Sedimentary Rock

Sediment

Sedimentary rock is formed from sediment. Sediment is smaller pieces of rocks or smaller pieces of formerly living things.

Turning rocks, and parts of formerly living things, into sedimentary rock takes a very long time and involves many processes.

The processes include weathering, erosion, deposition, compaction, cementation. Compaction and cementation are also known as lithification.

Let’s break all of this down (see what I did there?).

Weathering

Weathering is what causes material like rock to break down over time. At one time the arch was completely filled in until weathering caused the opening.

Weathering’s root word is weather. Heat,

cold, rain, snow, wind, etc. all help to break

down larger objects into much smaller

pieces. That includes formerly living things

that are exposed to weather over a long

period of time.

Erosion

Smaller particles are much easier to move than larger particles. If weathered material is exposed to wind and/or moving water (liquid or solid) the sediment gets moved from one place to another.

Deposition

When the moving sediment stops moving, it gets deposited (left, put down). That’s called deposition.

Compaction & Cementation

As sediment builds up, so does the pressure. Sediment that piles up on top of other sediment, along with any water, creates a lot of pressure.

Remember that most rocks have minerals within them. That means that sediment has a lot of mineral content. The minerals act as a kind of glue that begins to cement the smaller pieces of sediment together. The glue (cementation) along with the immense pressure (compaction) turns the sediment into layers of rock.

Sedimentary rock layer formation takes a very long time.

Layer formation takes millions of years.

Types of Sedimentary Rocks - Clastic Rock

We focus on 3 categories of sedimentary rocks; clastic, chemical, and organic.

Clastic sedimentary rocks, like the big one in this picture, form when different size rock fragments get lithified. The particles can be very small (too small for you to see individually) or relatively very large (too big for you to lift!).

Examples of clastic rocks include conglomerate (large rounded rock fragments), breccia (large sharp edged rock fragments), shale, and sandstone.

Types of Sedimentary Rocks - Organic Rock

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Organic sedimentary rocks are made up of formerly living material from plants and animals that are weathered, eroded, deposited, compacted and cemented.

Coal and limestone are two very common organic sedimentary rocks. Coal is primarily made from deposited plant material over millions of years.

Coal is still used as a fossil fuel and still mined in Ohio,

but coal is quickly losing ground to natural gas and

renewable energy.

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Limestone is created in oceans when many sea

creatures that have shells and/or skeletons made of

calcite decompose.

Types of Sedimentary Rocks - Chemical Rock

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Chemical rocks form when minerals crystallize from solution. Think about lakes, seas, and underground water that dries up.

Limestone is an example of a chemical rock that forms from solution. Scientists believe that Ohio was once covered by a shallow lake. They believe that explains why Ohio has limestone and sandstone. Hey didn’t you say that limestone is an organic sedimentary rock?

Rock salt (halite) is another example of a chemical rock.

Sedimentary Rock Uses

Sedimentary rocks are very useful as building materials and tools.

The White House is made of sandstone.

Limestone is ground up to make cement.

Flint can be used for arrowheads.

Salt, energy, other building materials…can you think of some?