All rocks at the earth's surface disintegrate slowly by weathering. Solid rock decomposes to sediment: gravel, sand, and clay. Rain, streams, winds, glaciers, and gravity erode this sediment, carry it off and deposit it elsewhere. Eventually the loose particles become cemented to form sedimentary rock by the process of lithification.
-Clastic Sedimentary Rocks-
Formed from fragments of other rocks.
(Clastic = particles)
Course Clastic- contains large particles of sediment
Conglomerate- Type of Clastic Sedimentary rock.
Medium Clastic- contains intermediate particles of sediment
Sandstone is a clastic sedimentary rock that has contain small specks of sediment and is rough to the touch.
Fine Clastic- contains fine or small particles of sediment
Shale is a fine clastic sedimentary rock that smells of mud when wet.
-Chemical Sedimentary Rocks-
Contains usally one important mineral that has precipated out from a solution.
Chemical sedimentary rocks are formed from the precipitation from water.
Water can dissolve minerals: calcite, gypsum, halite and quartz.
When the water evaporates the mineral in solution crystallizes and precipitates to form limesotne, gypsum, rock salt and chert respectively.
Ex. Halite (table salt)
Ex. Chert (pure quartz)
-> cher was used by cavemen as sharp tools.
-Organic Sedimentary Rocks-
Remains of Animals and Plants.
form by lithification(the conversion of loose sediment to solid rock) of organic sediment.
Limestone may form form the precipitation of calcite form water, but the calcite comes from shelled animals and reef building animals.
Ex. Coal(is buned for heat -> to make elctricity)
Coal comes from the fossilized remains left from ancient plants.
(Shell) Limestone comes from the fossilized remains of shelled creatures.
Sorting Sediment
Sediments are sorted by rivers and wind. Fast moving river can carry more and larger particles because it has higher energy, as the speed of the river slows down (due to varying steepness),large particles are deposited first. There are two terms that are related to sorting sediment.
well-sorted refers to sediment clasts/particles that are approximately the same size.
poorly-sorted refers to sediment clasts/particles that are many different sizes.
Sediments left behind from rivers and wind can give a lot of information! By observing the direction of the cemented sediments due to the ripples of the river, one can find out such things as the direction of the river, speed of river flow etc.
Quartz grains become rounded as they bump and grind past each other.
Well-sorted:refers to sediment/clasts particles that are apporiximately the same size.
Poorly-sorted: sediment/clasts particles of many different sizes.
CLASTIC SEDIMENTARY ROCKS
- Sedimentary rocks made from clasts.
- Clastic sediment is a mixture of fragments of previously existing rocks.
Sedimentary structures
Nearly all sedimentary rocks contain sedimentary structures, features that develop during of shortly after deposition of the sediment.
Here is a list of different Sedimentary Structures.
Bedding:layering that develops as sediment is deposited. Bedding occurs because sediments is almost always deposited in layer by layer process
Ripples:
- Symmetrical Ripples: Water flowing over sand creates parallel ripples and is preserved when the sand lithifies; water flowed in both directions.
- Asymmetrical Ripples: Formed when a river flowed in only one direction.
Crossbedding: an arrangement of small beds lying at an angle to the main sedimentary layering.
Mud cracks: polygonal cracks that forms when mud shrinks as it dries.
Fossils: remains of plants and animals.
Comments (0)
You don't have permission to comment on this page.