Slides 4c: Igneous

Question 1

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

Answer 1

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

Question 2

How are igneous rocks classified?

Answer 2

based on their viscosity aka. silicate content

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

Question 3

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

Answer 3

intrusive –> Coarse-grained

extrusive –> fine-grained

Question 4

Magmas vary chemically due to ____

Answer 4

• initial source rock compositions.
• partial melting.
• assimilation.
• magma mixing.

Question 5

_______ dictates initial magma composition.

Answer 5

Source rock

Question 6

What type of magma is formed from mantle source?

Answer 6

ultra-mafic and mafic magmas

Question 7

What type of magma is formed from crustal source?

Answer 7

mafic, intermediate, and felsic magmas.

Question 8

What is partial melting?

Answer 8

Upon melting, rocks rarely dissolve completely.

Instead, only a portion of the rock melts.

Question 9

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

Answer 9

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

Question 10

Removing a partial melt from its source creates____

Answer 10

• felsic magma.

- mafic residue.

Question 11

What is assimilation?

Answer 11

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

Question 12

What is magma mixing?

Answer 12

• Different magmas may blend in a magma chamber.

* The result combines the characteristics of the two.

Question 13

What happens when magma mixing is not complete?

Answer 13

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

Question 14

What is fractional crystallization?

Answer 14

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

Question 15

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

Answer 15

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.

Question 16

What is Bowens Reaction Series?

Answer 16

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.

Question 17

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.

Answer 17

D

Question 18

Why are there different magma compositions?

Answer 18

• initial source rock compositions.
• partial melting.
• assimilation.
• magma mixing.

Question 19

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

Answer 19

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.

Question 20

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

Answer 20

• 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.

Question 21

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

Answer 21

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

Question 22

In intrusive settings magma invades preexisting wall rock by _______

Answer 22

• percolating upward between grains

- forcing open cracks.

Question 23

What is the wall rock and what are its regions?

Answer 23

magma-intrusive contact reveals high heat.

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

Question 24

Geologists categorize intrusions by shape and these categories are _____

Answer 24

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

Question 25

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

Answer 25

- tend to have uniform thicknesses. - have two major subdivisions. Sill—injected parallels to rock layering Dike—cuts across rock layering

Question 26

How do dikes and sills modify invaded country rock?

Answer 26

They cause the rock to expand and inflate. | They thermally alter the country rock.

Question 27

How are dikes formed?

Answer 27

- cut across preexisting layering (bedding or foliation). - spread rocks sideways. - dominate in extensional settings.

Question 28

How are sills formed?

Answer 28

- are injected parallel to preexisting layering. | - are usually intruded close to the surface.

Question 29

Both dikes and sills exhibit wide variability in ____

Answer 29

- size. - thickness (or width). - lateral continuity.

Question 30

What is a laccolith?

Answer 30

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

Question 31

With erosion, progressively deeper features are exposed such as ______

Answer 31

- Vertical dikes - Horizontal sills - Mushroom-shaped laccoliths

Question 32

How do intrusive settings affect the landscape?

Answer 32

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

Question 33

How does continued uplift and erosion exposes the pluton?

Answer 33

- Intrusive rocks are usually more resistant to erosion. | - Thus, intrusive rocks often stand high on the landscape.

Question 34

________ 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

Answer 34

D

Question 35

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.

Answer 35

A

Question 36

``` 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 ```

Answer 36

C

Question 37

What are plutons?

Answer 37

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

Question 38

What are batholiths? Give an example

Answer 38

multiple plutons, 100’s km long x 100 km wide | ex: Sierra Nevada Batholith

Question 39

Explain the plutonic activity

Answer 39

- 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.

Question 40

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

Answer 40

- 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 .

Question 41

Where and why are the igneous rocks used in life?

Answer 41

Igneous rock is used extensively as building stone. - Office buildings - Kitchens Why? - Durable (hard) - Beautiful

Question 42

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

Answer 42

- 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.

Question 43

How is crystalline classified?

Answer 43

Classification is based on composition and texture.

Question 44

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

Answer 44

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.

Question 45

How is glassy texture formed and what is the fracture?

Answer 45

Solid mass of glass or crystals surrounded by glass - Fracture conchoidally - Result from rapid cooling of lava

Question 46

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

Answer 46

More common in felsic igneous rocks - Obsidian—felsic volcanic glass - Pumice—frothy felsic rock full of vesicles; it floats. - Scoria—glassy, vesicular mafic rock

Question 47

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

Answer 47

Pyroclastic—fragments of violent eruptions - Tuff—volcanic ash that has fallen on land - Volcanic breccia—made of larger volcanic fragments