What is the difference between clastic and chemical sedimentary rocks

Why do sedimentary rocks have layers? What happens when limestone comes in contact with an acid? Why are sedimentary rocks the only rock type to contain fossils? Why are sedimentary rocks stratified? Why are sedimentary rocks important?

What are some examples of sedimentary rocks that react with acid? See all questions in Sedimentary Rocks. Impact of this question views around the world. Mineral collections and instructive books are also available. The specimen in the photo is about two inches five centimeters across.

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Diamonds: Learn about the properties of diamond, its many uses, and diamond discoveries. Calcite can also form on land in a number of environments. Tufa forms at springs Figure 6. Similar material precipitates within limestone caves to form stalactites , stalagmites, and a wide range of other speleothems.

Dolomite CaMg CO 3 2 is another carbonate mineral, but dolomite is also the name for a rock composed of the mineral dolomite although some geologists use the term dolostone to avoid confusion. All of the dolomite found in ancient rocks has been formed through magnesium replacing some of the calcium in the calcite in carbonate muds and sands.

This process is known as dolomitization , and it is thought to take place where magnesium-rich water percolates through the sediments in carbonate tidal flat environments. In some cases, chert is deposited along with limestone in the moderately deep ocean, but the two tend to remain separate, so chert beds within limestone are quite common Figure 6.

In other situations, and especially in very deep water, chert accumulates on its own, commonly in thin beds. Some ancient chert beds — most dating to between and Ma — are also combined with a rock known as banded iron formation BIF , a deep sea-floor deposit of iron oxide that is a common ore of iron Figure 6. BIF forms when iron dissolved in seawater is oxidized, becomes insoluble, and sinks to the bottom in the same way that silica tests do to form chert.

The prevalence of BIF in rocks dating from to Ma is due to the changes in the atmosphere and oceans that took place over that time period. Photosynthetic bacteria i. Clastic or detrital sedimentary rocks are made from pieces of bedrock, sediment, derived primarily by mechanical weathering.

Clastic rocks may also include chemically weathered sediment. Clastic rocks are classified by grain shape , grain size , and sorting. Chemical sedimentary rocks are precipitated from water saturated with dissolved minerals. Chemical rocks are classified mainly by the composition of minerals in the rock. Lithification turns loose sediment grains, created by weathering and transported by erosion, into clastic sedimentary rock via three interconnected steps.

Deposition happens when friction and gravity overcome the forces driving sediment transport, allowing sediment to accumulate. Compaction occurs when material continues to accumulate on top of the sediment layer, squeezing the grains together and driving out the water. The mechanical compaction is aided by weak attractive forces between the smaller grains of sediment.

Groundwater typically carries cementing agents into the sediment. These minerals, such as calcite, amorphous silica, or oxides, may have a different composition than the sediment grains. Cementation is the process of cementing minerals coating the sediment grains and gluing them together into a fused rock. Diagenesis is an accompanying process of lithification and is a low-temperature form of rock metamorphism see Chapter 6 , Metamorphic Rock.

During diagenesis, sediments are chemically altered by heat and pressure. A classic example is aragonite CaCO 3 , a form of calcium carbonate that makes up most organic shells. When lithified aragonite undergoes diagenesis, the aragonite reverts to calcite CaCO 3 , which has the same chemical formula but a different crystalline structure. In sedimentary rock containing calcite and magnesium Mg , diagenesis may transform the two minerals into dolomite CaMg CO 3 2.

Diagenesis may also reduce the pore space, or open volume, between sedimentary rock grains. The processes of cementation, compaction, and ultimately lithification occur within the realm of diagenesis, which includes the processes that turn organic material into fossils. Detrital or clastic sedimentary rocks consist of preexisting sediment pieces that come from weathered bedrock.

Most of this is mechanically weathered sediment, although some clasts may be pieces of chemical rocks. This creates some overlap between the two categories, since clastic sedimentary rocks may include chemical sediments. Detrital or clastic rocks are classified and named based on their grain size. Detrital rock is classified according to sediment grain size , which is graded from large to small on the Wentworth scale see figure.

Grain size is the average diameter of sediment fragments in sediment or rock. Grain sizes are delineated using a logbase-2 scale [ 9 ; 10 ]. For example, the grain sizes in the pebble class are 2. These include boulders, cobbles, granules, and gravel. Sand has a grain size between 2 mm and 0. Sediment grains smaller than sand are called silt. Silt is unique; the grains can be felt with a finger or as grit between your teeth, but are too small to see with the naked eye.

Sorting describes the range of grain sizes within sediment or sedimentary rock. It is important to note that soil engineers use similar terms with opposite definitions; well-graded sediment consists of a variety of grain sizes, and poorly graded sediment has roughly the same grain sizes. When reading the story told by rocks, geologists use sorting to interpret erosion or transport processes, as well as deposition energy. For example, wind-blown sands are typically extremely well sorted, while glacial deposits are typically poorly sorted.

These characteristics help identify the type of erosion process that occurred. Coarse-grained sediment and poorly sorted rocks are usually found nearer to the source of sediment, while fine sediments are carried farther away. In a rapidly flowing mountain stream, you would expect to see boulders and pebbles.

In a lake fed by the stream, there should be sand and silt deposits. If you also find large boulders in the lake, this may indicate the involvement of another sediment transport process, such as rockfall caused by ice- or root-wedging. Rounding is created when angular corners of rock fragments are removed from a piece of sediment due to abrasion during transport.

Well-rounded sediment grains are defined as being free of all sharp edges. Very angular sediment retains the sharp corners. More rounded grains imply a longer erosion time or transport distance, or more energetic erosional process.

Mineral hardness is also a factor in rounding. Composition describes the mineral components found in sediment or sedimentary rock and may be influenced by local geology, like source rock and hydrology.

Other than clay, most sediment components are easily determined by visual inspection see Chapter 3 , Minerals.