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6-Understanding Planet Earth > Slides 4c: Igneous > Flashcards

Slides 4c: Igneous Flashcards

1
Q

What are the major types of magma and how are they classified?

A

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
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2
Q

How are igneous rocks classified?

A

based on their viscosity aka. silicate content

mafic —-(increasing silica content)—–> felsic

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3
Q

How do intrusive and extrusive igneous rocks differ in terms of their grain characteristics?

A

intrusive –> Coarse-grained

extrusive –> fine-grained

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4
Q

Magmas vary chemically due to ____

A
  • initial source rock compositions.
  • partial melting.
  • assimilation.
  • magma mixing.
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5
Q

_______ dictates initial magma composition.

A

Source rock

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6
Q

What type of magma is formed from mantle source?

A

ultra-mafic and mafic magmas

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7
Q

What type of magma is formed from crustal source?

A

mafic, intermediate, and felsic magmas.

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8
Q

What is partial melting?

A

Upon melting, rocks rarely dissolve completely.

Instead, only a portion of the rock melts.

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9
Q

How does the composition of the rock affects the melting time?

A

Si-rich minerals melt first;
Si-poor minerals melt last.
Partial melting, therefore, yields a silica-rich magma.

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10
Q

Removing a partial melt from its source creates____

A
  • felsic magma.

- mafic residue.

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11
Q

What is assimilation?

A
  • Magma melts the wall rock it passes through.
  • Blocks of wall rock (xenoliths) fall into magma.
  • Assimilation of these rocks alters magma composition.
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12
Q

What is magma mixing?

A
  • Different magmas may blend in a magma chamber.

* The result combines the characteristics of the two.

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13
Q

What happens when magma mixing is not complete?

A

•Often magma mixing is incomplete, resulting in blobs of one rock type (xenolith) suspended within the other.

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14
Q

What is fractional crystallization?

A

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

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15
Q

What are the changes that occur with cooling during igneous rock formation and what are the outcomes?

A

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.
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16
Q

What is Bowens Reaction Series?

A

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.
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17
Q

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.

A

D

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18
Q

Why are there different magma compositions?

A
  • initial source rock compositions.
  • partial melting.
  • assimilation.
  • magma mixing.
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19
Q

What are the two major categories of the igneous environments? What are the based on? What are their characteristics?

A

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.
20
Q

What are the steps of formation of igneous rocks in extrusive settings?

A
  • 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.
21
Q

What kind of eruptions occur in extrusive settings? What are its characteristics?

A

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

22
Q

In intrusive settings magma invades preexisting wall rock by _______

A
  • percolating upward between grains

- forcing open cracks.

23
Q

What is the wall rock and what are its regions?

A

magma-intrusive contact reveals high heat.

  • Baked zone—rim of heat-altered wall rock
  • Chill margin—rim of quenched magma at contact
24
Q

Geologists categorize intrusions by shape and these categories are _____

A
  • Tabular (sheet)—planar with uniform thickness
  • Blister-shaped—a sill that domes upward
  • Balloon-shaped—blobs of melted rock
25
Q

What are tubular intrusions like? What are the two major subdivisions?

A
  • tend to have uniform thicknesses.
  • have two major subdivisions.
    Sill—injected parallels to rock layering
    Dike—cuts across rock layering
26
Q

How do dikes and sills modify invaded country rock?

A

They cause the rock to expand and inflate.

They thermally alter the country rock.

27
Q

How are dikes formed?

A
  • cut across preexisting layering (bedding or foliation).
  • spread rocks sideways.
  • dominate in extensional settings.
28
Q

How are sills formed?

A
  • are injected parallel to preexisting layering.

- are usually intruded close to the surface.

29
Q

Both dikes and sills exhibit wide variability in ____

A
  • size.
  • thickness (or width).
  • lateral continuity.
30
Q

What is a laccolith?

A

Some intrusions start to inject between layers but then dome upward, creating a blister-shaped intrusion known as a laccolith

31
Q

With erosion, progressively deeper features are exposed such as ______

A
  • Vertical dikes
  • Horizontal sills
  • Mushroom-shaped laccoliths
32
Q

How do intrusive settings affect the landscape?

A
  • With erosion, progressively deeper features are exposed
  • Continued uplift and erosion exposes the pluton
  • Unroofing takes long periods of geologic time
33
Q

How does continued uplift and erosion exposes the pluton?

A
  • Intrusive rocks are usually more resistant to erosion.

- Thus, intrusive rocks often stand high on the landscape.

34
Q

________ 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

A

D

35
Q

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

A

36
Q 
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
A

C

37
Q

What are plutons?

A

irregular or blob-shaped intrusions, 10 m to 10 km across

38
Q

What are batholiths? Give an example

A

multiple plutons, 100’s km long x 100 km wide

ex: Sierra Nevada Batholith

39
Q

Explain the plutonic activity

A
  • 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.
40
Q

What are Large Igneous Provinces? What are the types of magma that is erupted and characteristics?

A
  • 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 .
41
Q

Where and why are the igneous rocks used in life?

A

Igneous rock is used extensively as building stone.

  • Office buildings
  • Kitchens

Why?

  • Durable (hard)
  • Beautiful
42
Q

How do we describe the igneous rocks in terms of the size, shape, and arrangement of the minerals?

A
  • 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.

43
Q

How is crystalline classified?

A

Classification is based on composition and texture.

44
Q

What are the three types of textures that can be seen on crystalline and how are they formed?

A

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.
45
Q

How is glassy texture formed and what is the fracture?

A

Solid mass of glass or crystals surrounded by glass

  • Fracture conchoidally
  • Result from rapid cooling of lava
46
Q

How is glassy texture classified? What type of igneous rocks represent these categories?

A

More common in felsic igneous rocks

  • Obsidian—felsic volcanic glass
  • Pumice—frothy felsic rock full of vesicles; it floats.
  • Scoria—glassy, vesicular mafic rock
47
Q

How is pyroclastic texture classified? What type of igneous rocks represent these categories?

A

Pyroclastic—fragments of violent eruptions

  • Tuff—volcanic ash that has fallen on land
  • Volcanic breccia—made of larger volcanic fragments

6-Understanding Planet Earth (18 decks)

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