Formation of Sedimentary Rocks

Formation of Sedimentary RocksSedimentary rocks account for approximately 5% of the crust. The upper 16 km of Earth.

Regardless of this low percentage, approximately 75% of all rocks exposed at the surface is sedimentary rock.

1) Erosion and weathering

2) Transportation

3) Deposition

4) Lithification

Factors leading to the formation of sedimentary rocks include;

Reference: Tarbuck and Lutgens Pages 41 - 42

Erosion and Weathering In reference to the Rock Cycle, the origin of sedimentary rocks begins with the processes of weathering and erosion.

Pre-existing rocks can be broken down both mechanically or chemically to produce sediment.

Mechanical Weathering happens when a material is broken into smaller pieces which still retain the characteristics of the original material. Example: freeze – thaw action.

Water freezing cracks fragments of rock

Mechanical weatheringMechanical weathering Frost wedging

Erosion and Weathering In reference to the Rock Cycle, the origin of sedimentary rocks begins with the processes of weathering and erosion.

Pre-existing rocks can be broken down both mechanically or chemically to produce sediment.

Chemical Weathering happens when a material is altered by chemical action and a new material is produced which is chemically different than the original material. Examples: Oxidation reactions - Iron rusting Decomposition of limestone (statues and headstones)

Chemical weathering:

Processes that break rock components and

internal structures of minerals, making new

minerals

WATER: most important agent

Most important processes:

dissolution

oxidation

hydrolysis

Chemical weathering

Dissolution

Dissolving minerals by aliquid agent (such as water)Dissolving minerals by aliquid agent (such as water)

How?How?

Many minerals are water-soluble example: Halite (salt)

Adding acid (H+) increases corrosiveness example: carbon dioxide and rain

Chemical weathering

Oxidation

Chemical reaction that causes loss of electronsChemical reaction that causes loss of electrons

ExampleExample

Iron (Fe) + water (H2O) oxygen (O) combines with Fe to form hematite (Fe2O3)

Oxidation decomposes important minerals: mineral formula where

olivine (Mg,Fe)2SiO4 Earth’s mantle

pyroxene (Mg,Fe)SiO3 Earth’s mantle

hornblende Ca2(Fe,Mg)5Si8O22(OH)2 Continental rocks

Chemical weathering

Hydrolysis

Decomposition of graniteDecomposition of granite

Granite (mainly quartz + potassium feldspar) +carbonic acid (water + carbon dioxide)

Liberates potassium from feldspar - important plant nutrient

makes kaolinite - this left over mineral is clay - very stable - main inorganic part of soil

quartz - very stable, glassy appearance - carried to sea: beach sand, sand dunes - cements to form sandstone

Chemical weathering

Alterationschemical weathering that causes physical chemical weathering that causes physical changes to rockschanges to rocks

Rock corners are eroded more rapidly than sides

(more surface area at corners)

Chemical weathering

Alterations

“ spheroidal weathering ”

T&L Figure 5.11

Chemical weathering

Transportation of Sediment Erosional agents (such as, water, wind, and ice,) influenced by gravity, transport the sediment to new locations where it is deposited.

Sediment is broken down during the transport phase. This process is called Abrasion.

Three main agents tend to transport the sediment, these being;

2) Ice (glaciers)

1) Water (rivers and groundwater)

3) Wind

Wind Blown Sand

Deposition of Sediment

In dry areas of the world, sediment (sand and silt) can accumulate in areas called deserts.

Eventually the sediment is deposited in lakes, river valleys, seas, and more frequently the oceans.

Lithification of Sediment

Following deposition, sediment may become lithified, which means turned to solid rock.

Sediment is commonly lithified by two processes;

Compaction: when the sediment size is small (silt and clay), the weight from the layers of sediment above can compact the lower layers forming solid sedimentary rock.

1)

Compaction

Silt and Clay

Sand

Lime (Calcite)

Round Pebbles

Shale

Sandstone

Limestone

Conglomerate

Shale

Lithification of Sediment

Following deposition, sediment may become lithified, which means turned to solid rock.

Sediment is commonly lithified by two processes;

Cementation: Over long period of time mineral matter dissolved in groundwater pass through pore spaces and cement (stick) sediment particles together to form solid sedimentary rock.

2)

Sediment Particles Cement

Water with dissolved minerals, (calcite or silica) pass through pore spaces