F2 key idea 2 (igneous)

Question 1

primordial heat

Answer 1

leftover heat from the earth’s formation
Accretion (material colliding to form the earth)
core formation (gravitational potential) / differentiation into layers

Question 2

radiogenic heat

Answer 2

produced by radioactive decay of isotopes
found mainly in the crust and mantle.

as unstable parent atoms change into unstable daughter atoms, thermal energy is transferred into the surrounding earth

Question 3

convection

Answer 3

the underlying mantle is moving upward, at mid-ocean ridges and hot spots

process by which thermal energy is transferred by substances due to buoyancy differences
hot material expands, density reduces and the material rises
while colder, denser material sinks under the influence of gravity

Question 4

conduction

Answer 4

process by which thermal energy is transferred through a substance with no overall movement of that substance.
transferred atom to atom down a thermal gradient

Question 5

plutons

Answer 5

large igneous intrusive bodies.
if they are greater than 100km2 then they’re called batholiths, usually made of several plutons

Question 6

batholith

Answer 6

large igneous intrusion which may be a collection of plutons

Question 7

dykes

Answer 7

are discordant and often vertical or near vertical intrusions
intruded along zones of weakness in the country rock, such as faults or joints

Question 8

sills

Answer 8

are concordant (parallel) to the beds
some occasionally cut across the beds in steps from one to the other, to form a transgressive sill
often forms at more shallow depths

cooled intrusively, slower than sills so form coarser crystals

two baked and chilled margins, above and below

rips up pieces of country rock from above and below (xenoliths)

will not be weathered due to being protected by the surrounding country rock

Question 9

lava flows

Answer 9

cool at the surface so will cool rapidly, having much finer crystals than sills

along contact with the country rock, baked and chilled margin at base

rips up pieces of country rock from below (xenoliths)

weathering and erosion of upper surface due to being exposed to earths processes

Question 10

hypabyssal
plutonic

Answer 10

hypabyssal - when igneous rocks form at relatively shallow depths from the surface

plutonic - when igneous rocks form deep below the surface

Question 11

glassy texture

Answer 11

means there are no crystals and the rock resembles a block of glass
as a result of very rapid cooling where crystals had no time to form

Question 12

equicrystalline texture

Answer 12

all the crystals are the same size but may be fine, medium or coarse
the larger the crystal size, the longer the time of cooling

fine - extrusive
medium - hypabyssal
coarse - plutonic
both hey-abyssal and plutonic = intrusive

Question 13

vesicular texture

Answer 13

gas bubbles are trapped in lava as it cools rapidly, leaving holes where the gas was present
the vesicles (holes) are usually oval or ellipsoid in shape and elongated parallel to the direction of flow
common is basalt and pumice

Question 14

amygdaloidal texture

Answer 14

vesicles are later infilled by minerals deposited from percolating groundwater (calcite and quartz)

each infilled hole is called an amygdale
larger ones, partially infilled with crystals growing towards the centre are called geodes (most common in basalt)

Question 15

flow banding texture

Answer 15

occurs where layers of dark and light minerals form due to the separation of minerals within a lava flow, aligns parallel to the flow direction

Question 16

porphyritic texture

Answer 16

forms when a rock has two stages of cooling, resulting in two distinct sizes of crystals

large crystals - phenocrysts forming first by cooling slowly
surrounded by a finer groundmass, which cooled more quickly

Question 17

euhedral crystal shape

Answer 17

crystals are well formed with good crystal faces, uninterrupted by the growth of neighbouring crystals

as a result of very slow cooling, in deep, plutonic intrusions

Question 18

subhedral crystal shape

Answer 18

crystals have some well-formed and some poorly-formed faces, as magma cools the growing crystals touch each other

Question 19

anhedral

Answer 19

crystals show poorly-formed faces
indicates hypabyssal intrusion cooling as the emergent crystals tend to crowd each other and no single crystal can evolve as euhedral

Question 20

pillow structure

Answer 20

formed by submarine eruptions of basalt lava
the exterior chills and crystallises rapidly against cold sea water

outside layer will have glassy texture and vesicles trapped below surface

Question 21

Aa lava

Answer 21

flows with a rough blocky, jagged surface

Question 22

pahoehoe lava

Answer 22

flows with smooth or ropy surface
less viscous than Aa and as the lava Colls, becomes even more viscous, so that pahoehoe lava flows may become Aa lava flows

Question 23

columnar jointing

Answer 23

forms when lava flows cool slowly, associated with thicker flows.
as lava cools, discrete cooling centres develop and the rock contracts towards these centres, producing a polygonal pattern of vertical joints.

Question 24

magma at divergent plate margins

Answer 24

plates are pulled apart
as it stretches and thins, the asthenosphere upwells closer the the surface and pressure is reduced

decompression of the ultramafic peridotite causes partial melting, producing mafic magmas (e.g. basalts, mostly erupting as pillow lavas)

Question 25

magma at convergent plate boundaries subduction zones

Answer 25

plates collide where plates collide and the oceanic plate is subducted, water in the crust is carried down into the hot mantle water then released into the mantle rock above the plate, lowing the melting point of minerals in the mantle rock resulting in only partial melting (flux melting)

Question 26

magma at oceanic-oceanic plate margins

Answer 26

when the overriding plate is oceanic the erupting magma will be mafic to intermediate in composition, as initially the magma only has thin oceanic crust to rise through (not much material to interact with) if magma rises quickly at shallow depths it will be basaltic in larger, thicker crusts (e.g. island arcs) intermediate magmas can evolve (andesitic in composition)

Question 27

magma at continental- oceanic plate margins

Answer 27

when the overriding plate is continental the magma must rise through thicker silicic crust which may partially melt due to the increasing temperature this rising mafic magma mixes with silicic materials, giving intermediate to silicic magma the two components have different viscosities, so mixing is difficult some will reach the surface as intermediate volcanoes, most will be intruded to form granite batholiths due to differing viscosities

Question 28

magma at continental-continental plate margin

Answer 28

neither continent will be subducted high pressure and mass of sediments (which have been deformed to form fold mountains) combine and force the base of the crust down partial melting at the base produces silicic magma as the magma rises it intrudes to form granite batholiths no volcanoes as the magma is too viscous to rise to the surface

Question 29

magma at hot spots (inside plates)

Answer 29

convection in the mantle slowly transports heat from the core to the earths surface mantle plumes carry the heat upwards in narrow, rising columns which spreads out when the plume head meets the base of the rigid lithosphere lower pressures allow decompression melting of mantle peridotite, in a concentrated zone of asthenosphere, forming enormous volumes of basaltic magma may erupt to form flood basalts

Question 30

viscosity and silica content

Answer 30

mafic magma has low viscosity because the silica content is low consequently, basaltic lavas are more fluid low viscosity allows the gas to escape so eruptions are quiet and non explosive (effusive) intermediate and silicic magmas have a high viscosity because the silica content is high, producing thick lava that moves slowly the high viscosity does not allow gas to escape easy so eruptions are explosive when the gas finally does break through

Question 31

pyroclastic flows

Answer 31

fragmented volcanic rock or ash that is ejected during an explosive eruption ash lapilli - pebble sized bits of lava or pumice volcanic blocks - solid chunks of rock volcanic bombs - material ejected from volcano as liquid globules which solidify in the air and deposited as solid particles

Question 32

lahars

Answer 32

volcanic mudflow - mixture of water, volcanic ash, and debris that rushes down the slopes of a volcano cause by rainfall mixing with loose volcanic ash melting snow or glaciers during eruption lakes or rivers can be generated by pyroclastic flows

Question 33

risk analysis

Answer 33

destroy properties and agricultural land blast damage can be destructive for areas close by, people in the danger zone killed, trees flattened and buildings collapsed ash fall affects areas far from the volcano, causing damage to property as the mass of the ash cause roof collapse

Question 34

distribution of volcanic material

Answer 34

coarse bombs and blocks are dropped close to the vent (found in circular pattern around vent) finer pyroclastic material carried further away, getting finer and thinner with distance rising columns of ash during eruption allow its distribution high into the stratosphere, jet streams then transport ash around the world higher the wind velocity, the further material travels

Question 35

types of volcano

Answer 35

shield volcano - gentle slopes formed by low-viscosity basaltic lava flows stratovolcano (composite) - steep-sided formed from altering layers of lava flows, ash and other debris, typically associated with more viscous intermediate magmas cinder cone - small, steep-sided cones built from pyroclastic fragments ejected during moderately explosive eruptions

Question 36

volcanic monitoring

Answer 36

ground deformation - tilt meters, uplifting or bulging means the magma is rising and building up gravity anomalies - magma is less dense than solid rock so when rising, replaces denser rock, decreasing gravity thermal anomalies - satellites equipped with infrared sensors detect heat emitted from earths surface gas emissions - as magma rises, more gas is released, indicating moving closer to surface seismic activity - magma movement causes earthquakes, forcing way up through cracks and fractures