Chapter 8:Sediments & Sedimentary Rocks

Utah sandstone (USA)

Intro. Video for Flowchart

Introduction to Sediments

Rocks exposed at the Earth’s surface are constantly being altered. One way is weathering (or the destructive process that changes the physical and chemical nature of rock). Tightly bound crystals in the rock can be loosened and changed to new minerals by weathering.

There are two types of weathering:

1. Mechanical weathering – breaks the rock into smaller pieces. This is usually caused by frost action (water freezing and breaking apart rocks) and the decreased pressure that rocks are exposed to at the surface. In other words, bodies of rock that are formed within the Earth often come to the surface where there is less pressure causing the rocks to split open.

2. Chemical weathering – Rocks formed deep in the Earth are exposed to oxygen, water and other environmental conditions (such as acid rain) at the surface.

Erosion is also important. Erosion is the “picking up” or physical removal of rock particles by things such as streams and glaciers. Weathering helps break down rock that is then easily eroded.

What is sediment?

• Sediments are a general term for loose, solid particles.

• There are two possible origins for sediments:

1. weathering & erosion of existing rocks

2. chemicals precipitating (falling out of) water

• Sediments include beach sand, boulders in glaciers, pebbles in a stream, and dust from the air.

• Sediments come in various shapes and sizes. They include the following:

Diameter Sediment type

>256 mm Boulder

64-256 mm Cobble

2-64 mm Pebble

0.0625-2.0 mm Sand

0.004-0.0625 mm Silt

>0.004 mm Clay

• Sediments are rounded (removal of sharp edges over time), sorted (coarse sediment at the beginning of a river; sand, silt, and clay at the mouth), and deposited (sediments come to rest).

How do sediments change into sedimentary rock?

Sedimentary rocks are formed in three steps:• Layers of sediment are deposited at the bottom

of seas and lakes. • Over millions of years the layers get squashed

by the layers above. • The salts that are present in the layers of

sediment start to crystallize out as the water is squeezed out. These salts help to cement the particles together.

There are two processes that form sedimentary rocks:

1. Lithification is the general term for the processes that convert loose sediment into sedimentary rock. Lithification happens because of compaction (packing lose grains together) and cementation (a cement or “glue” holds the grains together). The cement is usually made of calcite or silica. A sedimentary rock that is made of sediment grains bound by a cement into a rigid framework is said to have a clastic texture.

2. Some rocks form by crystallization. This happens when crystals grow and develop from solution (ex. salt precipitating or falling out of salty water). With crystalline structure, the rock is held together with crystals instead of cement.

Continued:

What are the main types of sedimentary rock?

There are three different types of sedimentary rocks:

1. Clastic sedimentary rock

2. Chemical sedimentary rock

3. Organic sedimentary rock

Clastic Sedimentary Rock

• These are formed from sediments that are fragments of pre-existing rocks. These sediments are then cemented together. The rock fragments can be identifiable pieces of rock (ex. pebbles) or tiny grains of minerals.

Examples of Clastic Sedimentary Rock

Breccia

(coarse, angular fragments)

Conglomerate

(coarse, round fragments)

Sandstone

(cemented sand grains)

Shale

(cemented fine-grained silt)

Chemical Sedimentary Rock

• These rocks are formed when minerals precipitate from (or fall out of) a watery solution.

Examples of Chemical Sedimentary Rock

Limestone

(made of calcite [CaCO3] from shells, coral, & algae)

Dolomite

(similar to limestone but with Mg)

Chert

(fine grains of silica)

Evaporites (ex. Rock salt)

(formed from crystals that precipitate during evaporation)

Organic Sedimentary Rock

• These are rocks that accumulate from the remains of organisms (plants, animals, and algae).

• Organic Sedimentary Rocks are rocks that are formed with once living material such as seashells or plant material. These rocks look black like coal or very sandy gritty and look much like a granola bar.

Examples of Organic Sedimentary Rocks

Organic sedimentary rock with seashells

Anthracite, otherwise know as coal

Sedimentary Rock Structures

• Sedimentary structures are features found within sedimentary rocks that usually form after settling, but before lithification. These structures provide geologists with valuable information about how sediments were transported, and what conditions were like at that time.

Examples of Sedimentary Rock Structures

Bedding

(visible horizontal lines)

Examples of Sedimentary Rock Structures

Cross bedding

(not horizontal)

Examples of Sedimentary Rock Structures

Ripple marks

Examples of Sedimentary Rock Structures

Trilobite fossil

(from U.K.)

Petrified wood

(from U.K.)

T. Rex fossil

from Montana, USA

Quiz

• Sedimentary Rock Quiz

http://www.proprofs.com/quiz-school/story.php?title=sedimentary-rocks

Complete the quiz, print out the completion certificate, write your name on the front and submit to the teacher. Thanks!