Chapter 3: Igneous Rocks and Intrusive Activity

Question 1

What is magma? How does magma differ
from lava?

Question 2

In what basic settings do intrusive and extrusive igneous rocks originate?

Question 3

Differentiate between felsic and mafic igneous rocks.

Question 4

In what way are the dark (ferromagnesian) silicate minerals different from the light (nonferromagnesian) silicate minerals?

Question 5

What are the factors affecting crystal size?

Answer 5

1. amount of silica present
2. the rate at which molten rock cools
3. amount of dissolved gases in magma

Question 6

What does a porphyritic texture indicate
about an igneous rock?

Question 7

Why are the crystals in pegmatites so large?

Question 8

The classification of igneous rocks is based largely on two criteria. Name these criteria.

Question 9

How are granite and rhyolite different? In what way are they similar?

Question 10

Compare and contrast each of the following pairs of rocks:
a. granite and diorite
b. basalt and gabbro
c. andesite and rhyolite

Question 11

How do tuff and volcanic breccia differ from other igneous rocks such as granite and basalt?

Question 12

How magmas form?

Answer 12

1. when an increase in temperature causes a rock to exceed its melting point
2. in zones of upwelling a decrease in pressure (without the addition of heat) can result in decompression melting
3. the introduction of volatiles (principally water) can lower the melting temperature of hot mantle rock sufficiently to generate magma.

Question 13

Differentiate intrusive from extrusive rocks

Question 14

In crystallization, what does slow cooling forms?

Answer 14

large crystals

Question 15

In crystallization, rapid cooling of magma produces?

Answer 15

solid mass consisting of tiny intergrown crystals

Question 16

In crystallization, what forms when molten material is quenched instantly?

Answer 16

a mass of unordered atoms, referred to as glass

Question 17

consequences of the mineral composition of an igneous rock

Answer 17

chemical make-up of the parent magma and the environment of crystallization.

Question 18

Describe felsic rocks

Answer 18

composed mostly of the light-colored silicate miner als potassium feldspar and quartz

Question 19

Describe intermediate rocks

Answer 19

rich in plagioclase feldspar and amphibole

Question 20

Describe mafic rocks

Answer 20

contain abundant olivine, pyroxene, and calcium-rich plagioclase feldspar. They are high in iron, magnesium, and calcium, low in silica, and dark gray to black in color.

Question 21

What does the texture of igneous rocks imply?

Answer 21

refers to the overall appearance of the rock based on the size, shape, and arrangement of its mineral grains.

Question 22

Factor that influences the texture of igneous rocks

Answer 22

the rate at which magma cools

Question 23

What are the common igneous rock textures? Briefly describe each.

Answer 23

1. Aphanitic (fine grained)
2. Phaneritic (coarse-grained)
3. Porphyritic
4. Glassy

larger crystals are called phenocrysts and matrix of smaller crystals are groundmass

Question 24

Significance of Bowen’s Reaction Series

Answer 24

illustrates the sequence of mineral formation within magma.

Question 25

What did N.L. Bowen discovered?

Answer 25

that as magma cools in the laboratory, those minerals with higher melting points crystallize before minerals with lower melting points

Question 26

This process happens when magma cools in the laboratory, those minerals with higher melting points crystallize before minerals with lower melting points

this happens during the crystallization of magma

Answer 26

crystal settling

Question 27

The process of developing more than one magma type from a common magma

Answer 27

magmatic differentiation

Question 28

Once a magma body forms, its composition can change through the incorporation of foreign material, this process is called?

Answer 28

assimiliation or magma mixing

Question 29

How can magma be generated?

Answer 29

Through:
1. raising a rock’s temperature
2. decrease in pressure (causes decompression melting)
3. introduction of volatiles (water) - can lower a rock’s melting point sufficiently to generate magma.

Question 30

produces a melt made of the low-melting-temperature minerals, which are higher in silica than the original rock

Answer 30

partial melting

magmas generated by partial melting are nearer to the felsic end of the compositional spectrum than are the rocks from which they formed.

Question 31

How are intrusive igneous bodies classified?

Answer 31

according to their shape and by their orientation with respect to the country or host rock, generally sedimentary or metamorphic rock

The two general shapes are tabular (sheet-like) and massive.

Question 32

Intrusive igneous bodies that cut across existing sedimentary beds

Answer 32

discordant

Question 33

intrusive igneous bodies that form parallel to existing sedimentary beds

Answer 33

concordant

Question 34

tabular, discordant igneous bodies produced when magma is injected into fractures that cut across rock layers.

Answer 34

dikes

Question 35

Nearly horizontal, tabular, concordant bodies form when magma is injected along the bedding surfaces of sedimentary rocks.

Answer 35

sills

sills closely resemble buried lava flows.

Question 36

the largest intrusive igneous bodies, sometimes make up large linear mountains, as exemplified by the Sierra Nevada

Answer 36

batholiths

Question 37

similar to sills but form from less fluid magma that collects as a lens-shaped mass that arches overlying strata upward.

Answer 37

Laccoliths

Question 38

best-known and most important ore deposits are generated from?

Answer 38

hydrothermal (hot-water) solutions

Question 39

thought to originate from hot, metal-rich fluids that are remnants of late-stage magmatic processes.

Answer 39

hydrothermal deposits

These ion-rich solutions move along fractures or bedding planes, cool, and precipitate the metallic ions to produce vein deposits. In a disseminated deposit (e.g., much of the world’s copper deposits), the ores from hydrothermal solutions are distributed as minute masses throughout the entire rock mass.