Igneous Rocks: Forged By Fire

-Igneous rocks (from the Latin ignis, or fire) form as molten rock cools and solidifies.

- Igneous rocks and metamorphic rocks, derived from igneous “parents,” make up about 95 percent of the Earth’s crust.

- The mantle, which accounts for more than 82 percent of the Earth’s volume, is also composed of igneous rock. Thus, Earth can be described as a huge mass of igneous rocks covered with a thin veneer of sedimentary rocks and having a relatively small iron-rich core.

Igneous rocks form from a material called magma, which is created when intense heat and pressures melt solid rock located in the crust and upper mantle of the Earth.

Where Do Igneous Rocks Come From?

General characteristics of magma:

•Parent material of igneous rocks

•Forms from partial melting of rocks inside Earth

•Magma that reaches the surface is called lava

- Igneous rocks that form below the Earth’s surface are called intrusive igneous rocks (or plutonic). The word “plutonic” comes from Pluto, the name for the Greek god of the underworld.

- They form when magma enters a pocket or chamber underground that is relatively cool and solidifies into crystals as it cools very slowly.

- Igneous rocks can be described as intrusive and plutonic (formed from magma inside the Earth), or extrusive and volcanic (formed from lava above the Earth’s surface).

How Does Magma Make Igneous Rocks?

Magma consists of three components:

A liquid portion, called melt, that is composed of mobile ions

Solids, if any, are silicate minerals that have already crystallized from the melt

Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

Characteristics of Magma

Characteristics of Magma- A major portion of all magma is silica, which is a compound of silicon (the second most abundant element on Earth), and oxygen (the most abundant element on Earth). Magma also contains gases, which expand as the magma rises.

- Magma that is relatively poor in silica flows easily, so gas bubbles move up through it and escape fairly gently.

- Magma that is high in silica resists flowing, so expanding gases are trapped in it. Pressure builds up until the gases blast out in a violent, dangerous explosion.

• Role of heat

- Temperature increases within Earth’s upper crust (called the geothermal gradient) average between 20oC to 30oC per kilometer

- Rocks in the lower crust and upper mantle are near their melting points

- Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may induce melting

Characteristics of Magma

• Role of pressure

- An increase in confining pressure causes an increase in a rock’s melting temperature or conversely, reducing the pressure lowers the melting temperature

Characteristics of Magma

- When confining pressures drop,

decompression

melting occurs

• Role of volatiles

– Volatiles (primarily water) cause rocks to melt at lower temperatures

– This is particularly important where oceanic lithosphere descends into the mantle

Characteristics of Magma

- Assimilation: Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma

Characteristics of MagmaSometimes as it is forming, magma can change its composition. This may result in different types of magma coming out in one volcanic event.

- Magmatic differentiation: Separation of a melt from earlier formed crystals to form a different composition of magma

- When scientists began observing different compositions of lavas coming from various volcanic events, this led them to wonder if there was a way to use this information to learn more about the various igneous rocks that form.

• One scientist, N.L. Bowen, used this information to demonstrate that as a magma cools, minerals crystallize in a systematic fashion based on their melting points.

• Bowen’s reaction series, as it is called today, shows us temperature and mineral content are the main determinants of how crystal structures will change and evolve (and ultimately, which type of chemical composition an igneous rock will have).

Geology and Chemistry Connect!

-During crystallization (as the magma cools), the composition of the liquid portion of the magma continually changes. At the higher temperatures associated with mafic and intermediate magmas, the general progression can be separated into two branches: the continuous branch and the discontinuous branch.

Bowen’s Reaction Series Explained

The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich to more sodium-rich.

-The discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica.

Bowen’s Reaction Series Explained

-The weird thing that Bowen found concerned the discontinuous branch.

-At a certain temperature a magma might produce olivine, but if that same magma was allowed to cool further, the olivine would "react" with the residual magma, and change to the next mineral on the series (in this case pyroxene). Continue cooling and the pyroxene would convert to amphibole, and then to biotite.

-Mighty strange stuff, but if you consider that most silicate minerals are made from slightly different proportions of the same 8 elements (O, Si, Al, Fe, Ca, Na, K, Mg), all we're really doing here is adjusting the internal crystalline lattice to achieve stability at different temperatures. Really no big deal.

Bowen’s Reaction Series Explained

- Understanding the way magma forms and solidifies helps us know WHY we have different textures and compositions for various igneous rocks.

So, What Does This Mean For Igneous Rocks?

Igneous rocks are composed primarily of silicate minerals

•Dark (or ferromagnesian) silicates

Properties of Igneous Rocks

Olivine Pyroxene Amphibole Biotite Mica

•Light (or nonferromagnesian) silicates

Quartz Muscovite Mica Feldspar

Properties of Igneous Rocks

Granitic composition:– Composed of light-

colored silicates– Felsic (feldspar and

silica) in composition– Contains high amounts

of silica (SiO2)

– Major constituents of continental crust

– Granitic magmas are higher in silica and therefore more viscous than other magmas

– Because of their viscosity, they lose their mobility before reaching the surface and tend to produce large plutonic structures

Main Differences of Igneous Rocks

Basaltic composition:

Composed of dark silicates and calcium-rich feldspar

Mafic (magnesium and ferrum, for iron) in composition

More dense than granitic rocks

Comprise the ocean floor as well as many volcanic islands

- Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones

- Large outpourings of basaltic magma are common at Earth’s surface

Main Differences of Igneous Rocks

• Other compositional groups:

• Intermediate (or andesitic) composition

– Contain at least 25 percent dark silicate minerals

– Associated with explosive volcanic activity

• Ultramafic composition– Rare composition that is

high in magnesium and iron

– Composed entirely of ferromagnesian silicates

Main Differences of Igneous Rocks

Texture in igneous rocks is determined by the size and arrangement of mineral grains, which is usually determined by when/how a rock and crystals were formed.

Physical Properties of Igneous Rocks

• Rate of cooling– Slow rate promotes

the growth of fewer but larger crystals

– Fast rate forms many small crystals

– Very fast rate forms glass

• Amount of silica (SiO2) present

• Amount of dissolved gases

Factors That Determine Crystal Size

• Aphanitic (fine-grained) texture

– Rapid rate of cooling of lava or magma

– Microscopic crystals– May contain vesicles

(holes from gas bubbles)

• Phaneritic (coarse-grained) texture

– Slow cooling– Crystals can be identified

without a microscope

Types of Igneous Textures

Aphanitic Basalt

Phaneritic Diorite

• Porphyritic texture– Minerals form at different

temperatures as well as differing rates

– Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass

• Glassy texture– Very rapid cooling of molten

rock– Resulting rock is called

obsidian

Types of Igneous Textures

Porphyritic Andesite

Glassy Obsidian

• Pyroclastic texture– Various fragments ejected

during a violent volcanic eruption

– Textures often appear to more similar to sedimentary rocks

• Pegmatitic texture– Exceptionally coarse

grained– Form in late stages of

crystallization of granitic magmas

Types of Igneous Textures

Pyroclastic Tuff

Pegmatite