Unit 3 - Rock Types Igneous Rocks

Basic Rock Classifications ● Igneous ● Sedimentary ● Metamorphic

Igneous Rocks An igneous rock is formed when magma or lava cools and solidifies Molten rock in the Earth is magma  Magma is buoyant, rises to surface, & sometimes breaks through Magma reaches Earth’s surface it is called lava

Igneous Rocks Two types of igneous rocks ● Plutonic/Intrusive – cools underground ● Volcanic/Extrusive – cools above ground

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

Plutonic rocks Volcanic necks Shallow intrusion formed when magma solidifies in throat of volcano

Plutonic rocks Dikes Tabular intrusive structure that cuts across any layering in existing rock

Plutonic rocks Sills Tabular intrusive structure that parallels layering in existing rock

Plutonic rocks Plutons Large, blob-shaped intrusive body formed of coarse- grained igneous rock, commonly granitic Small plutons are called stocks, large plutons are called batholiths

Geothermal Gradient – the rate at which temperature increases with depth in the Earth *important for the formation of igneous rocks

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

Igneous Rock Classification Igneous rocks are classified two ways 1. Texture 2. Mineral composition

Igneous Rock Classification - Texture

5 Types of Textures 1. Phaneritic (coarse-grained) 2. Aphanitic (fine-grained) 3. Glassy (compact and frothy) 4. Vesicular 5.Porphyritic

Textures - Phaneritic (coarse-grained) forms far below surface Slow Cooling Large Crystals Examples: Gabbro, diorite, and granite

Textures - Aphanitic (fine-grained) fast cooling magma/lava forms at or near surface can’t see individual crystals (small) Examples: Basalt, andesite, and rhyolite

Texture - Glassy (compact and frothy) Very Rapid Cooling No Crystal Structure Example: Obsidian

Texture - Vesicular Has many cavities at its surface and inside Forms from gas bubbles in the magma Example: Pumice * Can be aphanitic vesicular or glassy vesicular rocks

Texture - Porphyritic 2 cooling stages: magma cooled slowly for a while then erupted and cooled quickly. minerals crystallize at different temperatures and or rates Different sized crystals Example: Porphyritic Granite

Worksheet - Homework Igneous Rock (Texture) Worksheet

Igneous Rock Classification Igneous rocks are classified two ways 1. Texture 2. Mineral composition

Igneous Rock Classification - Chemical Composition mainly silicate minerals determined by composition of magma from which it crystallized magma mainly 8 elements: Si, O, Al, Ca, Na, K, Mg, Fe Al - light coloured Mg - dark coloured

Bowen's Reaction Series The geologist N.L. Bowen (Norman) found that minerals tend to form in specific sequences in igneous rocks These sequences are assembled into temperature gradient chart

Bowen’s Reaction Series

Igneous Mineral Composition Types 1. Mafic 2. Felsic 3. Intermediate 4. Ultramafic

Igneous Mineral Composition Types - Mafic Have ~50% silica, by weight, Contain dark colored minerals with iron, magnesium and calcium Examples: Intrusive/extrusive - gabbro/basalt

Mafic Chemical Composition

Igneous Mineral Composition Types - Felsic Have >65% silica, by weight, Contain light-colored minerals that are abundant in silica, aluminum, sodium and potassium Examples: Intrusive/extrusive - granite/rhyolite

Felsic Chemical Composition

Igneous Mineral Composition Types - Intermediate rocks have silica between (50-65%) those of mafic and felsic rocks Examples: Intrusive/extrusive - diorite/andesite

Intermediate Chemical Composition

Igneous Mineral Composition Types - Ultramafic Have the least amount of <45% silica, by weight, Are composed almost entirely of dark-colored ferromagnesian (Fe and Mg) minerals Example: peridotite (intrusive)

Special Note Different minerals crystallize from magma at different temperatures. (Bowen’s Reaction Series) As minerals crystallize from magma the remaining composition is forever changed. Therefore, it is possible for more than one igneous rock to form from the same magma depending on the cooling environment.

Worksheet - Homework Igneous Rock (Composition) Worksheet

Diamonds STSE - Kimberlite What is a kimberlite? kimberlite is a ultramafic, igneous rock which occurs as small volcanic pipes, dykes, and sills. Kimberlite commonly contains inclusions of ultramafic rocks.

Kimberlite and Diamonds they are formed deep within the mantle, at are erupted rapidly and violently, It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.

Kimberlite indicators formed under high pressure and temperature within the mantle

Complete STSE Complete Lab 4 Part 1