Chapter 3 Igneous rocks

2) contains a mixture of one or more minerals Definition of a rock -> different from that of a mineral. A rock: 1) is solid 2) contains a mixture of one or more minerals 3) occurs naturally as part of our planet

minerals rock mineral minerals collection of one or more A collection So far we have: minerals rock mineral collection of one or more minerals A collection of one or more types of atoms

Molten rock Some definitions inside the Earth is magma when magma reaches Earth’s surface(outside) it is called lava

Figure 4.1

An igneous rock is formed when magma or lava cools and solidifies

Igneous rocks that form at the surface(lava) Volcanic (extrusive) Igneous Igneous rocks that form at the surface(lava) Plutonic (intrusive) Igneous Igneous rocks that form deep inside the earth (magma)

Plutonic rocks To see them, they must be uplifted to surface And softer surrounding rock eroded away Magma body

Plutonic rocks For us to ever see them, they must be uplifted to the surface And softer surrounding rock eroded away

Plutonic igneous rock example: El Capitan Yosemite Nat. Park, California

As magma cools, atoms arrange in an orderly crystal structure crystallization

Note: different crystal sizes

Coarse-grained igneous rock Close up of crystals Note the variety of crystal sizes

Texture a. size b. shape c. arrangement the - of interlocking crystals

cooling rate crystal size Slow cooling larger crystals Fast cooling Coarse texture Fast cooling small or no crystals Fine texture

Coarse-grained Fine-grained Large crystals in matrix Glassy

fast cooling magma/lava forms at or near surface sometimes holes present can’t see individual crystals forms far below surface slow cooling intergrown crystals Aphanitic Fine-grained Phaneritic Coarse-grained very rapid cooling ions unable to unite in orderly crystalline structure magma cooled slowly for a while then erupted. minerals crystallize at different temperatures and or rates Porphyritic Glassy Large crystals in matrix

Porphyritic Texture

Aphanitic Texture

Texture Rock types Coarse grained Peridotite, Gabbro, Diorite, Granite Fine grained Komatiite, basalt, andesite, rhyolite Glassy Obsidian, Pumice, Scoria Vesicular Pumice, Scoria Prophyritic Andesitic porphyry Phenocrysts –large very crystals.

mainly silicate minerals determined by composition of magma from which it crystallized magma mainly 8 elements: Si, O, Al, Ca, Na, K, Mg, Fe Most abundant constituents

Figure 4.7

DARK silicates(simatic) LIGHT silicates(sialic) Two major silicate mineral groups: DARK silicates(simatic) LIGHT silicates(sialic) Silicate group Rich in Poor in Examples DARK (Simatic) Fe and/or Mg Si olivine pyroxene amphibole biotite LIGHT (Sialic) Si, K, Na, Ca Fe and/or Mg quartz muscovite feldspars

Bowen’s Reaction Series In a magma body: crystals form as magma cools heavy crystals sink to bottom the composition of the magma changes variety of igneous rocks from the same source

Bowen’s reaction series Figure 4.23

Igneous rocks are classified from A) texture B) mineral composition depends on: how fast/slow magma cools depends on: chemical makeup of parent magma

mafic felsic high in Mg, Fe. Dark, dense Igneous rocks types mafic felsic magnesium + ferrum high in Mg, Fe. Dark, dense feldspar + silica (quartz) high in Si. Lighter, less dense

zooming in rhyolite volcanic f e l s i c granite plutonic

obsidian volcanic f e l s i c volcanic pumice

zooming in andesite volcanic i n t e r m e d i a t e diorite plutonic

zooming in basalt volcanic m a f i c gabbro plutonic

volcanic m a f i c scoria

magnesium + ferrum feldspar + silicate magma recipes 3 main types magnesium + ferrum high in Mg, Fe. Dark, dense “mafic” basaltic andesitic granitic feldspar + silicate high in Si, lighter, less dense “felsic”

magma recipes basaltic andesitic granitic 3 main types “mafic” magnesium + ferrum high in Mg, Fe. Dark, dense Most common volcanic Igneous rock. “mafic” basaltic andesitic granitic Most common plutonic Igneous rock “felsic” “felsic” feldspar + silicate high in Si, lighter, less dense

magma recipes basaltic andesitic granitic 3 main types Partial melt of: “mafic” basaltic andesitic granitic mantle rock: peridotite Mantle peridotites + felsic crustal rocks Si-rich continental crust rocks “felsic” “felsic”

Kimberlite What is a kimberlite? kimberlite is a potassic, ultramafic, igneous rock which occurs as small volcanic pipes. Kimberlite commonly contains inclusions of upper mantle ultramafic rocks. olivine, illmenite, pyrope, diopside,, enstatite and chromite.

Kimberlites are formed deep within the mantle, at between 150 and 450 kilometres depth, from exotic mantle compositions, and are erupted rapidly and violently. It is this depth of melting and generation which makes kimberlites prone to hosting diamond.

Most kimberlites are confined to the ancient cratons Most kimberlites are confined to the ancient cratons. The ages of most kimberlites are Late Mesozoic Era (Jurassic-Cretaceous).

Diamonds in the kimberlite

Kimberlite Indicator Minerals Cr-pyrope (purple colour,), eclogitic garnet (orange-red), Cr-diopside (pale to emerald green), Mg-ilmenite (black, conchoidal fracture), chromite (reddish-black), and olivine (pale yellow-green) are the most commonly used kimberlite indicator minerals