- While approximately 95% of Earth's crust is igneous rock material, sedimentary
rocks comprise approximately 75% of the Earth's exposed crust. They
are a thin veneer on Earth's surface but they are not the dominant rock type
at depth.
- The majority of Sedimentary Rocks form underwater in oceans, estuaries,
rivers, lakes and inland seas.
- Lithification of loose sediments into solid rock is a process that varies
in time. It may take tens to hundreds of thousands of years up to many millions
of years for rock formation. A lot depends upon the burial depth of the sediments,
how much compaction is taking place and whether or not minerals that will
act as a natural cement are present in solution in circulating water bodies
or groundwater.
- Some Clastic Rocks are formed on land and not in a water environment. For
example, in giant sand dunes wind blown sand grains may compact over time
to form sandstone.
- Glacial Till can become Sedimentary Tillite with enough compaction caused
by overlying layers of sediment.
- Most Sedimentary Rocks are Clastic in that they are composed
of rock fragments weathered from larger rocks.
- Bioclastic Sedimentary Rocks are composed of the remains of once
living organisms such as shells and corals.
- Clastic and Bioclastic Sedimentary Rocks are formed by the processes of
Compaction and Cementation.
- Evaporation and Precipitation are processes that form Chemical
Sedimentary Rocks and they are lithification processes specific to a water
environment.
- Sedimentary Rocks that today are visible on Earth's surface most likely
formed underwater millions of years ago only to be exposed at Earth's surface
by uplifting forces, weathering and erosion or the regression of seas (lowering
of Sea Level).
- One area of the world where Sedimentary Rocks are presently forming is in
the area of the Mississippi River Bird Foot Delta in the Gulf of Mexico.
