Summer SchoolGeosciences

Geology

Lecture 6 Igneous Rocks

Igneous Rocks

• Ignis is the Latin word for fire

• Igneous rocks are those formed by the consolidation, or crystallization, of magmas resulting in a mass of interlocking crystalline units or super-cooled glass

Igneous Rocks

• Magma is defined as a molten fluid, which is formed within the Earth’s crust or mantle. Magma is comprised of a complex of molten silicates with water and gasses in solution

Igneous Rocks

Classification

Igneous rocks are classified according to their:

• Rock chemistry

• Size of crystals forming the rock

Igneous RocksRock Chemistry

• Silica (chemically SiO2) is the dominant constituent in virtually all igneous rocks. Therefore classification is on the basis of silica variation:

• Ultrabasic <45% silica

• Basic 45 – 55% silica

• Intermediate 55 – 65% silica

• Acidic 65-80% silica

Igneous Rocks

Crystal Size

• If the majority of crystals have average dimensions of:

• >/= 5mm then the rocks are coarse-grained

• 1 – 5 mm rocks are medium-grained

• </= 1mm rocks are fine-grained (includes non-crystalline/glassy rocks)

Fine-grained

Obsidian

Medium-grained Granite

Coarse-grained pegmatite Granite

Igneous Rocks

• Many Igneous Rocks are PORPHYRITIC, i.e. they contain some crystals (Phenocrysts) which are much larger than the bulk of the rock. When classifying these rocks ignore the phenocrysts.

Igneous Rocks

Igneous Rocks may be of two types:

• Extrusive – those formed at the Earth’s surface

• Intrusive – those formed within the Earth’s crust

Igneous Rocks

Grain Size and rate of cooling

• Lavas tend to be fine-grained

• Hypabyssal rocks tend to be medium-grained

• Plutonic rocks tend to be course-grained

Igneous Rocks

Extrusive Rocks include:

• Lavas – which are formed from the consolidation of erupted magma

• Pyroclastic rocks – fragmented material and welded rocks produced by explosive igneous activity

• Hyaloclastites – rocks formed by the interaction of lava and water, e.g. sea or lakes

Igneous RocksIntrusive Rocks are divided into:• Hypabyssal rocks – formed from

relatively small bodies of magma, occurring along planes of weakness or fractures at comparatively shallow depths within the Earth’s crust

• Plutonic Rocks – formed from large intrusions, sometimes with areas of 1000s of kms and huge volumes of magma emplaced at considerable depth within the Earth’s crust

Igneous Rocks

• The centre of a lava may be medium-grained because it has taken longer to cool than the surface

• So the crystalline form of an igneous rock is controlled mainly by its rate of cooling

Igneous Rocks• Where magma is intruded deep within

the Earth’s crust, SLOW cooling - formation of COARSE-grained rock

• Where magmas are intruded into the Earth’s crust at shallow levels, MODERATE rates of cooling form MEDIUM-grained rock

• Where magmas are extruded onto the Earth’s surface, or into water, RAPID cooling results in FINE-grained rock

Igneous Rocks

• Using the classification diagram enables rock names to be assigned on the basis of % SiO2 and grain size

• There are no sharp boundaries between rocks of different chemistry, e.g. granites grade into syenites without a break

Igneous Rocks• The chemistry of an Igneous Rock is reflected

in its mineralogy

• Acid rocks contain ‘free’ silica – not combined with other oxides – and occurs as the mineral Quartz

• Basic rocks are relatively poor in silica, and no quartz is present, they have high content of Iron, Magnesium and Calcium oxides

Igneous RocksOther important igneous terms:

• Colour Index (CI) depends on the proportion of dark (Mafic or ferro-magnesian) minerals present

• Texture – the shapes of individual mineral grains and the relationship between them

Igneous Rocks

• Texture Terms:

• Holocrystalline – the rock is entirely crystalline

• Vitreous – containing abundant glass

Igneous Rocks

• Euhedral – grains showing perfect or near perfect crystal form

• Anhedral – showing no crystal form

• Sub-hedral – with a recognisable but imperfect crystal form

• Porphyritic – with large phenocrysts

• Equigranular – grains of approximately equal size

Igneous Rocks• Vesicular – has

cavities formerly occupied by gas bubbles

• Amygdaloidal – has cavities infilled with secondary mineral

• Xenolithic – has fragment of a foreign rock type

Igneous Rocks

Order of crystallization – the order in which the crystals in an igneous rock grew e.g. in porphyritic rock, the phenocrysts grew first and were subsequently encased in the later crystallizing groundmass

Igneous Rocks

Reading

• Press and Siever Ch. 14

• Thompson and Turk chapter on igneous rocks