What is the crystal size for coarse igneous rocks?
\>5mm
2
New cards
What is the crystal size for medium igneous rocks?
5-1mm
3
New cards
What is the crystal size for fine igneous rocks?
4
New cards
Where do coarse crystal igneous rocks form?
intrusive e.g a pluton or batholith
5
New cards
Where do medium crystal igneous rocks form?
hypabassal e.g a dyke or sill
6
New cards
Where do fine crystal igneous rocks form?
extrusive e.g lava flow
7
New cards
Where do glassy igneous rocks form?
extrusive e.g lava flow
8
New cards
Name the coarse silicic rock
granite
9
New cards
Name the coarse intermediate rock
diorite
10
New cards
Name the coarse mafic rock
gabbro
11
New cards
Name the coarse ultramafic rock
peridotite
12
New cards
Name the medium silicic rock
microgranite
13
New cards
Name the medium intermediate rock
micro diorite
14
New cards
Name the medium mafic rock
dolerite
15
New cards
Name the fine silicic rock
rhyolite
16
New cards
Name the fine intermediate rock
andesite
17
New cards
Name the fine mafic rock
basalt
18
New cards
Name the glassy silicic rock
obsidian
19
New cards
What is the silica content in silicic rocks?
\>66%
20
New cards
What is the silica content in intermediate rocks?
66-52%
21
New cards
What is the silica content in mafic rocks?
52-45%
22
New cards
What is the silica content in ultramafic rocks?
23
New cards
What are the silicic minerals?
orthoclase feldspar, plagioclase feldspar, quartz
24
New cards
What rocks are composed of quartz, orthoclase feldspar and biotite mica
granite and rhyolite
25
New cards
What rocks are composed of plagioclase feldspar, amphiboles and pyroxenes?
diorite and andesite
26
New cards
What rocks are composed of plagioclase feldspar, pyroxenes and olivine?
gabbro and basalt
27
New cards
What rock is composed of olivine and pyroxene?
peridotite
28
New cards
What is the temperature where melting begins?
700 degrees celcius
29
New cards
What elements increase when there is a higher silica content? (silicic)
silica, potassium and sodium
30
New cards
What elements increase when there is a lower silica content? (mafic)
magnesium, iron and calcium
31
New cards
define equicrystalline
when magma cools at the same rate so the crystals are the same size
32
New cards
define glassy texture
no visible crystals as magma cooled incredibly fast so the rock resembles glass
33
New cards
How does vesicular texture form?
when magma rises to the surface, the pressure decreases so the solubility of the magma decreases and gases in the solution come out. But as the magma is cooling rapidly, the vesicles of gas are trapped in the solidifying magma
34
New cards
How does amygdaloidal texture form?
when the vesicles in a vesicular rock are filled with mineral deposits (amygdales) from ground water as the vesicular rocks are very porous so ground water can flow through easily
35
New cards
How does flow banding form?
light and dark minerals form layers due to the separation of minerals then align parallel to the flow of the lava or with convection currents
36
New cards
How does porphyritic texture form?
large phenocrysts form first at depth and slowly move to cooler crust where cooling is quicker and are surrounded by fine crystals
37
New cards
What is the liquidus?
the boundary showing the temperature where the las solid particle melts
38
New cards
What is the solidius?
the boundary showing temperature where the first particle melts
39
New cards
What is partial melting?
when some of the minerals in a rock melt as they have a lower melting point
40
New cards
Why is there no partial melting of the mantle in a normal situation?
the temperature and pressure increases deeper into the crust so the melting point of peridotite increases so the rock cannot melt to form magma
41
New cards
Why does partial melting occur at a mid-ocean ridge?
crust is stretched due to the plates pulling apart which puts the asthenosphere higher in the crust where the pressure is reduced. so peridotite partially melts due to decompression
42
New cards
Why does partial melting occur at hot spots?
as the mantle plume rises from deep in the mantle where it is hotter (it rises as it is less dense) towards the crust, where the pressure is lower but the magma is very hot. so partial melting occurs
43
New cards
Why does partial melting occur at subduction zones?
the subducting plate brings down rocks with minerals that contain water, lowering the solidus so that partial melting can occur
44
New cards
What crystal size would a lava flow have?
fine as it cools quickly as it is extrusive
45
New cards
What is a batholith?
a large intrusion made from a group of plutons
46
New cards
What crystal size would a batholith have?
coarse as it cools slowly as it is intrusive
47
New cards
What is a dyke?
a discordant hypabassal intrusion that is several m thick
48
New cards
What does discordant mean?
it cuts through the bedding planes
49
New cards
define hypabassal
small intrusion several m thick
50
New cards
What is a sill?
a concordant hypabassal intrusion several m thick
51
New cards
what does concordant mean?
it goes along the bedding planes
52
New cards
What is a transgressive sill?
a sill that follows along the bedding planes but has small parts that cut through the bedding planes
53
New cards
What crystal size would a dyke and sill have?
medium as they are intrusive but they are not very big
54
New cards
What is a chilled margin?
the edge of the intrusion which is cooled by the country rock
55
New cards
What is a baked margin?
the edge of the country rock with is melted by the heat of the intrusion
56
New cards
What is fractional crystallisation?
The process by which the composition of the magma can change by olivine (for example) settles at the bottom reducing the amount of mafic minerals so the composition becomes more silicic
57
New cards
Describe how columnar jointing forms
a lava flow at least 3m thick cools slowly and contracts which forms hexagonal columns. the cooling originates at equally spaced centres and proceeds in all directions, the contraction causes cracks called joints to form vertically down.
58
New cards
Describe how pillow lavas form
underwater, the outer skin of the extruded magma forms rapidly. pressure builds from behind the pillow causing it to break through forming another pillow. they are rounded in shape and the outer pillows are the youngest
59
New cards
define stoping
the process of magma moving through the country rock by fracturing the surrounding rock
60
New cards
define assimilation
the melting of country rock into the magma composition
61
New cards
What is a xenolith?
fragments of pre-existing rock contained within an igneous rock
62
New cards
Define viscosity
the runniness of the magma
63
New cards
What is the parent magma?
the original magma before a composition change
64
New cards
What is the daughter magma?
the new magma after stoping with a changed composition due to the addition of silicic country rocks
65
New cards
Why does assimilation and magma fracturing occur with mafic magma only?
the magma is not viscous so it can move into the fractures and it is at a higher temperature so it can melt the country rock
66
New cards
Describe the properties of a sill
- two baked margins and two chilled margins - possible xenoliths - differentiation of magma and gravity settling - medium average crystal size - dolerite most common rock - no weathering, regular upper surface
67
New cards
Describe the properties of a lavaflow
- one chilled margin and one baked margin - no xenoliths - flow banding, vesicles and phenocrysts - fine average crystal size - basalt the most common rock - lava fragments in overlying sedimentary rock from erosion - irregular upper surface due to weathering - reddened surface from iron minerals reacting with air
68
New cards
what minerals are on the discontinous side of bowens reaction series?
olivine, pyroxene, amphibole, biotite (old people are boring) as the temperature lowers
69
New cards
what is the bowens reaction series divided into?
3 parts, the discontinuous side, the continuous side and the lower temperature minerals
70
New cards
what mineral is on the continuous side of the bowens reaction series?
plagioclase feldspar, it begins rich in calcium and further down in the cooler area it is sodium rich
71
New cards
what is the discontinuous series of the bowens reaction series?
different minerals with different structural forms
72
New cards
what is the continuous series of the bowens reaction series?
the same mineral but enriched with different elements
73
New cards
what happens to the mineral ratio of elements in olivine, then pyroxene then amphibole then biotite (the discontinuous bowens reaction series)
they increase in silica and oxygen and have less iron and magnesium so they become more silicic and less mafic in composition
74
New cards
what are the mafic elements in the mafic minerals?
iron, magnesium, calcium
75
New cards
what are the silicic elements found in the silicic minerals?
sodium, potassium, silicon dioxide
76
New cards
which of the minerals in the bowens reaction series would weather the fastest?
olivine weathers the fastest as it is the most unstable at the lower temperatures at the surface.
77
New cards
which of the minerals in the bowens reaction series would weather the slowest?
quartz as it is a low temperature mineral that forms at conditions closest to the surface so it is the most stable and resistant to weathering. it also has a hardness of 7 and no cleavages as it has no cations in its framework, increasing its resistance to weathering
78
New cards
what are the 3 minerals formed at the bottom of the bowens reaction series at the lowest temperatures?
orthoclase K feldspar, muscovite mica, quartz
79
New cards
what is zoning?
the crystal of plagioclase feldspar begins to form at high temperature where calcium rich plagioclase is more stable. as the magma then cools the temperature lowers and the crystal continues that growth but with increasing sodium richness as it is more stable at the lower temperature. this means that there is calcium rich at the centre and sodium rich on the outside of the plagioclase feldspar crystal
80
New cards
what is a eutectic system?
when crystallisation occurs in both the discontinuous and the continuous but the minerals that crystallise out are immiscible (they cannot mix) so they form 2 solid phases
81
New cards
what happens in a phase diagram?
the melt cools as it reaches the liquidus line where the crystals of the higher ratio in the composition will crystalise out at 100% of that mineral. this depletes the magma in that mineral and enriches it with the other. the line will then migrate down the liquidus line until the eutectic point is reached. here the 2 crystal types will crystalise out in that ratio e.g 40:60 due to the lower temperature. this continues till all of the crystals have formed and the original composition ratio is reached. it will return from the addition of the earlier formed crystals of the higher ratio mineral
82
New cards
what is ophitic texture?
random crystals stacked on top of each other
83
New cards
what happens in a continuous phase diagram?
the melt reaches the liquidus line and crystals will begin to form, they will be richer in one element e.g calcium. as these crystals form that calcium is removed from the melt and it will become richer in another element e.g sodium. so the composition migrates down the liquidus towards the sodium richer end of the diagram. then the plagioclase crystals react with the melt and Ca is substituted for Na as the temperature decreases. as the last crystal forms will have the same composition as the original melt
what processes cause mafic magma to form all types of magma?
fractional crystallisation, gravity settling, filter pressing, assimilation and magma mixing
86
New cards
what is fractional crystallisation and how does it differentiate magma?
as the mafic magma cools the higher temperature minerals crystallise out e.g olivine and pyroxene. these minerals are rich in elements like iron and magnesium. higher temperature calcium plagioclase also forms. so the magma is depleted in calcium, iron and magnesium and is enriched in silica, potassium, sodium as earlier formed minerals have less of these elements. over time this causes the magma to become more silicic
87
New cards
what is gravity settling and how does it differentiate magma?
these higher temperature minerals olivine and pyroxene that contain high proportions of iron and magnesium have a higher density than the surrounding melt. so they then sink to the base of the intrusion and form a cummulate layer. this depletes the magma of magnesium, iron and calcium and enriches it with, K, Na and SiO2. this makes the magma more silicic in composition
88
New cards
what is filter pressing and how does it differentiate magma?
during the process of crystallisation of magma, the crystals and liquid exist as a slush. as a result of the overlying weight of the crystals the liquid is squeezed out forming a layer. this liquid is then depleted in elements of iron and magnesium (which form mafic minerals that are the crystals at the base at higher temperature) the composition is then enriched in the lower temperature mineral elements like Na, K and silicon dioxide
89
New cards
what is assimilation and how does it change magma composition?
as the magma expands through the country rock by assimilation, the mafic magma has a low viscosity and a high temperature. it increases in size and applies pressure to the surrounding country rock. it fractures forming xenoliths. they are assimilated into the magma. it alters the mafic parent magma to a more intermediate daughter magma. the mafic magma then sinks to the base of the intrusion and the silicic magma cools at the top of the intrusion
90
New cards
what is the process of magma moving through the country rock called?
stoping
91
New cards
how does magma mixing change the composition?
magmas that have 2 different composition that are normally immiscible can be combined in a magma chamber by convection currents to form a magma that has a intermediate composition of the 2 parent magmas
92
New cards
what are the PGE elements?
platinum group elements, platinum, palladium, iridium, osmium, rhodium, ruthenium. these economically mineable deposits are rare.
93
New cards
how do PGE elements form?
sulfur forms immiscible blobs that come out of solution. magma is then saturated in S2- ions. they sink and combine with chalcophiles to form sulfide ore minerals e.g CuFeS. in the absence of a siderophile, PGE elements will join with sulfur by acting like chalcophiles. they sink and form cummulate layers
94
New cards
what are the 3 main features of the pallisade sill?
- gabbro at the centre of the intrusion as the cooling rate was slowest so it has coarse crystals. it has a slightly lower amount of mafic elements (iron and magnesium) - the chilled margins cooled rapidly due to contact with the cold country rock. this is fine basalt and has the original composition of the intrusion as it cooled prior to any differentiation - there is also an olivine cummulate layer forms by gravity settling, magma becomes more silicic in composition
95
New cards
what is the Hekla volcano and what are its features?
- a large volcano in iceland - it is over a divergent plate boundary so it should form mafic magma but it forms intermediate and sometimes silicic. - as if there is a long interval between eruptions, fractionation causes more mafic minerals to crystallise out at the higher temperatures and settle at the bottom of the magma chamber. this forms a intermediate magma, the longer this process occurs the more silicic the magma composition
96
New cards
what are the features of the Skaergaard intrusion?
- formed when the north atlantic was opening by a single injection of magma - the marginal border series is a chilled margin with a fine crystal size so it cooled rapidly. but its not the same as the original composition as it was contaminated by the country rock - the lower zone is ultramafic with high proportion of olivine and pyroxene - the layered series has layers of high density olivine and pyroxene and lighter ca rich plagioclase deposited by gravity settling. then filter pressing causes the expulsion of differentiated liquid. the convection currents mix the magma and the process continues. over time the magma evolves and becomes more silicic further up as it is depleted in elements like magnesium and iron and cummulates form
97
New cards
how is magma formed during mountain building?
when two continental plates collide together and neither subducts. high pressure and overlying sediments push the base down. the base then melts partially and forms silicic magmas (granite intrusions)
98
New cards
what are ring dykes?
when the roof of the magma chamber collapses when the pressure decreases of magma in the underlying magma chamber. magma oozes into the fractures surrounding the collapse. this forms dykes that dip away from the underlying magma chamber
99
New cards
what are cone sheets?
when magma pressure is pushing upwards, it fractures the overlying rock. magma is forced into these cracks and forms cone sheets that dip towards the centre of the magma source
100
New cards
what is a diapir?
how magma moves through the crust. it is a lower density body of material that pierces through and rises through the overlying higher density material. it occupies a larger volume than when solid