GEO MID: Sedimentary

Sedimentary Rocks

• Formed from compacted and cemented sediments.

• Process: Weathering Transport (water, wind, ice) + Deposition → Lithification (compaction + cementation).

• Make up only 5% of Earth's outer crust, but about 75% of rocks at the surface.

• Contain fossils, helping scientists reconstruct Earth's history and past environments.

• Economic importance: sources of coal, oil, natural gas, metals, and construction materials.

Detrital Rocks

form from solid particles produced by weathering of other rocks.

Chemical rocks

form when dissolved substances precipitate from solution, either directly or through biological activity.

Detrital Rocks

• Composed mainly of clay minerals (from weathering) and quartz (resistant to weathering).

• Classified by particle size:

  • Gravel: Conglomerate (rounded) or Breccia (angular).

  • Sand: Sandstone.

  • Silt: Siltstone.

  • Clay: Shale (the most common sedimentary rock).

• Grain size indicates the environment of deposition:

  • Large grains suggest high-energy settings such as fast rivers or glaciers.

  • Small grains suggest low-energy settings such as lakes, swamps, or deep oceans.

Chemical Sedimentary Rocks

• Formed when minerals dissolved in water are left behind after precipitation or evaporation.

• Major examples include:

  • Limestone (CaCO3) - most abundant, often from shells and skeletons (biochemical).

  • Chert, Flint, Jasper, Agate - formed from silica.

  • Evaporites - Halite (rock salt) and Gypsum (used in plaster and drywall).

  • Coal - formed from plant material in oxygen-poor swamps; classified as organic.

• These rocks are important sources of building materials, salts, and fuels.

Coal Formation

• this is different from most chemical sedimentary rocks because it is made mainly of organic plant material, not minerals.

• Forms in swampy, oxygen-poor environments where plant remains do not fully decay.

• Stages of formation, with increasing heat and pressure:

  • Peat: partially decayed plant material.

  • Lignite: soft, brown coal. Compaction

  • Bituminous coal: compact, carbon-rich coal.

  • Anthracite: hardest and highest carbon content; actually metamorphic rock.

• this is an important energy resource used for electricity and industry.

Peat

partially decayed plant material

Lignite

soft, brown coal

Bituminous coal

compact, carbon-rich coal

Anthracite

hardest and highest carbon content, actually a metamorphic rock

Coal

an important energy source used for electricity and industry

Deposition

Clasts are dropped or settle out

Compaction

As more sediments accumulate above, clasts are forced closed together

Cementation

groundwater moves between the grains and leaves behind mineral deposits, bonding the grains to each other

Lithification

turns sediment into rock

Strata (beds)

distinct layers that record changes in conditions over time

Bedding Planes

flat surfaces separating strata, showing breaks between depositional events

Fossils

• preserved remains or traces of life

• provide evidence of past environments such as rivers, seas, or swamps

• serve as time indicators that help correlate rock layers across regions

External Processes

powered by the Sun, this shape the Earth’s surface

Weathering
Mass Wasting
Erosion

• these break down rock, transport material, and form sediments

• contrast to internal processes: mountain building, volcanism

Weathering

breakdown of rock at or near the surface

Mechanical Weathering

this is the physical breaking into smaller pieces

Chemical Weathering

the alteration into new compounds

Weathering

produces soil, nutrients, and shapes landscapes

Mechanical Weathering

• breaks rocks into fragments, increasing surface area for chemical attack

• key process:

  • frost wedging (water freezes/expands in cracks)

  • salt crystal growth in cracks

  • sheeting/unloading

    • exfoliation domes

  • Biological activity: roots, burrowing animals, humans

Chemical Weathering

• alters mineral structures into stable surface compounds

• Water is the main agent (often with carbonic acid)

• Ex:

  • Oxidation: rust on iron-rich rocks

  • Carbonic acid dissolves calcite (marble, limestone)

  • Granite → feldspar turns to clay; quartz resists and remains

• Produces clays, soluble salts, silica

Quartz

In products of weathering:

• Residual Products: Quartz grains

• Material in Solution: Silica

Feldspars

In products of weathering:

• Residual Products: Clay minerals

• Material in Solution: Silica, K+, Na+, Ca2+

Amphibole

In products of weathering:

• Residual Products: Clay minerals, Iron oxides

• Material in Solution: Silica, Ca 2+, Mg 2+

Olivine

In products of weathering:

• Residual Products: Iron Oxides

• Material in Solution: Silica, Mg2+

Rock Characteristics

mineral composition, cracks, joints

Climate

warm + wet = fastest chemical weathering

Different weathering

rocks weather at different rates, creating dramatic landscapes

Soil

• mix of mineral matter, organic matter, water, air

• supports life by cycling nutrients and storing water

• forms where the atmosphere, hydrosphere, lithosphere, and biosphere meet

• dynamic and sensitive to environmental changes

Texture

In soil properties, this is the proportions of sand, silt, clay (loam = best for plants)

Structure

In soil properties, arrangement into clumps (platy, prismatic, blocky, spheroidal)

Texture and Structure

In soil properties, this control water movement, nutrient storage, and erosion resistance

Parent Material

source of weathered minerals from which soil develops

Residual soils

form on bedrock

Transported soils

form on sediments carried by water, wind, gravity, or ice.

Factors in Soil Formation

• time

• climate

• plants and animals

• topography

Time

soil needs this to develop

Young

soils that strongly reflect the parent material

Mature

soils that is thiciker, less like parent rock

Climate

most important factor in soil formation

Temperature and Rainfall Control

• type of weathering (mechanical vs chemical)

• rate and depth of soil development

Hot and wet climate

thick chemically weathered soilds

Cold and dry climate

thin, mechanically weathered soils

Plants and Animals

• provide organic matter (mainly from plants)

• decomposition forms humus, improves fertility and water retention

• microorganisms recycle nutrients and fix nitrogen

• earthworms and burrowers mix soil, improve air and water movement

Topography

shape of land influences soil development

Steep Slopes

thin soils, high erosion, low fertility

Flat/undulating land

best soil development

Slope Orientation

this affects sunlight, moisture, and vegetation

O Horizon

organic matter

A Horizon

topsoil (mineral and humus)

E Horizon

leached, light-colored zone

B Horizon

subsoil, clay & minerals accumulate

C Horizon

partly altered parent material

Aridosols

dry desert soils

Inceptisols

young soils with little development

Soil Erosion

natural but greatly accelerated by humans

Agents

water (raindrops, sheet erosion, rills, guilles) & wind

2.5x faster

rates now ??? than before humans

Threat

topsoil lost faster than it forms = lower fertility

Erosion

removes fertile topsoil, reducing productivity

Sediment

fills rivers & reservoirs, causing floods and water loss

Chemicals

• fertilizers, pesticides

• pollute water and harm life

Conservation Methods:

Contour Plowing, terracing, crop rotation, cover crops, windbreaks, and reduced tillage

Goal

protect soil, food supply, and ecosystems

Secondary Enrichment

weathering can concentrate metals

Bauxite

main ore of aluminum

Tropical Rainy Climates

where is Bauxite formed?

Aluminum-Rich Bauxite

Intense weathering removes soluble elements, leaving aluminum-rich bauxite

Environmental Concern

tropical forest destruction, poor recovery of soils