1 - Clastic sediments and sedimentary rocks

What % of rocks on Earth are sedimentary

75%

Features of continental crust

Andesitic/intermediate

20-30% Quartz

20-70% Feldspars

10-30% Mica

Features of Oceanic crust

Basaltic/Basic

0-50% Olivine

40-80% Pyroxenes

0-30% Plagioclase

Features of the upper mantle

Peridotite/ultrabasic

50-100% Olivine

0-50% Pyroxene

Describe the Goldich Weathering series

Olivine (least resistance to weathering)

Pyroxenes

Amphiboles

Biotite

Muscovite

K-Feldspar

Quartz (most resistance to weathering)

Types of physical weathering

Frost/freeze-thaw weathering

Exfoliation (heat-cool, peeling)

Action of plants

Types of chemical weathering

Hydrolysis

Hydration

Carbonation

Oxidation

Ion exchange

Chelation

Impact of chemical weathering

Water mediated formation of new minerals with different volumes

Factors controlling rate of weathering and regolith formation

High rainfall

High temperature

Topography

Where is the highest weathering rates

Tropics (hot + wet)

Clay negative feedback

Clay produced in weathering → impermeable material → prevents water reaching bedrock → limits weathering

Weathering and erosion

Generating regolith then removing the regolith

Facilitators of erosion

Gravity → hills, falls, landslides and runnoff

Main building blocks of andesites

Quartz

Feldspars

Mica

Features of Quartz

Inert

Hard (Moh’s 7)

Forms most common course grains or clasts

No cleavage

Weathering of feldspars and micas

Feldspars and micas are metastable → React readily with water and weak acids formed from hydrolysis → break down into clay (unlike quartz)

Features of clay

Sheet silicates with a single perfect cleavage

Soft (1-2.5)

Form a fine matrix/mud

Comprises shales and mudstones

Features of clastic sediments

Grains/clasts → visible to the naked eye → usually quartz

Clay → appears as a speckly mush

For sediments to form rock they must:

undergo diagenesis → cemented at high temperatures/pressures

For Carbon to be locked into the rock:

It must be buried in a sedimentary basin

Increasing distance and time leads to more texturally mature rocks:

Grainsize decreases

Grain angularity decreases

Grain sphericity increases

Grain sorting increases

Loss of feldspar, mica and lithic clasts in favour of quartz and clay matrix

Basic summary of diagenesis

Growth of crystals which precipitate out of water to form cement, holding the grains together