IGNEOUS ROCKS

We can see from direct observation of volcanoes that some rocks crystallized from melt. Molten tock is called magma. But all these rocks not same. Some coarse, some fine grained, some dark looking, some light colored. Thus, basic variables are texture and mineral composition

Texture

Volcanic (extrusive)rocks fine grained because cools quickly. Experiments prove that faster cooling, them the smaller the grain size. If cooling v rapid then amorphous solid volcanic glass forms.

Porphyritic texture is when large grains(phenocrysts) form by slow cooling, then matrix is fine grained because cools rapidly

Plutonic/ intrusive rocks. Coarse grained igneous rock also found how formed. Can see in field that invades and cuts surrounding rock along fractures. Contact baked. Conclusion is that formed at depth Intrusive = plutonic igneous rock.

Chemical and Mineralogical Composition.

Mafic (dark) and felsic (light) minerals

Mafic: Olivine, Pyroxene, Amphibole, Biotite

Felsic: Quartz, Alkali feldspar (k-Feldspar, pink), plagioclase feldspar (Na), muscovite,

Mafic rocks are those with a large proportion of mafic minerals, -dark colored. Ultramafic - no light colored minerals. Felsic rocks those with felsic minerals. Intermediate - those inbetween

Classification of Igneous Rocks

To see classification table click here

Physical features of Igneous rocks

Extrusive - volcanic

Lava:. Tongue like form of flows. Columnar joints, pillow lavas, vesicles, scoria, pumice

pyroclastic rocks: ash, lapilli, bombs, blocks

pyrocalstic deposits: tuff, volcanic breccia

Intrusive - plutonic

"minor" dikes - discordant, sills- concordant

"major" pipes, stocks, batholiths

other freatures: xenoliths, roof pendants

Origin of magma - how do magmas form

Earth gets hotter as approach center ( by about 250C/km) so this suggests has origin in mantle. But how do we get mantle to melt? Also mantle seems to be peridotite, but most magmas are not that mafic

Experiments show that with increasing temperature melting occurs partially. Partial melting of 10% of peridotite gives basalt liquid.

But how do we get mantle to melt?

Decompression melting If can lower pressure, then can lower melting point. Can get mantle to flow upward, them pressure decreases. This happens in Mid-ocean ridges and in mantle plumes (Hot spots)

Hydrous melting Water also decreases melting point in silicates, if can intruduce water into mantle, then again can cause melting. This happens in sudbuction zones. Metamorphic recrystallization in the descending ocean floor slab releases small quantities of water that promote melting in mantle above slab

Fractional Crystallization and subsequent modification of magmas

Magmas produced from mantle are mainly basaltic, and we need to explain other igneous rock types - magmatic differentiation

Magmas may pond in their rise to the surface and sit at intermediate levels in a magma chamber there beging to crystallize

As temperature falls, different minerals crystallize. Which minerals crystallize at what temperature is described by Bowens reaction series. When mafic minerals crystallize they leave theliquid more felsic. If such mafic crystals sink to the bottom of the magma chamber, the magma erupted will tend to have an intermediate or even felsic compostion. This process is known as fractional crystallization.

(if time)

Igneous rocks and mineralization

Differentiation mafic intrusions Bushveldt Complex platinum and chromite

Hydrothermal mineralization: gold, silver, copper, zinc, iron, cobalt, molybdenum etc.

veins (lodes), disseminated