Ways that volume changes occur within fissures
1) frost weathering (H2O -> ice)
2) precipitate salts in fissures
3) biological (tree roots)
Types of physical weathering
1) pressure release
burial = pressure
then uplift = expand and break along planes of weakness
2) Insolation weathering
thermal expansion and contraction -> differential expansion between surface and middle creates stress
3) Hydration weathering
water absorbed and released by clay minerals, swelling and shrinking of rock
4) volume changes within fissures
methods of chemical weathering
1) acid hydrolysis
carbonic acid reacts with primary mineral to replace metal cation with H+ ion, forming hydrous mineral
e.g. acid hydrolysis of silicate minerals
carbonic aid + feldspar = hydrated aluminosilicate (+ metal ion + bicarbonate)
2) oxidation
metals lose e- and rust
methods of biochemical weathering
plant roots and bacteria produce organic acids which break down rocks
e.g. fungi penetrate and take nutrients directly from mineral
how does goldich’s weathering series relate to Bowens reaction series
as you do down Bowens reaction series, minerals become less susceptible to weathering, as were formed/are stable at conditions closer to those at the surface
4 categories of sediments/sedimentary rocks
1) clastic deposits
- particles from pre-existing rocks
2) biogenic/organic deposits
- resulting from biological materials/biologially mediated processes
3) chemical deposits
- precipitates from solution
4) volcaniclastic deposits
- products of volcanic eruption of breakdown of volcanic rocks
examples of each of the 4 types of sediments/sedimentary rocks
1) clastic depostis - breccia, sandstone, mudstone
2) biogenic - limestone, coal, CaCO3
3) chemical deposits - evaporite, BIF
4) volcaniclastic - tuff
difference between a matrix and a cement
matrix = fine grained granular
cement = precipitated out of fluid after deposition, crystalline
physical descriptors of clastic sediments
- grain size + size distribution
- grain morphology
- grain packing (fabric)
what factors effect grain size distribution
- nature of sediment source
- efficiency of transport medium (only carry specific sizes, wind, water, glacier)
- energy in deposition
what are patterns of grain morphology with distance transported
- roundness = increase rapidly
- size = slow decrease
- sphericity = not much change
define textural maturity
immature = high matrix proportion, poorly sorted, angular grains
mature = less matrix, well sorted, rounded grains
high maturity = porous + permeable
what makes up clastic rocks
quartz
feldspars
lithic fragments
micas + clay minerals
heavy minerals (accessory grains)
other detrital (fossils)
characteristics of quartz in clastic rocks
- v common, hard, no cleavage
- derived from granite + gneiss
- can be monocrystalline or polycrystalline
characteristics of feldspars important in clastic rocks
- common, less hard than quartz, cleaved
- chemically liable to be replaced by clay minerals = indicator of weathering intensity + environment
characteristics of lithic fragments important in clastic rocks
- type depends on geology of source and durability
- key in breccia, coarse sandstones
what is the pettijohn classification of sandstones
ternary diagram showing the relative proportion of quartz, feldspars, and lithic fragments in a rock
what is the importance of micas and clay minerals in clastic rocks
- common in matrix
- clay minerals small and fine grained
- muscovite > biotite (stability)
- matrix creates another scale on pettijohn classification (amount of matrix)
importance and characteristics of heavy minerals in clastic rocks
- density > 2.85 g/cm^3
- stable oxides and silicates
- evidence for origin
e.g. garnet = met, apatite = igneous
how does composition and texture evolve with maturity
composition:
unstable minerals (feldspar) -> stable + LFs -> entirely quartz
texture:
matrix, poor sorting, angular
-> decreasing matrix, increasingly sorted, increasingly rounded
difference between conglomerates and breccia
conglomerate = rounded clasts
breccia = angular clases
what is diagenesis
physical and chemical changes that alter the characteristics of a sediment after deposition
unconsolidated sediment -> consolidated rock
how does packing change during diagenesis, and example with different grains
increase pressure by overlying sediments -> change packing to increase density -> decrease porosity
fluid expelled and volume is reduced
muds = very compressible due to layered structure
sand = compacted much less
when are sutured contacts between grains seen
when grains fuse together due to temperature increase during diagenesis
