Slides 4c: Igneous Flashcards
What are the major types of magma and how are they classified?
There are four major magma types based on % silica (SiO2).
- Felsic (feldspar and silica) 66–76% SiO2
- Intermediate 52–66% SiO2
- Mafic (Mg- and Fe-rich) 45–52% SiO2
- Ultramafic 38–45% SiO2
How are igneous rocks classified?
based on their viscosity aka. silicate content
mafic —-(increasing silica content)—–> felsic
How do intrusive and extrusive igneous rocks differ in terms of their grain characteristics?
intrusive –> Coarse-grained
extrusive –> fine-grained
Magmas vary chemically due to ____
- initial source rock compositions.
- partial melting.
- assimilation.
- magma mixing.
_______ dictates initial magma composition.
Source rock
What type of magma is formed from mantle source?
ultra-mafic and mafic magmas
What type of magma is formed from crustal source?
mafic, intermediate, and felsic magmas.
What is partial melting?
Upon melting, rocks rarely dissolve completely.
Instead, only a portion of the rock melts.
How does the composition of the rock affects the melting time?
Si-rich minerals melt first;
Si-poor minerals melt last.
Partial melting, therefore, yields a silica-rich magma.
Removing a partial melt from its source creates____
- felsic magma.
- mafic residue.
What is assimilation?
- Magma melts the wall rock it passes through.
- Blocks of wall rock (xenoliths) fall into magma.
- Assimilation of these rocks alters magma composition.
What is magma mixing?
- Different magmas may blend in a magma chamber.
* The result combines the characteristics of the two.
What happens when magma mixing is not complete?
•Often magma mixing is incomplete, resulting in blobs of one rock type (xenolith) suspended within the other.
What is fractional crystallization?
When a magma cools, mafic minerals that have a greater melting point will form first. As a result, the remaining magma becomes more felsic (Si-rich) as it cools –> fractional crystallization
What are the changes that occur with cooling during igneous rock formation and what are the outcomes?
Fractional crystallization—early crystals settle by gravity.
Melt composition changes as a result.
- Fe, Mg, Ca are removed as early mafic minerals settle out.
- Remaining melt becomes enriched in Si, Al, Na, and K.
What is Bowens Reaction Series?
N. L. Bowen—devised experiments cooling melts (1920s).
- Early crystals settled out, removing Fe, Mg, and Ca.
- Remaining melt progressively enriched in Si, Al, and Na.
Which is the correct order of adjectives describing magma composition?
←more silica less silica→
A.felsic / intermediate / ultramafic / mafic.
B.ultramafic / mafic / felsic / intermediate.
C.intermediate / felsic / mafic / ultramafic.
D.felsic / intermediate / mafic / ultramafic.
D
Why are there different magma compositions?
- initial source rock compositions.
- partial melting.
- assimilation.
- magma mixing.
What are the two major categories of the igneous environments? What are the based on? What are their characteristics?
Two major categories—based on cooling locale.
Extrusive settings—cool at or near the surface.
- Cool rapidly.
- Chill too fast to grow big crystals.
Intrusive settings—cool at depth.
- Lose heat slowly.
- Crystals often grow large.
What are the steps of formation of igneous rocks in extrusive settings?
- Lava flows cool as blankets that often stack vertically.
- Lava flows exit volcanic vents and spread outward.
- Low-viscosity lava (basalt) can flow long distances.
- Lava cools as it flows, eventually solidifying.
What kind of eruptions occur in extrusive settings? What are its characteristics?
Explosive ash eruptions
- High-viscosity felsic magma erupts explosively.
- Yield huge volumes of ash that can cover large regions
- Pyroclastic flow—volcanic ash and debris avalanche Races down the volcanic slope as a density current
Often deadly
In intrusive settings magma invades preexisting wall rock by _______
- percolating upward between grains
- forcing open cracks.
What is the wall rock and what are its regions?
magma-intrusive contact reveals high heat.
- Baked zone—rim of heat-altered wall rock
- Chill margin—rim of quenched magma at contact
Geologists categorize intrusions by shape and these categories are _____
- Tabular (sheet)—planar with uniform thickness
- Blister-shaped—a sill that domes upward
- Balloon-shaped—blobs of melted rock
What are tubular intrusions like? What are the two major subdivisions?
- tend to have uniform thicknesses.
- have two major subdivisions.
Sill—injected parallels to rock layering
Dike—cuts across rock layering
How do dikes and sills modify invaded country rock?
They cause the rock to expand and inflate.
They thermally alter the country rock.
How are dikes formed?
- cut across preexisting layering (bedding or foliation).
- spread rocks sideways.
- dominate in extensional settings.
How are sills formed?
- are injected parallel to preexisting layering.
- are usually intruded close to the surface.
Both dikes and sills exhibit wide variability in ____
- size.
- thickness (or width).
- lateral continuity.
What is a laccolith?
Some intrusions start to inject between layers but then dome upward, creating a blister-shaped intrusion known as a laccolith
With erosion, progressively deeper features are exposed such as ______
- Vertical dikes
- Horizontal sills
- Mushroom-shaped laccoliths
How do intrusive settings affect the landscape?
- With erosion, progressively deeper features are exposed
- Continued uplift and erosion exposes the pluton
- Unroofing takes long periods of geologic time
How does continued uplift and erosion exposes the pluton?
- Intrusive rocks are usually more resistant to erosion.
- Thus, intrusive rocks often stand high on the landscape.
________ is a tabular intrusion that cuts across layers, whereas a _______ is one that intrudes between layers.
A.Pluton/laccolith
B.Sill/dike
C.Dike/sill
D.Flow/batholit
D
If you find an outcrop of coarse-grained igneous rock, you are probably looking at . . .
A.slowly cooled magma in a large pluton that formed deep in the crust.
B.a frozen lava flow that cooled quickly at the Earths surface.
C.a rapidly cooled magma in a very thin dike.
D.a layer of pyroclass & debris.
A
In which of the following tectonic settings is magma formation not likely a consequence of decompression melting? A.mid-ocean ridge B.Rift C.Convergent plate boundary D.Hot spot
C
What are plutons?
irregular or blob-shaped intrusions, 10 m to 10 km across
What are batholiths? Give an example
multiple plutons, 100’s km long x 100 km wide
ex: Sierra Nevada Batholith
Explain the plutonic activity
- Plutons may amass into a batholith.
- Immense volumes of intrusives
- Form above subduction zones
- May add magma for tens of Ma
- Batholiths mark former subduction.
What are Large Igneous Provinces? What are the types of magma that is erupted and characteristics?
- Unusually large outpourings of magma
- Mostly mafic, include some felsic examples
- Mantle plume first reaches the base of the lithosphere.
- Erupts huge volumes of mafic magma as flood basalts
- *Low viscosity
- *Can flow tens to hundreds of kms
- *Accumulate in thick piles .
Where and why are the igneous rocks used in life?
Igneous rock is used extensively as building stone.
- Office buildings
- Kitchens
Why?
- Durable (hard)
- Beautiful
How do we describe the igneous rocks in terms of the size, shape, and arrangement of the minerals?
- Crystalline—interlocking crystals fit like jigsaw puzzle
- Fragmental—pieces of preexisting rocks, often shattered
- Glassy—made of solid glass or glass shards
** Texture directly reflects magma history.
How is crystalline classified?
Classification is based on composition and texture.
What are the three types of textures that can be seen on crystalline and how are they formed?
Fine-grained
- Rapid cooling
- Crystals do not have time to grow.
- Extrusive
Coarse-grained
- Slow cooling
- Crystals have a long time to grow.
- Intrusive
Porphyritic texture—a mixture of coarse and fine crystals
- Indicates a two-stage cooling history.
- Initial slow cooling creates large phenocrysts.
- Subsequent eruption cools remaining magma more rapidly.
How is glassy texture formed and what is the fracture?
Solid mass of glass or crystals surrounded by glass
- Fracture conchoidally
- Result from rapid cooling of lava
How is glassy texture classified? What type of igneous rocks represent these categories?
More common in felsic igneous rocks
- Obsidian—felsic volcanic glass
- Pumice—frothy felsic rock full of vesicles; it floats.
- Scoria—glassy, vesicular mafic rock
How is pyroclastic texture classified? What type of igneous rocks represent these categories?
Pyroclastic—fragments of violent eruptions
- Tuff—volcanic ash that has fallen on land
- Volcanic breccia—made of larger volcanic fragments
