Chapter 6 and Interlude B

Sediment

Loose fragments of rocks or minerals, shell and shell fragments, or mineral crystals that precipitate out of water

Weathering

processes that break down rocks into small pieces by chemical or physical/mechanicals means

What controls weathering?

1. Properties of parent rock

• mineralogy anf crystal structure - Are the minerals soluble in water? Is the structure susceptible to breaking down?

2. Climate : rainfall and temp

3. Presence or Absence of Soil : soil formation promotes more soil formation

4. Length of Exposure: the longer exposed, the more altered a rock becomes

Physical weathering

Breaking larger rocks into smaller rocks without changing the minerals

• along natural zones of weakness 

  • Sedimentary rocks: along bedding planes

    Foliated metamorphic rocks: along foliation/cleavage planes

    Non foliated rocks: along fractures formed by deformation and cooling/contraction

Jointing 

natural cracks formed in rocks due to the expansion of a rock undergoing exhumation and cooling contraction (ex: pressure expansion)

Exfoliation

when the outer layers of rock, approximately parallel to the surface, fracture off

Frost Wedging

• breakage caused by freezing water

• most effective in locations with frewuent freeze thaw cycles

Root Wedging

• plant roots work into cracks in the surrounding rock ,forcing it apart

Salt Wedging

when saltwaters get into cracks in the rock and evaporates, salt crystals form and exert a force on the surrounding rock (occurs in arid climates and coastal areas)

Thermal expansion

some minerals expand more than others for any given temperature change

• most effective in environments where there are large day night temp differences

• the rock expands during the warm day then contracts at night, causing cracks to form

Chemical Weather

chemical reactions that cause some minerals to break down or form new minerals

Dissolution

• when minerals dissolve in water

• mostly effects carbonate rocks and salts

• exL acid rain dissolution or marble statues

Hydrolysis

Carbonic acid reacts with some silicate minerals in rock to form clay minerals (which then get eroded away)

More resistant minerals do not weather and persist

Oxidation

Reactions during which an element loses electrons

Usually occurs when elements combine with oxygen

Common in iron bearing minerals (mafic minerals)

Weathering vs Erosion

weathering: breaking and transforming rock into sediments

erosion: removal of rock or sediment

Different types of erosion

1. waves

2. glaciers

3. wind 

4. rivers

5. landslides

Depositional Environments

where sediments accumulate

Properties of Sediments

• texture

  • grain size

  • grain shape

  • grain sorting

• composition

  • categories:

    • clastic

    • chemical/biochemical

    • organic

  • individual minerals

Texture

1. angular

2. subangular

3. subrounded

4. rounded

5. well rounded

Composition: clastic

made of weathered sediment (small pieces or minerals from pre existing rocks

Composition: biochemical/chemical

• not made from sediment that was physically weathered

• examples: a rock precipitated from a solution or a rock formed from pieces of shells

Based on the texture and composition you can interpret:

• the energy of deposition

  • high energy: river

  • low energy: lake

• sediment source

  • clastic sediment: weathered pieces of rock and minerlas

  • biochemical: shells of marine organisms