Sedimentary rock formation
happens from sediments.
It happens wherever
sediments are accumulated in the bottom of water bodies, or deposited
by winds in deserts or moving ice sheets in polar regions.
First, the sediment material is formed from rock by erosion, or from the
shells of dead organisms.
The eroded material is
by wind, water or moving ice.
All three carriers do the same - they
sort the eroded material by grain size. Heaviest particles are dropped
first, while the lightest ones travel furthest in wind or water stream,
or under moving ice, until they are finally deposited.
Over millions of years, the deposits
accumulate. As new layers are laid on the top,
the older ones get pushed deeper down, where rising pressures and temperatures help to
turn the sediments into rock in a process called lithification. In the
process, water is squeezed out of the rocks, so they dry and
Powdery sediments turn into rock by compaction
only. Sandy sediments, which contain the hard particles of quartz,
to be cemented
togehter by cements such as calcite, clay minerals, iron compounds or
Because of this, and because the sediments are first sorted by grain
size, sedimentary rocks generally tend to have quite homogenous grain size.
The fine grained ones particularly tend to be uniform in grain size,
while larger ones such as conglomerates and breccias have large
particles, but also fine grained cement, because they needed to be
If fossils happen to form and be buried under layers, they will be
embedded in sedimentary rock. This is the only type
that can have fossils. Igneous
rocks never have, while metamorphics
theoretically can but very seldom do in practice.