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Igneous Rocks. An igneous rock is formed when magma or lava cools and solidifies as a result of the process of crystallization. Remember: Definition of a Rock. Is a solid 2) Contains a mixture of one or more minerals 3) Occurs naturally as part of our planet. Molten Rock.

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Igneous

Rocks

Anigneous rock is formed when magma or

lava cools and solidifies as a result of the process of crystallization.


Remember: Definition of a Rock

Is a solid

2) Contains a mixture of one or more minerals

3) Occurs naturally as part of our planet


Molten Rock

In the Earth is magma. Magma is buoyant and therefore, rises to the surface. Sometimes it breaks through.

When magma reaches Earth’s surface, it is called lava.


Why Should We Care?

  • Igneous rocks make up the bulk of Earth’s crust.

  • Earth’s mantle is basically one huge igneous rock.

  • Igneous rocks are important economically.

  • Creates and provides striking landscape features.


Igneous rocks that form at the surface are volcanic or extrusive.

Igneous rocks that form deep down are plutonic or intrusive.


Magma body

Plutonic Rocks:

To see them, they must be uplifted to the surface

and softer surrounding rock must be weathered and eroded away.

El Capitan, Yosemite National Park, California


As magma cools, atoms of elements of minerals arrange themselves in an orderly crystal structure. (Crystallization)

Note the cooling and growth of crystals.


Coarse-grained Igneous Rock Texture (Phaneritic) themselves in an orderly crystal structure.

Close up of crystals.

Note the variety of crystal sizes.


Texture themselves in an orderly crystal structure.

Texture

a. size

b. shape

c. arrangement

of interlocking crystals


cooling rate themselves in an orderly crystal structure.

crystal size

Slow Cooling

Larger Crystals

Fast Cooling

Smaller or no Crystals


Coarse-grained themselves in an orderly crystal structure.

Phaneritic

Fine-grained

Aphanitic

Glassy

Large crystals in matrix

Porphyritic


Aphanitic or Fine-grained themselves in an orderly crystal structure.

Phaneritic or Coarse-grained

  • Fast cooling lava.

  • Forms at the surface.

  • Sometimes holes are present due to escaping gases.

  • Cannot see individual crystals

  • Forms far

  • below the surface.

  • Slow cooling magma.

  • Intergrown larger crystals.

Porphyritic

Glassy

  • Magma cooled slowly for a while then erupted and became lava.

  • Minerals crystallize at different temperatures and therefore, rates.

  • Large crystals (phenocrysts) in a matrix (groundmass).

  • Very rapid cooling.

  • Ions unable to unite in an orderly crystalline structure so therefore, no crystals.


  • Porphyritic themselves in an orderly crystal structure.

  • Texture


Aphanitic themselves in an orderly crystal structure. Texture Or Fine-grained


Texture Rock Types themselves in an orderly crystal structure.

Coarse-grained Peridotite, Gabbro, Diorite, Granite

Fine-grained Komatiite, Basalt, Andesite, Rhyolite

Glassy Obsidian, Pumice, Scoria

Vesicular Pumice, Scoria

Porphyritic Andesitic porphyry


Igneous Compositions themselves in an orderly crystal structure.

  • Mainly silicate minerals.

  • Determined by the composition of magma or lava from which it crystallizes.

  • Magma or lava mainly consists of 8 elements, which include: Si, O, Al, Ca, Na, K, Mg, and Fe.

  • WHEN YOU THINK OF IGNEOUS COMPOSITIONS, BE SURE TO THINK OF BOWEN’S REACTION SERIES.


Igneous Compositions themselves in an orderly crystal structure.

Magma cools slowly.

Lava cools fastly.

Solidification occurs due to the process of crystallization.

Silicate minerals form and there are two groups, which include: dark and light.


Silicate Group themselves in an orderly crystal structure.

Poor In

Examples

Rich In

DARK

(Simatic)

Olivine

Pyroxene

Amphibole

Biotite

Fe and/or Mg

Si

Si, K, Na, Ca

LIGHT

(Sialic)

Fe and/or Mg

Quartz

Muscovite

Feldspars


Magmatic Differentiation themselves in an orderly crystal structure.

In a magma body:

Crystals form as magma cools.

Heavy crystals sink to the bottom.

There is more than one type of magma as it continuously evolves as minerals crystallize.

A variety of igneous rocks crystallize from the same source due to the evolving magma.



Igneous Compositions Thing

Assimilation

As magmas migrate upwards, they may incorporate surrounding host rock.

Magma Mixing

Magma bodies could collide with other magma bodies and mixing could therefore occur.


Assimilation Thing

Example

Big chunks of olivine!


Naming Igneous Rocks Thing

Igneous rocks are classified based on:

  • Texture

  • Mineral Composition

Depends on: how fast/slow lava/magma cools.

Depends on: chemical makeup of parent lava/magma.


ma Thinggnesium + ferrum

feldspar + silica (quartz)

Igneous Rocks Types

  • mafic

  • felsic

high in Mg, Fe. Dark, dense

high in Si. Lighter, less dense


granite Thing

rhyolite

Naming Igneous Rocks

zooming in

volcanic

f e l s i c

plutonic


Naming Igneous Rocks Thing

obsidian

volcanic

f e l s i c

volcanic

pumice


Naming Igneous Rocks Thing

zooming in

andesite

volcanic

i n t e r m e d i a t e

plutonic

diorite


Naming Igneous Rocks Thing

zooming in

basalt

volcanic

m a f i c

gabbro

plutonic


volcanic Thing

m a f i c

scoria


magma recipes Thing

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 Thing

3 main types

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


Kimberlite Thing

Kimberlite is a volatile-rich, potassic, ultramafic, igneous rock that occurs as small volcanic pipes, dykes, and sills.

Kimberlite commonly contains inclusions of upper mantle-derived ultramafic rocks (due to formation and upward travel through the mantle).

Minerals associated with kimberlite include: olivine, ilmenite, pyrope, almandine-pyrope, diopside, phlogopite, enstatite, and chromite.


The general consensus reached on kimberlites is that they are formed deep within the mantle, between 150 and 450 kilometers deep, from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable carbon dioxide and volatile components. It is this depth of melting and generation that makes kimberlites prone to hosting diamonds.


Most kimberlites are confined to the ancient cratons (or areas underlain by the cratons). The ages of most kimberlites are in the Late Mesozoic Era (Jurassic-Cretaceous periods).


Diamonds in the Kimberlite areas underlain by the cratons). The ages of most kimberlites are in the Late Mesozoic Era (Jurassic-Cretaceous periods).


Kimberlite Indicator Minerals areas underlain by the cratons). The ages of most kimberlites are in the Late Mesozoic Era (Jurassic-Cretaceous periods).

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.


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