How do Sedimentary Rocks Form?

By J. Piccirillo

• For thousands, even millions of years, little pieces of our earth have been eroded--broken down and worn awayby wind and water (ice included).

• These little bits of our earth are washed downstream by

( E…. )where they settle to the bottom of the rivers, lakes, and oceans.

• Layer after layer of eroded earth sediments have been deposited and buried on top of each other.

• These layers are pressed down ( ) more and more through time, until the bottom layers slowly turn into rock.

• While compacting, these sediments are also glued

( ) together by dissolved minerals oozing from the sediments

• Eventually all the weight of Sediment squeezes all or most of the water out.

• Later on through many years of Earth moving upward or seas retreating or evaporating the compacted sediment turns into Hard Rock when exposed above the earth’s surface.

http://www.classzone.com/books/earth_science/terc/content/investigations/es0602/es0602page02.cfm?chapter_no=investigation

• Conglomerates• Breccia• Sandstones• Siltstones• Shale/mudstones

TYPES OF SEDIMENTARY ROCKS

Clastic rocks Chemical & Organic rocks

Evaporitic rocks

These rocks are formeddue to evaporation of saline water (sea water)eg. Gypsum, Halite (rock salt)

Carbonate rocks

Form basically from CaCO3 (Calcite) – both by chemical leaching and by organic source (biochemical) eg. Fossil Limestone; Dolostone (MgCO3)

Organic rocks

Form due to decomposition of organic remains under temperature and pressure eg. Coal/Lignite etc.

• Formed from broken rock fragments weathered and eroded by river, glacier, wind and sea waves. These clastic sediments are found deposited on floodplains, beaches, in desert and on the sea floors.

CLASTIC ROCKS

Clastic rockssolidify

• Clastic rocks are classified on the basis of the grain size: conglomerate, sandstone, shale etc.

GRAIN SIZE

Gravel< 256 - 2 mm

Sand2- 0.062 mm

Clay<0.0004 mm

Silt0.062 - 0.0004 mm

Boulder: >256mm

Cobble: 64-256 mm

Pebble: 4-64 mm

Granule: 2-4mmFine gravel

Non- Clastic or Chemical or Crystalline EVAPORITIC ROCKS

These rocks are formed within the depositional basin from chemical substances dissolved in the seawater or lake water.

GypsumHalite

(NaCl)CaSO4.2H20

Economic importance of Evaporites

• SALT: other then daily use of salt for cooking, it is used

• For production of Paper,• Soap• Detergents• Antiseptics• As chemical for dyeing etc.

• GYPSUM: is used for plaster and in manufacturing construction materials.

• Limestone: It is a non-clastic rock formed either chemically or due to precipitation of calcite (CaCO3) from organisms usually (shell).  These

remains will result in formation of a limestone. • Limestones formed by chemical precipitation are

usually fine grained, whereas, in case of organic limestone the grain size vary depending upon the type of organism responsible for the formation

-Chalk: which is made up of foraminefera is very fine grained

-Fossiliferous Limestone: which medium to coarse

grained, as it is formed out of cementation of Shells.

Non- Clastic CARBONATE ROCKS

• used as a building stone and in the manufacture of lime, carbon dioxide, and cement.

• Massive and compact lst. Are competent to support CE-structure

• However, if it occur in huge thickness then it is not advisable, because of its typical CAVING character.

Chalk

• Let’s look at some sedimentary rocks with your teacher using the ESRT.

Conglomerate(has large and small grains mixed)

Conglomerate

Sandstone Varieties

Fossil Sandstone

(from clay) shows some layering

• Shale

Limestone - chemical variety..from precipitated calcite out of seawater.. deposited on the bottom beds of evaporated seas and lakes and from sea animal shells

Rock Gypsum(looks like cake and flakes)

Halite (rock Salt..looks crystally)

Fossil Limestone….organic variety…Bioclastic

Fossil Limestone