Igneous Rocks

what size crystals does lava form?

lava cools rapidly–makes small crystals or no crystals

what determines crystal size?

Crystal size is dependent on COOLING RATE––linked to where rocks cool within Earth

rocks formed from cooling lava: lava cools rapidly–makes small crystals or no crystals

magma cools more slowly → coarse grained (e.g. granite)

Intrusive rocks

rock crystallizes from melt below the Earth’s surface; cools slowly; results in large/coarse crystals (e.g. granite, gabbro)

Extrusive rocks and what are the two types?

rock crystallizes from melt on Earth’s surface; cools quickly; results in small crystals, fine-grained or glassy texture (e.g. rhyolite, basalt)

two types: formed from lava or pyroclasts

lavas

type of extrusive rock formed from flowing lava; smooth or ropey lava flows that solidify into fine-grained rock

Pyroclasts

form from explosive volcanic eruptions; Fragments of lava thrown into the air; can range from tiny ash to large boulders

Porphyry Texture

Igneous rock with mixed grain sizes — large crystals (phenocrysts) embedded in a fine-grained or glassy groundmass

1. Large crystals start growing slowly deep underground (in the magma chamber).

2. Then the magma erupts to the surface and cools quickly, forming the fine-grained groundmass.

ex) andesite porphyry

examples of felsic rocks, key minerals

Intrusive: granite

Extrusive: rhyolite

key minerals: quartz, kspar, plagioclase, muscovite, biotite, hornblende

examples of intermediate rocks

Intrusive: diorite

Extrusive: dacite, andesite

key minerals: mix of quartz, plagioclase, muscovite, biotite, hornblende, pyroxene

examples of mafic rocks

Intrusive: gabbro

Extrusive: basalt

key minerals: Pyroxene, Olivine, Ca-Plag, (no quartz)

example of ultramafic rock

peridotite (intrusive rock that contraisn Olivine + Pyroxene)

How do you differentiate felsic, intermediate, mafic, and ultramafic rocks?

felsic: high silica content (~70%), light color, found in continental crust, high viscosity, lower melting T

intermediate: medium silica content (~60%), gray color, found in volcanic arcs

mafic: low silica content (~50%), dark color, found in oceanic crust, low viscosity, higher melting T

ultramafic: very low silica content (<45%), very dark color, found deep in mantle

felsic rocks have higher…

silica content, K and Na content, viscosity

felsic rocks have lower…

melting T, density, Fe, Mg, Ca content

3 things that can cause rock to melt:

1) Increase in temperature to raise it above the melting temperature of the rock. (happens at magma chambers)

2) Decrease in pressure → decompression melting; less pressure = easier to melt. (Happens at mid-ocean ridges and hotspots)

3) Addition of water → flux melting; lowers the rock’s melting temperature, allowing it to melt even if it isn’t getting hotter.

Partial melting

• Not all minerals melt at the same time.

• Felsic minerals (high in silica) melt first because they have lower melting points.

• Mafic minerals (low silica, rich in Fe and Mg) melt last because they have higher melting points.

• As a result, partial melting produces magma that is more felsic (silica-rich) than the rock it came from.

Assimilation

As magma rises through the crust, it can melt and absorb pieces of surrounding rock (wall rock), which changes the composition of the rock

magma differentiation

A process where different minerals crystallize at different temperatures that leads to the formation of rocks with varying compositions from the same parent (original) magma

Happens through partial melting, crystal settling, and assimilation

what is the geothermal gradient?

• the rate at which Earth’s temperature increases with depth.

• On average: 25–30°C per kilometer in the crust.

• At depths where the temperature reaches about 650–1280°C, rocks can start to melt—but only if pressure or other factors allow it.

Magma chambers

Magma chambers are large pockets of molten rock stored within the lithosphere

fractional crystallization

Last minerals to melt are the first ones to crystallize. Mafic minerals crystallize first, giving the remaining melt a more felsic composition.

process of progressive crystal formation and settling

Bowen’s Reaction Series

shows the order in which minerals crystallize from a cooling magma — from high temperature → low temperatur

continuous branch of Bowen’s reaction series

One mineral changes composition gradually as temperature drops

plagioclase feldspar series: at high T, it’s Ca-rich (mafic); at low T, it becomes Na-rich (felsic).

discontinuous branch of Bowen’s reaction series

each new mineral has a different crystal structure and composition

ex) Olivine → Pyroxene → Amphibole → Biotite

how does magma make room for plutons?

1)    Wedging open the overlying rock

2)    Breaking off large blocks of rock; xenoliiths–large blocks of country rock in magma

3)    Melting surrounding rock

what are plutons? what are the “big” plutons called? “small” plutons?

Plutons: large igneous bodies of one to hundreds of cubic kilometers

Batholiths cover at least 100 km², a collection of plutons

Stocks are small plutons

sill vs. dike

Sills are planar intrusions parallel to the layers in the host rock

Dikes are planar intrusions that crosscut the layers in the host rock; essential role in magma transport

what are veins? 

deposits of minerals, precipitated from a fluid, e.g. quartz, calcite

what are pegmatites?

are coarse grained granite dikes or veins that crystallize from a water-rich melt.

what melting process commonly occurs at mid-ocean ridges? (divergent plate boundary)

Decompression melting — as plates pull apart, pressure decreases and mantle partially melts

typically mafic rocks (basalt, gabbro)

melting process in subduction zones

Flux melting — water and other volatiles from the subducting plate lower the melting temperature of the mantle above.

typically Intermediate to felsic rocks (Andesite, Diorite, Rhyolite, Granite)

Hot spots (mantle plumes)

Decompression melting — hot mantle plume rises and melts as pressure decreases.

typically Mafic rocks (Basalt, Gabbro) at oceanic hot spots; Felsic rocks possible in continental hot spots