(3) Rock Cycle and Igneous Rocks

Question 1

Minerals definition

Answer 1

• A naturally occurring inorganic solid with a crystaline structure.
• All minerals crystallize from ‘solutions’ and are in equilibrium with conditions at which they form

Question 2

Rocks *

Answer 2

Are composed of 1 or more mineral

Question 3

Magma

Answer 3

• Is hot molten material from the Earth’s interior
• Mixture of liquid elements/compounds/gases
• Temperature range ~600degC - 1800degC
• Created from complete melting of materials

Question 4

Igneous Rocks

Answer 4

• Form when magma COOLS and CRYSTALLIZES (solidifies)
• Composed of interlocking crystals/grains usually random oriented
• Wide range of compositions

Question 5

Igneous rocks can be subdivided into 2 groups

Answer 5

• Extrusive/volcanic igneous rocks

- Intrusive/plutonic igneous rocks

Question 6

Extrusive igneous rocks

Answer 6

• Solidify fast at/near the surface

- Usually fine grained (

Question 7

Intrusive Igneous rocks

Answer 7

• Solidify slowly beneath the surface

- Usually fine coarse grained (>1mm)

Question 8

Weathering

Answer 8

• At the surface rocks may break-down by chemical and mechanical processes
• The broken or dissolved materials may then be eroded (removed) and transported to other sites and be deposited (forming)

Question 9

Sediments

Answer 9

• Unconsolidated materials derived from previously formed rocks
• Fragments = clastic or detrital sediments
• Organic materials = bioclastic sediments
• Inorganic precipitates = chemical sediments

Question 10

Sedimentary Rocks are caused by:

Answer 10

When sediments are lithified due to compaction and/or cementation

Question 11

Metamorphic rocks are caused by:

Answer 11

When preiviously formed rocks are subjected to metamorphism (heat, pressure, circulating fluids)

Question 12

Igneous rocks are formed by:

Answer 12

Magma

Question 13

Magma in Asthenosphere

Answer 13

• It is partly molten
• Liquid rocks ~10% less dense than solid rock, so it rises towards the surface
• Forceful rising also occurs by ‘squeezing’

Question 14

Chemical composition of Igneous rocks

Answer 14

• Most common igneous rocks are composed of silicate materials such as olivine and quartz
• Oxygen and silicon are the most abundant elements in the crust, and are the structural/chemical basis of the silicate mineral, therefore SiO2 content is important

Question 15

Chemical analysis of composition

Answer 15

• Because chemical analysis’s are presented as weight % oxides we use SiO2
• ~40-70% of common igneous are SiO2 by weight
• Quartz is 100% silicate

Question 16

Categories of igneous rocks based on silica

Answer 16

• Felsic: >65% silica, mainly felspar and silica, light coloured
• Intermediate: ~55-65% silica, mainly Na/K and some quartz
• Mafic: ~50% silica, mainly Mg and Iron, dark coloured
• Ultramafic:

Question 17

Cooling of Magma

Answer 17

• As magma rises it cools and crystallization begins
• The crystals grow over a range of temperatures
• The type of mineral formed is controlled by the temperature and initial composition of the magma
• Can evolve
• Early formed minerals tend to be UM
• As the magma cools and crystallizes the minerals formed evolve to become more felsic

Question 18

Melting temperatures of Ultramafic, Mafic, and Felsic

Answer 18

• U: 1800deg
• M: 1200deg
• F: 600 - 800deg

Question 19

Bowen’s Reaction Series for Discontinuous series

Answer 19

• Mainly the Fe and Mg rich minerals
• Early formed minerals become reabsorbed into the melt as the next mineral is formed
• During resorption the SOT joins together into more complex forms
• Minerals will absorb/recombine as long as the temperature is high enough
• Series: Olivine - Pyroxene - Amphibole - Biotite
• After biotite, all iron and Mg are used up, magma is now enriched with silica

Question 20

Bowen’s Reaction Series for Continuous series

Answer 20

• Continuous change between Ca and Na rich forms of plagioclase feldspar with decreasing temperature

Question 21

Last magma to crystallize becomes…

Answer 21

• Enriched in K, Al, and SiO2, so can become quartz

Question 22

After magma crystallizes

Answer 22

• The composition of the rock should reflect the composition of the magma
• NOT always the case

Question 23

Differentiation

Answer 23

• Ingredients separate from originally homogeneous mixture
• (A) fractional crystallization: earlier formed crystals are heavier than the magma and may sink to the bottom floor of the magma chamber (crystal settling)
• (B) remove magma from the crystals (filter pressing)
• Magma mixing

Question 24

Heat sources to melt rocks

Answer 24

• Geothermal gradient (temperature increases with depth)
• Radioactivity
• Friction
• Mantle rock/heat conduction

Question 25

Diverse compositions of crystallized magma (not differentiation)

Answer 25

• Partial melting: reverse of crytallization (low temperature minerals melt first) (minerals high in K, Al, SiO2)
• Assimilation: melting surrounding rocks (left with xenoliths (unmelted rock) in the magma)

Question 26

Classification of Igneous rocks based on textures

Answer 26

• Grain size mostly
• Fine grained: due to rapid xtallization most grains are too small to see without magnification
• Coarse grained: slow cooling beneath surface, visible grains
  Porphythic: very large grains, two stage cooling or extreme conditions
• Glassy: very rapid cooling, no grains visible