Earth Materials: Chp 3: Igneous Rocks

Convergent Boundary Divergent Boundary

Magma: molten rock below Earth’s surface Lava: molten rock at the surface

Composition Type of magma Silica Location Viscosity Colour content felsic = silisic >65% cont’l cr thick light Intermediate 53–65% cont-ocean mafic = basaltic 45-52% ocean-cont ultra mafic<45% ocean thin dark

How magma forms 1. Lower pressure Caused by decrease in pressure as plates move apart at divergent boundaries solid liquid Temperature Pressure High pressure Low pressure

Pressure

2. Higher Temperatures at magma plumes: localized heating caused by increased radioactivity or the “blanket effect” (large thick continental plates insulate resulting in heat build up) Continental plate Magma plume

Magma Plume

3. Water (yes water) lowers the melting temperature of rocks 1100 C liquid wet dry rock temperature solid 1500 C pressure Oceanic plate Cont’l plate Water leaks down here

Magma Melts: wet vs. dry

Modification of Magma = how magma becomes more felsic/silisic 1. Fractional Crystallization (crystal settling) High temeprature minerals high in Mg, Fe; low in SiO 2 form first. The minerals then sink/settle out of the magma (crystal settling). The magma becomes more felsic/silsic as the plume rises.

Bowen’s Reaction Series Crystallize first

Magma Modification: Fractional Crystalization

2. Wall Rock Assimilation As hot magma rises it melts and mixes with silisic/felsic country rock become more silisicous. Xenolith = fragment of country rock that falls into the magma and doesn’t melt (that’s how diamonds reach the surface!) Silisic country rock Mafic

Magma Modification: Wall Rock Assimilation

Xenoltih

3. Magma mixing When two magmas of different compositions (more/less silisic) join and mix to form an intermediate mixture. rare

Igneous Rock TYPES: 1. Intrusive (plutonic) Solidify slowly underground Large crystals (visible to naked eye) – PHANERITIC texture 2. Extrusive (volcanic) Solidify quickly at or near surface Small crystals = aphanitic texture

Igneous Rock Types Intrusive Extrusive aphanitic PHANERITIC

TEXTURE 1. PHANERITIC (coarse) (5 rocks) Minerals can be seen without magnification Slow cooling = larger crystals Intrusive 4 intrusive rocks ID with chart 1b Pegmatic: very large crystals (+5 mm) (1 rock = pegmatite) Very slow cooling Usually granitic composition

Granite

Diorite + Dalmation Rock

Gabbro

Peridotite

Pegmatite

2. aphanitic (fine): minerals too small to be seen without magnification (need lens/scope) Fast cooling Extrusive 3 extrusive rocks ID with chart

2a. Volcanic glass (obsidian) cools veery fast 2b. Vesicular (scoria & pumice) rocks that freezes before the gas can escape leaving numerous small vesicles (cavities) = “frozen milkshake” 2c. Pyroclastic = fragmental texture (Volcanic breccia & welded tuff) Material blown out of volcano, settles and become rock

Composition TextureFelsicMafic Vesicular Pumice Scoria/vesicular (bubbly) “styrofoam” basalt Glassy Obsidian (volcanic glass,usually black) Pyroclastic Volcanic Breccia (welded cinders, blocks & bombs) Tuff/Welded Tuff (welded ash)

2d. Porphyritic (porphyry) Large crystals (phenocrysts) surrounded by a small crystals (groundmass) Forms by: a)magma cools slowly at depth as an intrusive body forming initially the large crystals b)Then magma is quickly extruded to the surface where it cools quickly freezing the large crystals in a fine ground mass

Rhyolite

Andesite

Basalt

Volcanic Glass = Obsidian (Glassy Texture)

Pumice (Vesicular Texture)

Scoria = Vesicular Basalt (Vesicular Texture)

Welded Tuff (Pyroclastic Texture) Look for layering that formed as ash was laid down

Volcanic Breccia = “Dog Puke” (Pyroclastic Texture)

Porphyry = Choc-chip cookie (Porphyritic Texture)

Therefore all igneous rocks are classified by: 1. composition (ultra-mafic felsic) = minerals present 2. texture (PHANERITIC aphanitic)

HW Do WS 3.2 Do WS 3.2 Do Lab 4.1 Do Lab 4.1