summer school geosciences geology lecture 6 igneous rocks
TRANSCRIPT
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Summer SchoolGeosciences
Geology
Lecture 6 Igneous Rocks
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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
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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
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Igneous Rocks
Classification
Igneous rocks are classified according to their:
• Rock chemistry
• Size of crystals forming the rock
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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
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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)
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Fine-grained
Obsidian
Medium-grained Granite
Coarse-grained pegmatite Granite
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Igneous Rocks
• Texture Terms:
• Holocrystalline – the rock is entirely crystalline
• Vitreous – containing abundant glass
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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
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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
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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
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Igneous Rocks
Reading
• Press and Siever Ch. 14
• Thompson and Turk chapter on igneous rocks