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 Earths crust.- The mantle, which accounts for more than 82 percent of the Earths 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 rocksForms from partial melting of rocks inside EarthMagma that reaches the surface is called lava

- Igneous rocks that form below the Earths 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 Earths surface). How Does Magma Make Igneous Rocks?

Magma consists of three components:A liquid portion, called melt, that is composed of mobile ionsSolids, if any, are silicate minerals that have already crystallized from the meltVolatiles, 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 Earths 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 rocks melting temperature or conversely, reducing the pressure lowers the melting temperatureCharacteristics 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 magmas composition by the incorporation of foreign matter (surrounding rock bodies) into a magmaCharacteristics 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. Bowens 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.

Bowens Reaction Series ExplainedThe 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.Bowens 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.

Bowens 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 mineralsDark (or ferromagnesian) silicatesProperties of Igneous RocksBiotite MicaLight (or nonferromagnesian) silicatesQuartzMuscovite MicaFeldspar

Properties of Igneous Rocks

Granitic composition:Composed of light-colored silicatesFelsic (feldspar and silica) in compositionContains 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 structuresMain Differences of Igneous Rocks

Basaltic composition:Composed of dark silicates and calcium-rich feldsparMafic (magnesium and ferrum, for iron) in compositionMore dense than granitic rocksComprise 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 Earths surfaceMain Differences of Igneous Rocks

Other compositional groups:

Intermediate (or andesitic) compositionContain at least 25 percent dark silicate mineralsAssociated with explosive volcanic activity

Ultramafic compositionRare composition that is high in magnesium and ironComposed entirely of ferromagnesian silicatesMain 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 coolingSlow rate promotes the growth of fewer but larger crystalsFast rate forms many small crystalsVery fast rate forms glass

Amount of silica (SiO2) present

Amount of dissolved gases

Factors That Determine Crystal Size

Aphanitic (fine-grained) textureRapid rate of cooling of lava or magmaMicroscopic crystalsMay contain vesicles (holes from gas bubbles)

Phaneritic (coarse-grained) textureSlow coolingCrystals can be identified without a microscope

Types of Igneous TexturesAphanitic BasaltPhaneritic Diorite

Porphyritic textureMinerals form at different temperatures as well as differing ratesLarge crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass

Glassy textureVery rapid cooling of molten rockResulting rock is called obsidian

Types of Igneous TexturesPorphyritic AndesiteGlassy Obsidian

Pyroclastic textureVarious fragments ejected during a violent volcanic eruptionTextures often appear to more similar to sedimentary rocks

Pegmatitic textureExceptionally coarse grainedForm in late stages of crystallization of granitic magmas

Types of Igneous TexturesPyroclastic TuffPegmatite