L04 - Igenous Rocks and Volcanic Activity

what is an amorphous solid?

a type of solid that lacks definition in shape, pattern, and long-range order (i.e. glass)

describe how igneous rocks form

- crystallize from lava or magma

what is the difference between lava and magama?

- lava is above ground

- magma is below ground

what are intrusive/plutonic rocks?

a type of igneous rock that is formed below ground by the slow cooling of magma

- formed VERY DEEP below the surface

- take years -> millennia to form

do plutonic rocks have small or large crystals/what type of texture do they have?

- large (> 1mm)

- phaneritic texture

- e.g. granite

what are extrusive/volcanic rocks?

igneous rocks that form above ground through the cooling of lava

- quick cooling (days -> months)

- can be found up to 2km below the surface

- e.g. basalt

do extrusive/volcanic rocks have small or large crystals/what type of texture do they have?

- small (< 1mm), not visible to the naked eye

- anphanitic texture

what is two stage cooling?

- part of rocks cool very slowly, then cool very fast by coming out of a volcano

- have both anphanitic and phaneritic texture

what is the texture of rocks that undergo two stage cooling?

porphyritic texture

what factor of igneous rock formation influences texture?

cooling time

what does a light colour in igneous rocks indicate?

- that they are felsic

- feldspar + silica

- e.g. granite

what does a dark colour in an igneous rock indicate?

- that it is mafic

- magnesium + iron

- e.g. gabbro

what controls what type of silicates are found in igneous rocks?

- slightly controlled by chemistry

- strongly controlled by temperature of formation

what is Bowen's Reaction Series used for?

- describing what minerals will form from the melt (lava or magma) based on what temperature it is cooloing at

what is the order of reaction in Bowen's Reaction Series (left side)?

single tetrahedra -> ultramafic -> chain -> sheet -> 3D

olivine -> pyroxene, amphibole, biotite

what happens to temperature throughout Bowen's Reaction Series?

- decreases throughout

what is the order of reaction in Bowen's Reaction Series (right side)?

Ca rich to Na rich

what mineral composition is seen on the left side of Bowen's Reaction Series?

various silicates (simple to complex)

what mineral composition is seen on the right side of Bowen's Reaction Series?

variety of feldspar

where does liquid rock occur?

the crust

name the 3 ways magma can form

- increase temperature

- decrease pressure

- add volatiles or water

describe magma formation through decreasing pressure and where this may occur

- magma moves to a lower pressure -> decompression melting

- temperature stays the same

- lower pressure -> easier to melt

- occurs at divergent plate boundaries, as pulling plates apart reveals material below and lowers pressure

- occurs at hotspots, where hot material from deep in the mantle is brought to the surface

describe magma formation through adding volatiles/water and where this may occur

- adding water lowers the melting temperature

- occurs at subduction zones, where water comes off the descending plate and gets released into the mantle

- causes mantle to melt

what is a pluton?

mass of igneous rock that cools and crystallizes within the crust (any size or shape)

where are plutons crystallized?

- in rising bodies of magam called diapirs

- plutons cool at depth

- brought to the surface by plate tectonics/erosion

what is a batholith?

large (>100km^2) igneous bodies formed by several plutons formed together

what is a dyke?

- flat intrusive bodies that cut through layers or follow fractures

- can form near surface

- sometimes look like extrusive rocks (have small crystals)

what is a sill?

- flat intrusive body

- follow bedding and tend to be horizontal

what is the difference between a sill and a dyke?

- sills follow bedding and tend to be horizontal

- dykes cut bedding and tend to be vertical

what is a laccolith?

- mushroom shaped body

- magma gets injected between layers which inflates them (puffs them up)

what are the engineering applications of intrusive/plutonic rocks?

- generall strong, good for foundations

- allows steep, unsupported slopes

- can be almost impermeable to water

- not prone to alteration/weathering

- strong, intrusive igneous rocks can form extensive sets of fractures

how does magma rise in volcanoes?

- magma is often less dense than the surrounding rock

- moves upwards through the crust

- mostly fractures and breaks rock (only a little bit is by melting)

what are some reasons why volanoes differ?

- viscosity of lava

- magma compositions

- gas content

what controls the viscosity of volcanoes?

- temperature: hotter means less viscous

- composition of magma (silicate minerals are "sticky" and result in more viscosity)

describe mafic magma

- low viscosity

- lower silica content (45-55%)

- effusive

describe intermediate magma

- average temperature

- intermediate silica content (55-65%)

- average to high viscosity

- explosive

describe felsic magma

- usually relatively cool temperature

- high silica content (65-75%)

- very high viscosity

- highly explosive

- doesn't result in a lot of volcanoes because they all exploade

what is the typical range for gas content in volcanoes (%)?

1 to 10%

what happens to gas in volcanoes if magma is viscous?

- gases get trapped

- trapped gases expand near the surface

- pressure builds until an explosion

describe a volcano with high gas content and viscous magma

- explosive volcanism

- lots of pyroclastic material

- many hazards

describe a volcano with low gas content and runny magma

- effusive (non-explosive) volcanism

- lots of lava flows, less pyroclastic flow

- fewer hazards

describe a shield volcano

- mafic lava

- effusive

- has a central caldera

- flank eruptions can cause new lava flows

- only dangerous to buildings and not humans (slow moving lava)

what determines a volcano's form?

their composition and nature of eruption

describe composite/stratovolcanoes

- intermediate lava, explosive

- have ash/pyroclastic layers

- curved profiles, steeper at the top

- crater

- active lava flow near the top, old lava flow at the bottom

at what plate boundaries to stratovolcanoes usually form? what about mafic volcanoes?

- stratovolcanoes: subduction zones

- mafic volanoes: not a plate boundary, but at hotspots

describe how lava flow is a volcanic hazard

- low viscosity lava (basalts) can flow and spread laterally

- erupt at temperatures from 1200-1400 C

- road and property damage is common

- not dangerous to humans

describe how gasses are a volcanic hazard

- can erupt from volcanoes without warning

- suffocate entire regions

describe how pyroclastic materials are volcanic hazards

ash:

- pyroclasts (< 2mm diameter)

- form due to bubble formation and magma fragmentation

tephra:

- lateral flows of a turbulent mixture of hot gases and pyroclastic material (volcanic fragments, crystals, ash, pumice, glass shards)

- travel 300-400km/h over 10s of kms

- temperature: 500-700 C

- flow direction partially dictated by topography

describe how lahars are volcanic hazards

- lahars: volcanic debris flows

- mixture of water, pyroclastic material, debris

- travel up to 50km/h, often with little warning

- usually follow river channels

- travel further than lava and pyroclastic flows

explain the role of extrusive igneous rocks in construction

- typically less uniform than intrusive rocks

- more prone to weathering and breaking down

- may be subject to slope failure and provide weaker foundation support

- fractures are potential for 'lava tubes' to be problematic for tunnelling or a resevoir site

As Bowen's Reaction series progresses, how does the rock composition change?

ultramafic -> mafic -> intermediate -> felsic

As Bowen's Reaction series progresses, how does temperature change?

it goes from a high temperature to low temperature, as magma cools

What does Bowen's Reaction Series describe?

- describes the temperature at which minerals crystallize when cooled, or melt when heated.

- describes the sequence of crystallization of common silicate materials

- explains why certain types of minerals are found together while others are rarely associated with one another