Weathering and Rock Types

Weathering and Rock Types

What is Weathering?

• Weathering is the breakdown of rocks into smaller pieces.

• This occurs when the solid Earth is exposed to elements, leading to changes, breakdown, and the formation of new landforms.

Sedimentary Rocks

• Formed from fragments of other rocks compressed and hardened in layers (strata).

• Characteristics:

  • Form layers.

  • Soft (least resistant to weathering).

  • Contain fossils.

3 Main Rock Types

• Igneous: Formed when molten rock (magma/lava) cools and hardens.

• Sedimentary: Formed from fragments of other rocks compressed and hardened in layers (strata).

• Metamorphic: Developed from igneous or sedimentary rocks changed by heat or pressure.

Igneous Rocks

• Intrusive/Plutonic: Formed from magma.

• Extrusive/Volcanic: Formed from lava.

• Characteristics:

  • Crystalline.

  • Hard (most resistant to weathering).

  • Contain no fossils.

Metamorphic Rocks

• Developed from igneous or sedimentary rocks changed by heat or pressure.

• Characteristics:

  • Thinly layered

  • Soft to hard

Freeze Thaw (Ice Wedging/Frost Shattering)

• Water enters cracks in rocks, freezes, and causes the rock to crack.

• Process:

  • A: Rain or snow melts, and water seeps into cracks.

  • B: Temperatures drop below freezing, ice forms, increasing in volume and exerting pressure.

  • C: Repeated expansion/freeze-thaw breaks rocks apart.

Exfoliation

• Rock expands when heated (during the day) and contracts when temperatures decrease (at night).

• Repeated expansion and contraction break the rock in layers.

• Expansion makes parallel cracks while contraction makes cracks at right angles to the surface.

• Layers of rock peel off.

Scree

• Accumulation of broken rock fragments at the base of cliffs or steep slopes, resulting from physical weathering processes like freeze-thaw.

Chemical Weathering

• Involves the interaction of rock with mineral solutions (chemicals) to change the composition.

• Water interacts with minerals, creating chemical reactions and transforming rocks.

• Types:

  • Hydrolysis

  • Oxidation

  • Solution

  • Carbonation

Hydrolysis

• Rocks sitting in water for extended periods break down & have a clay-like texture.

• A-B + H_2O \rightarrow A-H + B-OH

Oxidation

• Oxygen reacts with iron in rocks, causing them to rust (red-orange color).

• Fe + O2 \rightarrow Fe2O_3

• Iron in rocks reacts with oxygen to form iron oxide, which is brittle like rust, causing the rock to break.

Solution

• Rocks in water react with acids and dissolve, indicated by small holes (e.g., coastal rocks).

Carbonation (Acid Rain)

• Water absorbs carbon dioxide from the air to form carbonic acid, which reacts with minerals.

Acid Rain Formation

• Burning fossil fuels releases sulfur, carbon, and nitrogen into the atmosphere.

• These combine with moisture to form sulfuric acid, carbonic acid, and nitric acid.

Biological Weathering

• Occurs when burrowing animals, humans, or plants interfere with rocks.

• Living organisms, like some plants and animals, produce weak acids that chemically weather rock.

• Lichens slowly break down granite boulders.

Factors Affecting Weathering Rate

• Climate: Expansion and contraction occur with extreme temperature differences. Chemical weathering needs water and heat; hot and wet conditions.

• Rock Type: Small particles weather faster than large particles. Rocks are made of minerals with different weathering resistances.

• Rock Structure: Cracks are weak points for plants, animals, or water. Porous or permeable rocks weather faster.

• Vegetation: Roots widen cracks in rocks.

• Relief: Greater erosion takes place.

• People: Pollution, gases, deforestation, farming accelerate weathering.