Rocks and Sediment
Rock Types: Igneous (cool from magma), Metamorphic (change through elevated pressures and
temperatures) and Sedimentary (particles eroded and transported, or formed from precipitation
out of seawater)
Sedimentary Texture-mainly important in CLASTIC sediment-transported particles
Roundness, grainsize, sorting
Grainsize depends mainly on velocity of the water transporting the sediment
Faster water = larger grainsize
Grainsize is usually measured in loose sediment with sieves.
WENTWORTH scale defines the sizes on a logarithmic scale due to the hydrodynamic
properties of grains
Gravel (boulders, cobbles, pebbles, granules) is larger than 2 mm diameter
Sand ranges from 2 mm to 1/16 mm in diameter
Mud (silt and clay) is finer than 1/16 mm
Sorting is a measure of the variation of grainsize. Certain environments tend to better
sort sediment than others.
Roundness is a function of how long the particles have been in transport, more rounding
with more time.
Composition depends on two things:
original source material (e.g. Hawaii has a lot of basalt, so we would expect basalt
sand and not quartz sand)
amount of weathering. Quartz is by far the most chemically stable rock-forming
mineral, so as the sediment weathers, the % of quartz increases as other minerals weather
away.
Overall description of the changes that take place with sediment is called the MATURITY of the
sediment. Most mature is sand sized particles that are round and made of quartz.
Carbonate sediment has a different set of rules as it is mainly formed by the biologic processes of
calcite secreting shells. Grainsize and shape are much more dependent on the original shape than
on any sort of weathering/rounding process. Carbonate sediment is usually found in areas away
from clastic sediment influx.