Continental crust key terms

Question 1

Bowins law

Answer 1

• more silicate in crust because of fractional crystallisation
  no mafic in upper crust
• increasing tempt decrease silica content: means more mafic (iron, magnesium and caesium) but reduces sodium and potassium

Question 2

ultra basic

Answer 2

periditities

Question 3

basic

Answer 3

gabbros & basalt

Question 4

intermediate

Answer 4

andesites & dolerites

Question 5

acid

Answer 5

lightly coloured granites, mildly radioactive

Question 6

Anaesthetic lava

Answer 6

high viscosity, often high silica

Question 7

hydrogenous mineral

Answer 7

mineral mainly made of hydrogen, most common minerals in crust and upper mantle
- anphibole, zoelights

Question 8

island arcs

Answer 8

Water in the subducted ocean lithosphere ‘boils off’ from hydrous minerals such as amphibole, zeolites and chlorite etc
 Released water and soluble elements rise
 Slab of subducted peridotite, basalt and sediments melts to produce basaltic magma with enrichment of soluble elements (e.g., K, Ba, and Pb)
 On way to the surface the melt cools with crystallization of silica poor minerals
- silica content of the remaining melt rises resulting in andesitic magma.

Question 9

geothermal gradient

Answer 9

the rate of increasing temperature with depth

Question 10

magma mixing: continental arcs

Answer 10

 High geothermal gradient above subducted plate
 Underplate of soft, partially molten continental crust
 Intruded basaltic magma causes partial melting of soft crust
 Mixing – intermediate compositions – andesite etc
 basaltic melt may heat up the overlying
arc, prompting partial melting
 Andesite probably formed by fractional crystallisation, magma mixing, partial melting and contamination by the subducted slab.

Question 11

upper asthenosphere

Answer 11

upper mantle, less viscous magma, melting from below lithosphere, more mafic

Question 12

Diorites and Andesites

Answer 12

formed above subduction zone

made of plagioclase felspar, augrite, horblade, amphibole quartz and olivine

Question 13

China Clay Formation

Answer 13

• occurs in funnel shaped bodies (St Austell granite)
• circulation of groundwater plus recharge from hydrothermal solutions
• feldspars alter to kaolinite
• china clays are by far the most economically valuable materical in Devon and Cornwall

Question 14

Greisen

Answer 14

• china clay formation

- where the feldspars have been hydrothermally altered to clay (kaolin)

Question 15

Hydrothermal Activity

Answer 15

• magma cools and solidifies = shrinks
• cracks and fissures open up
• mineral rich fluids
• concentrations of low temp and incompatible elements
• crystallise in viens

Question 16

Incompatible Elements

Answer 16

• have difficulty in entering cation sites of minerals because of their size/charge
• incompatible elements get concentrated in the magma (potassium, rubidium, barium, uranium)
• often found in minerals that are the last to crystallise (barites, apatite etc)

Question 17

Fractional crystallisation (island Arcs)

Answer 17

• water in the subduction ocean lithosphere ‘boils off’ from hydrous minerals such as amphibole, zeolites and chlorite etc.
• released water and soluble elements rise
• slab of subducted peridotite, basalt and sediments melts to produce basaltic magma with enrichment of soluble elements
• on way to the surface the melt cools with crystallisation of the silica poor minerals
• silica content of remaining melt rises resulting in andesitic magma

Question 18

Diorites and Andesites

Answer 18

• form above subduction zones
• 52-63% SiO2
• containing:
• Plagioclase
• augite (pyroxene)
• Hornblende/quartz/olivine
• wet ocean crust partial melting

Question 19

Granites and Rhyolites

Answer 19

• most of the continental crust
• forms in orogenic belts (Andes, Himalayas etc.)
• > 63% SiO2
• contain:
• Plagioclase
• Alkali feldspars (orthoclase)
• Quartz
• small amounts of mica, amphilboles, iron oxide etc.