Sedimentary Rocks

Chemical and Biochemical Sedimentary Rocks 2

About 90 percent of limestones are formed from biochemical sediments. Such sediments are the shells and skeletal remains of organisms that settle to the ocean floor. Another biochemical rock is chalk, the material used to write on a chalkboard.

Chemical and Biochemical Sedimentary Rocks 1

Chemical sedimentary rocks form when dissolved substances precipitate, or separate, from water solution. This precipitation generally occurs when the water evaporates or boils off, leaving a solid product. Examples of this type of chemical rock are some limestones, rock salt, chert, flint, and rock gypsum.

Clastic Sedimentary Rocks

Clastic sedimentary rocks can be grouped according to the size of the sediments in the rocks. When rounded, gravel-size or larger particles make up most of the rock, the rock is called conglomerate. If the particles are angular, the rock is called breccia. Sandstone is the name given to rocks when most of the sediments are sand-size grains. Shale, the most common sedimentary rock, is made of very fine-grained sediment. Siltstone is another fine-grained rock.

Compaction and Cementation

Compaction is a process that squeezes, or compacts, sediments. Compaction is caused by the weight of sediments. During compaction, much of the water in the sediments is driven out.

Features of Some Sedimentary Rocks 3

Fossils, which are the traces or remains of ancient life, are unique to some sedimentary rocks. Fossils can be used to help answer many questions about the rocks that contain them .For example, did the rock form on land or in the ocean? Was the climate hot or cold, rainy or dry? Did the rock form hundreds, thousands, millions, or billions of years ago? Fossils also play a key role in matching up rocks from different places that are the same age.

Clastic Sedimentary Rocks 1

Many different minerals are found in clastic rocks. The most common are the clay minerals and quartz. This is because clay minerals, like those that make up much of shale, are the most abundant products of chemical weathering. Quartz, which is a major mineral in breccia, is a common sedimentary mineral for a different reason. It is very durable and resistant to chemical weathering.

Features of Some Sedimentary Rocks 2

Ripple marks may indicate that the rock formed along a beach or stream bed. Mud cracks form when wet mud or clay dried and shrank, leaving a rock record of a dry environment.

Features of Some Sedimentary Rocks 1

Sedimentary rocks, like other types of rocks, are used to unravel what may have happened in Earth's long history. The many unique features of sedimentary rocks are clues to how, when, and where the rocks formed. Each layer of a sedimentary rock, for example, records a period of sediment deposition. In undisturbed rocks, the oldest layers are found at the bottom. The youngest layers are found at the top of the rocks.

Sediments 1

Sediments are deposited according to size. The largest sediments, such as the rounded pebbles in conglomerate, are deposited first. Smaller sediments, like the pieces of sand that make up sandstone, are dropped later. Some sediments are so small that they are carried great distances before being deposited.

Clastic Sedimentary Rocks

The first group includes rocks that are made of weathered bits of rocks and minerals

Chemical Sedimentary Rocks

The second group forms when dissolved minerals precipitate from water solutions.

Formation of Sedimentary Rocks

The word sedimentary comes from the Latin word sedimentum, which means "settling." Sedimentary rocks form when solids settle out of a fluid such as water or air.

What is the product of weathering?

Weathering causes larger rocks to be broken down into smaller particles, or sediments.

Weathering, Erosion, and Deposition

Weathering is any process that breaks rocks into sediments. Weathering is often the first step in the formation of sedimentary rocks. Chemical weathering takes place when the minerals in rocks change into new substances. Weathering also takes place when physical forces break rocks into smaller pieces. Living things, too, can cause chemical and physical weathering.

Deposition

When an agent of erosion—water, wind, ice, or gravity—loses energy, it drops the sediments.

Coal

a chemical sedimentary rock, is mostly made of organic matter

Which feature of a sedimentary rock, if present, would be most likely to help a scientist determine its age?

a fossil contained in the rock

Compaction

a process that squeezes, or compacts, sediments.

Cementation

akes place when dissolved minerals are deposited in the tiny spaces among the sediments. Much of the cement in the conglomerate can be seen with the unaided eye. The cement holding the sand grains together in the sandstone, however, is microscopic.

Coquina, a type of limestone, forms from shell fragments that settle on the ocean floor. What type of rock is coquina?

biochemical sedimentary rock

Sandstone forms from sand-size grains of weathered rocks or minerals. What type of rock is sandstone?

clastic sedimentary rock

Some sedimentary rocks have ripple marks and mud cracks. These features are clues to the _____.

environment where the rocks formed

Sedimentary rocks may form far away from where the sediments formed. What two processes bring sediments to a new location?

erosion and deposition

Erosion

involves weathering and the removal of rock.

From first to last, which is the order of processes that form sedimentary rock?

weathering, erosion, deposition, compaction, cementation