Sedimentary RocksSection 3.3

Formation of Sedimentary Rocks

Sedimentary comes from the Latin word sedimentum, which means settling

All sedimentary rocks begin to form when existing rocks are broken down into sediments

There are several major processes that contribute to the formation of sedimentary rocks:Weathering, Erosion and DepositionCompaction and Cementation

Formation of Sedimentary RocksWeathering is the breaking down of rock

into sedimentsErosion involves weathering and the

transportation of sedimentsDeposition is the process when water,

wind, ice or gravity loses energy causing sediments to dropLarger sediments are deposited

first, smaller sediments are dropped later

Formation of Sedimentary RocksCompaction is a process that squeezes,

or compacts, sediments, pushing water outCompaction is caused by the weight of

sedimentsCementation takes place when dissolved

minerals are deposited in the tiny spaces among the sediments

Classification of Sedimentary RocksSedimentary Rocks can be classified into

two main groups according to the way they formClastic – rocks that are made of

weathered bits of rocks and mineralsClassified by particle size

Chemical – forms when dissolved minerals precipitate from water solutionsBiochemical sedimentary rocks contain

shells and skeletal remains of organisms

Clastic Sedimentary RocksConglomerat

e

Sandstone

Breccia

Chemical Sedimentary Rocks

Rock Salt

Limestone

Chert

Flint

Biochemical Sedimentary Rocks

Features of some Sedimentary RocksSedimentary rocks give clues to how,

when and where the rocks formedLayers of sedimentary rock records a period

of sediment deposition. The oldest layers are found at the bottom

Ripple marks may indicate that the rock was formed along a beach or stream bed

Mud cracks are formed when wet mud or clay dried and shrank, leaving a record of a dry environment

Fossils found in sedimentary rocks can help answer questions about the rock

Ripple marks

Fossils

Layers of sedimentary rock

Mud cracks

Summary of Sedimentary RocksSedimentary rocks form as the result of

five major processWeathering produces particles called

sedimentsWind, water, ice and gravity erode and deposit sediments

Sediments are compacted and cemented to form sedimentary rocks

Sedimentary rocks can be classified according to how they form, by using rock formation, texture and composition

(Table 2 on page 79)