Igneous Rocks and Volcanic Activity
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How do Igneous rocks form?
Igneous rocks crystallize from lava or magma
Extrusive rock features and examples?
-Quick cooling tiny crystals. -Cooling time controls texture. -Cools quickly (days to months). -fine-grained (aphanitic) Ex: Basalt, Lava and ash
Intrusive rock features and examples?
-Slow cooling large crystals. -Cools slowly years to millennia -Large (visible) crystals develop (most > 1mm) -Coarse-grained (phaneritic) Ex. Magma and Granite
Composition of igneous rocks
-Mineral composition -Most rocks have silicate minerals (SiO4-) -Strongly controlled by temperature of formation and chemistry
Mineral composition. Which is light and which is dark?
Light colours = Felsic (Feldspar + Silica) Dark colours = Mafic (Magnesium + Iron Fe)
What is Bowen’s Reaction Series
describes which will form from the melt (lava or magma)
What is composition of minerals controlled by? Can different compositions form the same magma?
-strongly controlled by temperature of formation -Yes
How Do Magmas Form?
At depth, rock is under pressure and at high temperature
How to make magma (3 ways)
1) Increase Temperature 2) Decrease Pressure (decompression melting) 3) Introduction of Water/Volatiles
Where does magma form?
(Hot Spots, Divergent Margins, Subduction Zones)
Intrusive bodies (MUST KNOW) Hint: there’s 5
Plutons Batholith Dykes Sills Laccolith
What are plutons?
mass of igneous rock that cools and crystallizes within the crust (any size or shape) also they’re crystallized diapirs
What’s a diapir? (DHAMPIR??)
Rising bodies of magma are called Diapirs
How do intrusive (plutonic) rocks work with engineering? Why is it good and bad?
GOOD: -strong -good for foundations - Allows steep, unsupported slopes -Can be almost impermeable to water - Resistant: often not prone to alteration/weathering BAD: -can fracture
Describe magma in a volcano
Magma is often less dense than the surrounding rock: * Moves upwards through the crust * Mostly fractures and breaking rock (only a little by melting)
Are lava lakes rare or not?
Rare
Why Do Volcanoes Differ?
Volcanoes show a wide variety in their activity: * Shape * Type of eruption * Hazards * Explosivity
What is viscosity?
• The measure of a fluid’s resistance to flow • High viscosity – thick, slow flow • Low viscosity – runny, quick flow
2 ways viscosity of magma can be affected by:
Temperature of magma * The hotter the magma the less viscous it will be Composition of magma * Silicate (SiO2) minerals are “sticky” * Higher silica content * Higher viscosity
3 magma compositions and their properties:
Mafic Magma (45-55% silica) *High temp * Lower silica content * Low viscosity (runny) * Effusive Intermediate Magma (55-65% silica) * Average temperature * Intermediate silica content * Average to high viscosity * Explosive Felsic Magma (65-75% silica) * relatively cool temperature * High silica content * Very high viscosity * Highly explosive
How does a volcano blow up?
Gas Content * Gas content is typically 1-10% * If magma is viscous… * Gases get trapped * Trapped gases expand near surface * Pressure builds until… BOOM
Features of explosive volcanism
• Pyroclastic flows (fast-moving hot debris) • Ash clouds • Volcanic ash • Eruption columns
Features of Effusive (Nonexplosive) Volcanism
Low gas content and runny (not viscous) magma * “Tame” Volcano with lots of lava flows * Fewer hazards
Types of volcanoes
-Shield volcanoes (Mafic) Effusive Composite (flat asf) Strato-volcanoes (Intermediate) Explosive (tall asf)