovolcanoes) • Formed by alternating layers of molten rock and rock fragments • Usually conical, peak shaped • Explosive eruptions – Viscous magma  increases pressure

Mt. Shasta, CA

Shield Volcanoes • • • •

Generally expansive in size Lava spills out of vent(s) Low explosivity Characteristic by hotspots – ring of fire

Today’s Lab….. • Use characteristics to ID 8 igneous rock samples • Answer questions pertaining to photos in the lab book • Answer questions concerning volcanoes!

2 1 What type of volcano is this? Give three pieces of evidence! Describe a typical eruption for this volcano.

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