Sedimentary and Igneous Petrology - Molly’s

What is the crystal size for coarse igneous rocks?

\>5mm

What is the crystal size for medium igneous rocks?

5-1mm

What is the crystal size for fine igneous rocks?

Where do coarse crystal igneous rocks form?

intrusive e.g a pluton or batholith

Where do medium crystal igneous rocks form?

hypabassal e.g a dyke or sill

Where do fine crystal igneous rocks form?

extrusive e.g lava flow

Where do glassy igneous rocks form?

extrusive e.g lava flow

Name the coarse silicic rock

granite

Name the coarse intermediate rock

diorite

Name the coarse mafic rock

gabbro

Name the coarse ultramafic rock

peridotite

Name the medium silicic rock

microgranite

Name the medium intermediate rock

micro diorite

Name the medium mafic rock

dolerite

Name the fine silicic rock

rhyolite

Name the fine intermediate rock

andesite

Name the fine mafic rock

basalt

Name the glassy silicic rock

obsidian

What is the silica content in silicic rocks?

\>66%

What is the silica content in intermediate rocks?

66-52%

What is the silica content in mafic rocks?

52-45%

What is the silica content in ultramafic rocks?

What are the silicic minerals?

orthoclase feldspar, plagioclase feldspar, quartz

What rocks are composed of quartz, orthoclase feldspar and biotite mica

granite and rhyolite

What rocks are composed of plagioclase feldspar, amphiboles and pyroxenes?

diorite and andesite

What rocks are composed of plagioclase feldspar, pyroxenes and olivine?

gabbro and basalt

What rock is composed of olivine and pyroxene?

peridotite

What is the temperature where melting begins?

700 degrees celcius

What elements increase when there is a higher silica content? (silicic)

silica, potassium and sodium

What elements increase when there is a lower silica content? (mafic)

magnesium, iron and calcium

define equicrystalline

when magma cools at the same rate so the crystals are the same size

define glassy texture

no visible crystals as magma cooled incredibly fast so the rock resembles glass

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

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

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

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

What is the liquidus?

the boundary showing the temperature where the las solid particle melts

What is the solidius?

the boundary showing temperature where the first particle melts

What is partial melting?

when some of the minerals in a rock melt as they have a lower melting point

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

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

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

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

What crystal size would a lava flow have?

fine as it cools quickly as it is extrusive

What is a batholith?

a large intrusion made from a group of plutons

What crystal size would a batholith have?

coarse as it cools slowly as it is intrusive

What is a dyke?

a discordant hypabassal intrusion that is several m thick

What does discordant mean?

it cuts through the bedding planes

define hypabassal

small intrusion several m thick

What is a sill?

a concordant hypabassal intrusion several m thick

what does concordant mean?

it goes along the bedding planes

What is a transgressive sill?

a sill that follows along the bedding planes but has small parts that cut through the bedding planes

What crystal size would a dyke and sill have?

medium as they are intrusive but they are not very big

What is a chilled margin?

the edge of the intrusion which is cooled by the country rock

What is a baked margin?

the edge of the country rock with is melted by the heat of the intrusion

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

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.

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

define stoping

the process of magma moving through the country rock by fracturing the surrounding rock

define assimilation

the melting of country rock into the magma composition

What is a xenolith?

fragments of pre-existing rock contained within an igneous rock

Define viscosity

the runniness of the magma

What is the parent magma?

the original magma before a composition change

What is the daughter magma?

the new magma after stoping with a changed composition due to the addition of silicic country rocks

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

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

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

what minerals are on the discontinous side of bowens reaction series?

olivine, pyroxene, amphibole, biotite (old people are boring) as the temperature lowers

what is the bowens reaction series divided into?

3 parts, the discontinuous side, the continuous side and the lower temperature minerals

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

what is the discontinuous series of the bowens reaction series?

different minerals with different structural forms

what is the continuous series of the bowens reaction series?

the same mineral but enriched with different elements

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

what are the mafic elements in the mafic minerals?

iron, magnesium, calcium

what are the silicic elements found in the silicic minerals?

sodium, potassium, silicon dioxide

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.

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

what are the 3 minerals formed at the bottom of the bowens reaction series at the lowest temperatures?

orthoclase K feldspar, muscovite mica, quartz

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

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

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

what is ophitic texture?

random crystals stacked on top of each other

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 allow magma to differentiate?

fractional crystallisation, gravity settling, filter pressing

what processes cause mafic magma to form all types of magma?

fractional crystallisation, gravity settling, filter pressing, assimilation and magma mixing

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

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

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

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

what is the process of magma moving through the country rock called?

stoping

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

what are the PGE elements?

platinum group elements, platinum, palladium, iridium, osmium, rhodium, ruthenium. these economically mineable deposits are rare.

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

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

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

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

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)

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

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

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