G1 and 2 Flashcards
(11 cards)
magma mixing
magmas of two different compositions tend to be immiscible
various factors such as the density and thermal contrasts act to keep them separated
enclaves
an enclave is a blob or fragment of one type of magma or rock enclosed within another
often formed when two magmas of different compositions mix, but don’t completely blend
rounded edges, whereas a xenolith is a fragment of another rock, can have sharp, jagged edges
igneous at origin, whereas xenoliths can vary
contamination
stoping is the process where blocks of country rock are broken off by rising magma and incorporated into the magma as xenoliths
these xenoliths melt and become assimilated into the magma, they can contaminate and change its bulk composition
fractional crystalisation
as olivine and pyroxene form at high temperatures, they use iron and magnesium from the magma in their crystal lattices
high temp plagioclase crystals are rich in Ca
results in the magma becoming depleted in Fe, Mg and Ca
remaining liquid becomes rich in silica, K, Na and water because the early formed minerals are poor of these elements
changing composition of magma over time as it cools
gravity settling
crystals will be denser than the liquid magma and settle out
early formed minerals such as olivine and pyroxene, have a greater percentage of iron and are significantly denser, so sink to form a layer at the base of the intrusion or magma chamber, called a cumulate layer
the crystals that remain suspended in magma will react with that magma over time, following the continuous or discontinuous paths of Bowens reaction series
however, gravity settling removes crystals from the remaining liquid so they no longer react with the remaining magma, changing its original composition
filter pressing
during the crystallisation of magma, there is a point where crystals and liquid exist together as a slushy mass
due to the weight of the overlying crystals, the liquid gets squeezed out, forming a separate layer above
this liquid is depleted in the elements which are in early formed minerals and enriched in the elements which form felsic minerals (feldspar and silica)
turbidity currents
occur when an earthquake destabilises marine sediments which then flow downslope under the influence of gravity
can reach speeds of over 100km/hr and can be
extremely erosive at the head of the flow
as the flow moves down the continental rise/slope it travels at a higher velocity, but upon reaching the abyssal plain it will experience rapid deceleration
sediment will settle out in stages according to settling velocities of different sized particles
resultant deposits are called a turbidite typically exhibiting graded bedding in a sequence known as a bouma sequence
turbidity evidence
graphic logs
A - graded bedding, gravel and mud balls at the base, erosive base
B - coarse -grained planar bedded sands
C - fine sands, ripple marks (asymmetrical)
D - laminated silts and planar bedded fine sands
E - laminated muds, suspension deposits
Walther’s law
rocks that are deposited side by side today, will occur on top of one another in the rock record without any gaps/ unconformities
coastal environments
shallow silicic seas and beaches along the coast migrate laterally during marine transgressions (rise) and regressions (fall)
the former positions of the coastal environments can be seen in the vertical sequence of facies
diachronous
have the same lithology but vary in age along their extent
the point at which they cut across
