Geology Reference
In-Depth Information
Figure 7-8:
Poorly
sorted and well-
sorted sediments.
The distance that sediment travels and its mode of transportation leave vis-
ible changes in the particles, sometimes rounding them or scratching them. These
changes, along with the size of the particles, give a sedimentary rock its texture
and grain size. These are the characteristics geologists use to classify and name
sedimentary rocks.
After the sediments are transported and
deposited
(left in place) for a long period of
time, they go through various processes of
diagenesis,
or change.
The first stage of diagenesis is burial of the sediments. When the sediment is buried,
things start to heat up, and the pressure is increased. These conditions lead to
lithifica-
tion:
the process that turns sediments into rock. It starts with
compaction,
where the
pressure and weight of overlying sediments presses down, squeezing any extra space
out of the layers of sediment.
Following compaction is
cementation,
or gluing together the sediments into solid rock.
The individual grains are cemented together when water moves through the sediments,
carrying dissolved minerals. These minerals fill the tiny spaces between particles and
build crystals linking the grains together. Most sedimentary rocks are cemented by cal-
cite, silica, or iron oxide. (See Chapter 6 for details on minerals.)
Sedimentary rock-forming diagenesis occurs at relatively low temperatures
(about 150 to 200 degrees Celsius) close to the earth's surface. When temps are
