Hampden-Wilbraham Regional District

TASA The Wonders of Rocks and Minerals
Sedimentary Rocks Script

 How Sedimentary Rocks Form

Recall from the discussion of the rock cycle that sedimentary rocks form when rock particles are carried to a new location where they are deposited and eventually converted into solid rock.

Where steep, rocky slopes occur the process of weathering may produce large quantities of rock fragments that pile up to produce talus slopes.

Erosional agents such as running water, wind, waves, and ice pick up and carry these rock fragments to other locations.

Eventually, these solid particles and dissolved materials, called sediment, are deposited. Photographs of delta, glacial moraine, dunes and beach.

Next, the sediments are converted to rock either by compaction... (Click replay to compaction animation.)

…or by cementation as percolating water coats the sediment with mineral matter, and gradually cements the particles together. Sample: conglomerate

The result is sedimentary rocks such as shale, sandstone, and conglomerate. (Click replay to review types)

Some sedimentary rocks are not compacted or cemented, but form as solid masses of intergrown crystals. Rock salt is one example. (Click “Zoom Out”.)

These are called chemical sedimentary rocks. They usually form when water evaporates and leaves a deposit of chemicals that were dissolved in the water.

 

Conglomerate, Sandstone, and Shale

 Sedimentary rocks form as layer upon layer of sediment accumulates in horizontal layers.

These strata or beds are the single most characteristic feature of sedimentary rocks.

The sizes of particles in sedimentary rock varies greatly and can tell us much about the environment in which the rock was formed. Photo: glacial deposits and sand dunes

The angular pieces in breccia show little wear which indicate the particles were not carried very far from their source before they were deposited. Click “zoom in” and “zoom out” to view magnification

Currents of water and air sort particles by size. The stronger the current, the larger the particle size carried.

For example, in very turbulent water only large gravel-sized particles can be deposited, while smaller grains of sediment will continue to be carried downstream

The sedimentary rock conglomerate is made from hardened gravel-sized particles. The particles become rounded as they are carried by flowing water. Click “zoom in” and “zoom out” to view magnification

Wind and waves carry and deposit sand particles…

…where they may becomes the sedimentary rock sandstone. Quartz is the main mineral found in sandstone, which makes it a very durable rock. Click “zoom in” and “zoom out” to view magnification

The tiniest silt and clay-sized particles are deposited as mud in the quiet waters of a swamp, lake, or lagoon.

The sedimentary rock shale is formed from tiny particles of silt and clay. Shale sometimes contains fossils of plants and animals. Click “zoom in” and “zoom out” to view magnification

Because shale crumbles rather easily, it forms gentler slopes of weathered debris. By comparison, outcrops of more durable rocks like sandstone produce bold, steep cliffs.

Which of these sedimentary rocks was formed from a swampy environment? Choose: shale (Correct, this rock formed from muddy sediments and contains a fossil of a plant that once lived in or near a swamp or lake.) or sandstone (This rock formed from compacted sand particles in a beach or sand dune environment. Try again.)

Which of these sedimentary rocks contains particles that were carried farthest form their source? Click on the correct answer. Choose: conglomerate (Correct, these gravel-sized particles became rounded as they were carried from their source by streams.) or Breccia These angular particles suggest that this rock formed near the source of its sediments. Try again.

Which of these rocks formed from sediment with the smallest particles? Click on the correct answer. Choose: Shale (Correct, shale is formed from the sediment mud, which is made of the smallest silt and clay particles.) or Sandstone (Sandstone is formed from the sediment sand. Its particles are several times larger than silt or clay. Try again.)

 

Chemical Sedimentary Rocks

Some sedimentary rocks form from material dissolved in water. These are called chemical sedimentary rocks.

Limestone , the most abundant chemical sedimentary rock, is composed chiefly of the mineral calcite.

Most limestone has a biochemical origin. Examples include coquina, which consists of shell fragments from sea animals that use the calcite dissolved in seawater to build their shells. Click “zoom in” and “zoom out” to view magnification

Chalk is a form of limestone made of the hard parts of microscopic marine organisms. Click “zoom in” and “zoom out” to view magnification

Some limestones originated as coral reefs. Caption: El Capitan peak in Texas originated as a reef. Click “zoom in” and “zoom out” to view magnification

Coral reefs are made up of huge colonies of tiny water-dwelling organisms whose skeletons are composed of calcium carbonate. Caption: Shells and coral are examples of biochemical sediment.

A limestone with an inorganic origin is travertine, which is often seen decorating caves. Click “zoom in” and “zoom out” to view magnification

Thick evaporite deposits may form when seawater evaporates or, on a smaller scale, in desert basins.

These deposits include the mineral halite (sodium chloride, NaCl), the main ingredient of rock slat, another inorganic sedimentary rock. Click “zoom in” and “zoom out” to view magnification

Which or these two types of limestone has an inorganic origin? Click on the correct answer. Coquina (This limestone has a biochemical origin. It consists or shell fragments. Try again.) or Travertine (Correct, this limestone is formed in caves by the precipitation of calcite dissolved in water.)

Which of these rocks is composed mostly of the mineral calcite? Click on the correct answer. Chalk (Correct, this is a form of limestone made of the hard parts of microscopic marine organisms.) or Rock salt Rock salt is composed mostly of the mineral halite. Try again.)