Igneous = Fire. Igneous Rocks

Magma- Molten material below Earth’s surface (deep or shallow). It is composed mostly of Si and O (we call it silica, SiO 2 ) and dissolved gases. Why do we call this silica, and not quartz? Where is the image?

Lava- Molten rock material that has reached Earth’s surface. Some magmas become lavas, but not all.

Igneous rock- A silicate-rich rock that forms when molten rock (magma or lava) solidifies.

Two main types of igneous rocks: 1) Intrusive/Plutonic- Formed when magma solidifies deep underground. Includes granites (main rock of the continents). Never directly witnessed!

2) Extrusive/Volcanic- Formed when lava solidifies at the Earth’s surface. Includes basalts (main rock of ocean floors).

1) What are the two main categories of igneous rocks? 2) Where does each category form? 3) What is the name of the liquid that each solidifies from?

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) Coarse- grained (Intrusive) It is a binary classification system!

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) RhyoliteAndesiteBasaltKomatiite Coarse- grained (Intrusive) GraniteDioriteGabbroPeridotite

What is Texture? The size, shape and arrangement of crystal grains within a rock. Igneous rocks have an interlocking texture formed by growth of minerals from a melt. Crystal size is controlled by cooling rate.

Extrusive Textures Extrusive/volcanic rocks cooled quickly at or near Earth’s surface (little time for crystal growth). These are fine-grained or aphanitic (most crystals <1 mm).

Extrusive Textures Cont. An extreme case occurs when rocks are glassy (no crystals at all). An example is obsidian.

Intrusive Textures Intrusive/plutonic rocks cooled slowly deep within the Earth. These are coarse-grained or phaneritic (most crystals >1 mm).

Draw lines to match the appropriate geologic concepts and terms. Depth Cooling Rate Grain Size Textural Description Crystal Size Shallow Deep Slow Fast Small Coarse Phaneritic Aphanitic <1mm >1mm

You find a rock that has interlocking texture, is fine-grained and black. Name the rock.

Special Igneous Textures 1)Obsidian: An extrusive rock with glassy (ultra-fast cooled) texture and rhyolite composition (an unusually dark rhyolite).

Special Igneous Textures 2) Xenolith: A fragment of rock within an igneous rock that differs compositionally from the host rock. The host rock and zenolith inclusions formed from different magmas. 3) Vesicule: A bubble or hole formed by escaping gas (common in basalts).

4) Pegmatite: A very coarse grained igneous rock (crystal sizes > 5 cm) in which crystal growth was enhanced by the presence of fluids. Special Igneous Textures

5) Porphyritic: Igneous rock with large crystals (called phenocrysts) in a fine-grained matrix. Porphyritic rocks may represent a two-state cooling history: 1) slow cooling at depth followed by... 2) rapid uplift and fast cooling near Earth's surface.

Igneous Rocks Classified by Composition Four compositional categories: Felsic Mafic Intermediate Ultramafic A rock's color tells us about the minerals present and overall composition.

Mineral Composition vs Color Minerals With Light Elements (Si, O, Al, Na, K)… Tend to be __________ colored. Minerals With Heavier Elements (Ca, Mg, Fe)… Tend to be __________ colored. Felsic Mafic

Felsic Rocks Are: Rich in light-colored minerals (quartz, alkali feldspar, and some plagioclase feldspar). Compositionally rich in Si, Na, Al, and K (and poor in Fe and Mg). The dark-colored minerals biotite and amphibole are present, but only in small amounts).

Mafic Rocks Are: Rich in dark-colored minerals (plagioclase feldspar, pyroxene, olivine, biotite, and amphibole). Compositionally rich in Ca, Fe, and Mg (lower in Si, Na, Al, and K). There is no quartz or alkali feldspar!

Intermediate Rocks Are: Composed of roughly equal amounts of dark- and light- colored minerals.

Ultramafic Rocks Are: Composed almost exclusively of Fe and Mg-rich minerals from the mantle (olivine and pyroxene). Compositionally rich in Fe, Mg, and Ca, but poor in Si.

Warmup Without looking in your notes, fill in the following rock names. Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) SKIP THIS ONE Coarse- grained (Intrusive)

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) Rhyolite Coarse- grained (Intrusive)

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) Rhyolite Coarse- grained (Intrusive) Granite

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) RhyoliteAndesite Coarse- grained (Intrusive) Granite

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) RhyoliteAndesite Coarse- grained (Intrusive) GraniteDiorite

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) RhyoliteAndesiteGabbro Coarse- grained (Intrusive) GraniteDiorite

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) RhyoliteAndesiteGabbro Coarse- grained (Intrusive) GraniteDioriteBasalt

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) RhyoliteAndesiteGabbro(Komatiite) Coarse- grained (Intrusive) GraniteDioriteBasalt

Classification of Igneous Rocks The most useful system for classifying igneous rocks utilizes texture and composition (color). Felsic (light) Intermediate (medium) Mafic (dark) Ultramafic Fine- Grained (Extrusive) RhyoliteAndesiteBasaltKomatiite Coarse- grained (Intrusive) GraniteDioriteGabbroPeridotite

Classification of Igneous Rocks

Introduction to Bowen’s Reaction Series

Warmup (use your cell phones for 5 minutes). What temperature does water freeze at? F C What temperature does ice melt at? F C

Bowen’s Two Paths Two minerals we NEVER expect to find in the same rock are:

Bowen’s Reaction Series: Minerals crystallize in a predictable order over a large temperature range (and melt in the reverse order).

Lessons from Bowen’s Series 1) The chemistry of a magma controls the type of rock that can form from it.

Lessons from Bowen’s Series 2) First minerals to solidify: –Olivine –Pyroxene –Ca-rich plagioclase. This produces a mafic rock (rich in Fe, Mg, Ca) like basalt or gabbro.

Lessons from Bowen’s Series 3) As magma rises it will “evolve” into more a more felsic composition (rich in K, Na, and Si) producing granite or rhyolite.

Lessons from Bowen’s Series 4) During heating, the order of mineral melting will be reversed.

The Feldspar Triangle Alkali Feldspar is also known as orthoclase.

What Favors Melting? 1) Increase the Temperature! The temperature of the Earth increases from crust to core at approximately 30 C/km (the geothermal gradient). The core temperature is > 5000 C, and heat moves upward and melts the upper mantle and crust.

What Favors Melting? 2) Decrease the pressure! Since pressure favors solids, mineral melting temperatures decrease with decreasing pressure. This decompression melting occurs when hot mantle rock moves upward. P T

What Favors Melting? 3) Add water! Water reduces the melting point of rock. Dry granite melting temperature: 900°C.

What Favors Melting? 3) Add water! Water reduces the melting point of rock. Dry granite melting temperature: 900°C. Wet granite melting temperature: 700°C.

What Favors Melting? 4) Mix minerals! Mixtures of minerals always have lower melting points than the pure minerals would. Quartz melts at ~1650°C and K-Feldspar melts at ~1300°C. However, a 50/50 mixture of these two minerals will melt at ~1150°C.

Hints for the Igneous Rocks Lab

We will have a quiz on igneous rocks. Make SURE you know the difference between a rhyolite and basalt!

Magma Evolution Magmas (liquids) evolve from mafic  felsic. Magmas that solidify close to their source rock will be the most like the source rock. Magmas that solidify far from the source rock will be changed by magma differentiation. This occurs by 4 different processes: 1) Partial melting: Produces magmas more felsic than the source rock.

Magma Evolution 3) Assimilation: A hot magma can incorporates surrounding “country rock”. If mafic magma assimilates more felsic rocks continental crust an intermediate rock will result. 4) Magma mixing involves the mixing of more and less mafic magmas to produce a magma of intermediate composition.

Magma Evolution 2) Fractional crystallization: The magma composition changes by the removal of denser early-formed ferromagnesian minerals (crystal settling). The remaining magma becomes more felsic.

Intrusive Rock Bodies Intrusive rocks exist in intrusions that penetrate or cut through pre- existing country rock. We name these bodies based on: 1)Size 2)Shape 3)Geometric relationship to the country rock Intrusions are: A. Shallow if formed < 2 km deep (relatively fast cooling produces finer textures (but coarser than extrusive rocks). B. Deep if formed > 2 km deep (slow cooling produces coarse- grained rocks).

SHALLOW BODIES Dike: Tabular structure that cuts across the layering in the country rock.

SHALLOW BODIES Sill: Tabular structure that parallel to layering in country rock.

SHALLOW BODIES Volcanic Neck: Shallow intrusion formed when magma solidifies in the throat of a volcano (Ship Rock, New Mexico).

Plutons: Large, blob-shaped bodies formed when rising blobs of magma (diapirs) get trapped within the crust. DEEP BODIES

Stock: A small pluton (exposed over <100 km 2 ). Batholith: A large pluton (exposed over >100 km 2 ). The interfaces between intrusions and country rock are called contacts.

Where Does Most Igneous Activity Occur? Mainly at or near tectonic plate boundaries.

Mafic Rocks are common at divergent boundaries. The magma comes from the upper mantle (asthenosphere) and rises through thin crust. Little opportunity to differentiate  mafic. The ocean floors are basalt!

Felsic Rocks are Common on Continents. The thick continental crust means longer magma journey. More magma differentiation  more felsic. Continental crust is rich in granite!

Intermediate Rocks. Common at convergent boundaries. Partial melting of subducted ocean crust (mafic) produces basaltic magma; this evolves into more felsic magma by the assimilation of felsic crust.

Intermediate igneous rocks commonly form at convergent boundaries. Here, partial melting of subducted asthenosphere produces basaltic magma which evolves into more felsic magma by the assimilation of felsic crust.

Some igneous rocks form within plates (not at a plate boundary). Rising mantle plumes (of controversial origin) can produce localized hotspots and volcanoes as they rise through continental or oceanic crust.

End of Chapter 3