L10-12 igneous rocks

Question 1

Igneous rock formation

Answer 1

Cooled magma or lava
A mixture of liquid (melt), crystals and gas

Question 2

Igneous rock classification

Answer 2

Texture (phaneritic or phaneritic) and composition
Gabbro: course grained mafic
Basalt: fine grained mafic
Diorite: course grained intermediate
Andesite: fine grained intermediate
Granite: course grained felsic
Rhyolite: fine grained felsic

Question 3

Relation between the compositions of igneous rock and their minerals

Answer 3

Felsic: almost 3/4 silica

Intermediate: medium of everything

Mafic: less then half silica and more

Question 4

Relation between the textures of
igneous rocks and their occurrence

Answer 4

Growth: the growth of a crystal lattice increases in rate at high temperatures. slow cooling at high temperatures results in course grained rocks

Nucleation: nucleation of a crystal lattice increases in rate at lower temperatures. fast cooling at lower temperatures results in fine grained rocks

If temperature is low enough neither will occur and glass will form

Question 5

Intrusive igneous rock emplaced and formation

Answer 5

Dikes (crossing other features)
Sills (sideways)
Intrusive contacts (simple or complex)
Xenoliths (foreign rocks included in magma)
Chilled margins
Batholiths (large intrusions)

Question 6

Heat sources for the formation of magma

Answer 6

50% heat from the accretion & core-formation of earth
50% heat released by radioactive decay
Localised heat released by shearing of rock

Question 7

Principal mechanisms of partial melting and relation to plate tectonics

Answer 7

Decompression melting: occurs at divergent boundaries. asthenosphere rises making pressure drop. results in basalt eg pillow basalt

Hydrous melting: occurs at convergent boundaries in subduction zones. water is released from subducting plates which lowers the melting point of the overlying mantle. quite basaltic

Thermal melting: the movement of heat from one atom to another. slow and not very effective

Question 8

What determines the initial compositions of magmas?

Answer 8

The composition of rock that undergoes partial melting.

Other processes effect the composition of igneous rocks:
Fractional crystallization (different minerals crystalise at different temps yielding a distinctive sequence of crystallization)

Wall rock contamination (magma incorporates rock through which it ascends)

Mixing magmas (blend extreme compositions to get intermediates)

Liquid immiscibility (separation of initially
homogeneous liquids (applies mainly to
rare non-silicate magma types))

Question 9

Magma properties

Answer 9

Basaltic:
Composition: 45-55 wt% SiO2 high Fe, Mg, Ca low K, Na
Melting point: 1000-1300
Viscosity: low

Andesite
Composition: 55-65 wt% SiO2 intermediate Fe, Mg, Ca, K, Na
Melting point: 800-1000
Viscosity: medium

Rhyolite:
Composition: 65-75 wt% SiO2 low Fe, Mg, Ca high K, Na
Melting point: 650-800
Viscosity: high

Question 10

Bowens reaction series

Answer 10

crystallisation sequences for a wide range of magmas could be assembled into two composite reactions sequences

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Question 11

Define radioactive decay

Answer 11

Radioactive decay is a natural process in which the number of protons in an atom spontaneously changes transforming it into a different element

Question 12

How is radioactive decay used to date
minerals and rocks

Answer 12

Knowing the half-life for a decay pair - the time it takes for half of an amount of radioactive atoms to decay - the ratio of radiogenic to radioactive atoms
an be used to calculate the age of a mineral or rock

Question 13

The two radioactive-radiogenic
isotope pairs used most widely to date
igneous rocks

Answer 13

U-Pb is used to date zircon in intermediate to felsic igneous rocks & K-Ar to date K-bearing volcanic rocks including basalt