Weathering, Soil, Sendimentary, Metamorphic

rocksBy

Doba Jackson, Ph.D.

Associate Professor of Chemistry

Huntingdon College

• Rocks and minerals are disintegrated and decomposed by the processes of mechanical and chemical weathering. – The products of weathering include soluble salts,

ions in solution, and solid particles which can be eroded and become sedimentary rock or modified in place to become soils.

Mechanical and Chemical Weathering

Weathering and Erosion• How does weathering differ from erosion?

– Weathering is the mechanical and chemical alteration of Earth materials at or near the surface

– Erosion involves removing weathered materials from their place of origin-by running water or wind, for example.

Mechanical Weathering – Disaggregation of Earth Materials

• Mechanical weathering

includes the processes of

– Frost action– Pressure release– Thermal expansion and

contraction– Crystal growth– Activities of organisms.

Products of Mechanical weathering is chemically the same as it was prior to the weathering.

Mechanical Weathering – Freeze-Thawing

- When water freezes in cracks in rocks it expands and then it contracts when it thaws, thus exerting pressure and opening the cracks wider.

- Repeated freezing and thawing disaggregates rocks into angular pieces that may tumble downslope and accumulate as talus.

Mechanical Weathering – Pressure Release

Rock formed underground at higher pressure can undergo an expansion under lower pressure on the earth’s surface.

– Sheet joints are fractures that more or less parallel exposed rock surfaces, especially rocks now at the surface that formed under great pressure at depth.

Mechanical Weathering – Living Organisms, Plants, Trees,

• How do organisms contribute to mechanical and chemical weathering?

– Any plant or tree roots growing in cracks contributes to mechanical weathering by exposing the cracks inside the rocks and supplying water and other chemicals to them

Chemical Weathering – Decomposition of Earth Materials

• Chemical weathering processes include

– Solution– Oxidation– Hydrolysis

• Hot and wet environments accelerate chemical weathering.

• Chemical weathering occurs in all environments, except, possibly, permanently frozen polar regions.

Chemical Weathering – Dissolution or deposition in fluids

Dissolution of Carbonate rocks (Limestone, Dolomite)

• Solution/Dissolution – – rocks dissolve

• Carbonate Rocks– Rocks such as

limestone (CaCO3) are nearly insoluble in neutral or alkaline solutions, but they rapidly dissolve in acidic solutions

Oxidation of Iron, Magnesium and other metals

• Oxidation– – rocks rust

– Rocks such as sandstone may contain iron minerals that will breakdown when exposed to the atmosphere

Fe + O2 Fe2O3 Iron in any rock

Oxygen in the air

Iron Oxide(Rust)

Oxidation of Iron, Magnesium and other metals

• Oxidation– Iron Oxide (Fe2O3) can

further break down in the presence of water to Iron Hydroxide

Fe2O3 + H2O Fe(OH)3 Iron Oxide Water Iron Hydroxide

Hydrolysis of Rock by water to form Clays

• Hydrolysis – breakdown to clays– Potassium feldspar

• During hydrolysis hydrogen ions react with and replace positive ions in potassium feldspar

• The result is clay minerals and substances in solution such as potassium and silica.

KAlSi3O8 + CO2 + H2O Al2Si2O5(OH)4 + SiO2 + KHCO3 Orthoclase Clay (Kaolinite) Silica

How does mechanical weathering contribute to chemical weathering

Sandstone Arches at Arches National Park, Utah

Soil and Its Origin• Soils consist of weathered materials, air,

water, humus and also the plants which they support.

Soil and Its Horizons

• Soil formation produces horizons that are known in descending order as O, A, B, and C. – These horizons differ

from one another in texture, structure, composition and color.

Soil and Its Horizons

• Upper Layer (O) Humus – Consists of decayed

organic matter necessary for plant life. The dark color comes from Humus.

Soil and Its Horizons

• Topsoil (A) – Less Humus but lots

of microbes, insects, plant material. Also has lots of water, clays and quartz

Soil and Its Horizons

• Subsoil (B) – Little to no Humus.

Mostly degraded minerals

• Weathered Bedrock (C) – Weathered rock.

Cannot grow and living organisms.

What factors control soil formation?

– Soil formation is controlled by

• Climate• Parent material• Organic activity• Relief and slope• Time

Soil and Climate conditions

Introduction to Sedimentary Rocks

• What is sediment?– The two primary types of sediment are Detrital

and Chemical. – Detrital sediment

consists of solid particles, products of mechanical weathering.

– Chemical sediments consist of minerals precipitated from solution by inorganic processes and by the activities of organisms.

Sediment Sources, Transport, and Deposition

• Detrital sedimentary particles are classified according to grain (particle) sizes, in decreasing diameter: – Gravel (> 2mm)– Sand (1/16 to 2 mm)– Silt (1/256 to 1/16 mm)– Clay (less than 1/256 mm; as mud)

• During transport, abrasion effects reduce particle sizes (rounding).

What is a depositional Environment?

• Depositional environments are areas of sediment deposition.

Lithification: Converting Sediment into Sedimentary Rock

Lithification of sediment into sedimentary rock occurs by compaction and cementation.

Cementation of Sedimentary Rocks

Cementation is a process that glues the sediments together.

-Calcium Carbonate (CaCO3)

-Iron Oxide (Fe2O3)

- Iron Hydroxide (Fe(OH)3)

- Silica Dioxide (SiO2)

These materials are the products of chemical weathering of rocks!!!

• Conglomerate- a sedimentary rock consisting of an aggregate of gravel (> 2mm rocks) solidified together with rounded edges.

• Sedimentary Breccia- same as a conglomerate excepts it has sharp edges.

Conglomerate and Sedimentary Breccia

Sand and sandstone is the most common sedimentary rock

• Why is quartz the most common mineral in sandstone? • Quartz is a common mineral in many source rocks, and in addition it is

very stable chemically and quite durable mechanically.

Chemical Sedimentary Rocks

SeaShells• Protective exoskeleton of Crabs,

Lobsters, worms, Sea Urchins, etc.

Other Carbonate Rocks: Limestone & Dolostone

• Limestone is Calcium Carbonate but usually in a mixture with other minerals.

• Dolostone forms when magnesium replaces calcium in limestone.

Halites and Gypsum are formed when water evaporates

• Bedded rock salt (halite) and rock gypsum are chemical evaporite sediments formed by precipitation of minerals during the evaporation of water.

Coal: Biochemical sedimentary rock

• Coal is a biochemical sedimentary rock composed largely of altered land plant remains

Sedimentary Facies

• Sedimentary Facies- Bodies of sediment or sedimentary rocks which are recognizably different from adjacent sediment or sedimentary rocks and are deposited in a different depositional (sub) environment are known as sedimentary facies.

Marine Transgression

• A marine transgression occurs when sea level rises with respect to the land, resulting in offshore facies overlying nearshore facies.

• A marine regression, caused when the land rises relative to sea level, results in nearshore facies overlying offshore facies.

Marine Regression

• A marine regression, caused when the land rises relative to sea level, results in nearshore facies overlying offshore facies.

• Notice the different slant in the facies between a transgression and regression.

Important Resources in Sedimentsand Sedimentary Rocks

• What is oil shale? • Oil shale is a fine-grained sedimentary rock that

contains kerogen from which liquid oil and combustible gases can be derived.

• None is mined at present in the United States because oil and gas from conventional sources are cheaper. Oil shale and tar sands are increasingly important petroleum reserves.

Metamorphic Rocks

Introduction to Metamorphism

• What is metamorphism?• The transformation of rocks, usually beneath Earth's surface,

as the result of heat, pressure, and/or fluid activity, produces metamorphic rocks

• Why is metamorphism worth studying?• Metamorphism is an important process that is closely related

to plate tectonics, the growth of continents, and even climate change.

• It is also responsible for producing a number of economically valuable materials.

• Though largely hidden from view, metamorphism is part of the world around you. The world would be a much different and less interesting place without it.

Equilibrium and the Causes of Metamorphism

• Intrusive magmas or deep burial provide heat which causes metamorphism.

• Pressure is produced by overlying rocks (lithostatic) or is differential pressure produced by various stresses.

Equilibrium changes• Equilibrium: A system in which all competing forces are

balanced. • Minerals are no longer stable (no longer “in equilibrium”) as

conditions change. Minerals can react with one another to create more stable kinds of minerals (heterogeneous metamorphic reactions) or undergo rearrangements in their atoms to reduce the strain that builds up within them (crystal lattice reorientation and polymorphic transformation).

The Main Types of Metamorphism

• Principal types of metamorphism –

»Contact»Dynamic»Regional metamorphism

Contact Metamorphism• Contact metamorphism

– Contact metamorphism is when a body of magma alters surrounding rock. This provides intense heat which drives changes within the rock.

Contact metamorphism

Increased (or Differential) Pressure

Lithostatic pressure is a uniform field of pressure experienced by most rocks beneath Earth’s surface. Like the hydrostatic pressure experienced by divers underwater, the pressure acting on a rock embedded in the crust “feels” the same from all directions.

The Main Types of Metamorphism

• Dynamic metamorphism – Dynamic metamorphism is associated with faults

and areas where lots of pressure builds up in the crust, but the temperature is not very great, such as in the accretionary wedges at convergent plate boundaries.

The Main Types of Metamorphism

– Shock metamorphism is a type of dynamic (pressure-dominated) metamorphism associated with meteorite impacts. The pressure is extremely high relative to other natural processes that operate on Earth.

Regional Metamorphism– Regional metamorphism is the most common type of

metamorphism. As the name implies, regional metamorphism has a broad range. Temperature and pressure both act as driving forces for metamorphic reactions in regional metamorphism.

Classification of Metamorphic Rocks

• Metamorphic rocks are classified principally according to texture. The texture is

» Foliated - Foliated texture is produced by the preferred orientation of platy minerals.

» Nonfoliated - Nonfoliated textures do not exhibit preferred orientation of minerals.

Foliated Metamorphic Rocks • What is foliated texture, and what are some examples

of foliated rocks?– Foliated texture is produced by the preferred

orientation of platy minerals. – Amphibolite is another fairly common coarse grained

foliated metamorphic rock.

Schist and Gneiss

Non-Foliated Metamorphic Rocks

• What is a common nonfoliated metamorphic texture, and what are some examples of nonfoliated metamorphic rocks? – Nonfoliated textures do not exhibit preferred

orientation of minerals.– Common nonfoliated metamorphic rocks are

marble, quartzite, greenstone, and homfels.

Metamorphic Zones• What are isograds and metamorphic zones?

– Metamorphic rocks often can be arranged in metamorphic zones which reflect the pressure and temperature conditions of metamorphism

Some Economic Uses ofMetamorphic Materials

Marble