Minerals - The EarthÕs Building Blocks

A mineral is a naturally occurring, inorganic material with a definite crystal structure and chemical composition

No two minerals have both the same crystal structure and composition

Physical properties are the easiest (but least exact) way to identify them

 

Mineral Crystals

Each mineralÕs crystal structure is due to the regular repeating pattern of atoms which make up the chemical composition of the mineral

Example: Halite (NaCl)

 

Rocks

A rock is an aggregate or mixture of crystals from one or more minerals

 

Rock Types

Igneous Rocks - formed from the cooling of magma

Sedimentary Rocks - formed from the lithification of sediment

Metamorphic Rocks - formed by the alteration of other rocks through the addition of heat and /or pressure

 

Igneous Rocks

Formed from molten magma (= lava at the EarthÕs surface)

Intrusive or plutonic igneous rocks cool slowly far below EarthÕs surface.  Mineral crystals have time to grow big.

Extrusive or volcanic igneous rocks cool quickly at or near the EarthÕs surface.  Mineral crystals quench quickly and are small (microscopic) in size.

 

Sedimentary Rocks

Formed by lithification of sediment at the EarthÕs surface

Sediment may be either:

            Clastic:  detrital (rock fragments)

            Non-clastic:  chemically deposited

Characteristics  of sedimentary rocks

            Usually layered

            Often contain fossils

 

Metamorphic Rocks

Formed by changing any rock type by the addition of heat and/or pressure

Metamorphic Rock Classification

Non-foliated (recrystallized)

            Limestone – Marble

            Sandstone - Quartzite

Foliated (layered)

            Fine-grained- Slate - Schist

            Coarse - Grained rock – Gneiss

 

Rock Cycle

No single rock has remained unchanged since the amalgamation of the Earth ~ 4.5 Ga

Plate tectonics shows that oceanic crust is constantly being generated and destroyed

Similarly, continental crust is constantly being altered by tectonic forces, weathering, and erosion

These result in a rock cycle where all rocks are constantly being altered and changed into other rocks

 

Weathering

Weathering is the physical or chemical breakdown of rocks at the EarthÕs surface (critical element of rock cycle)

Physical Weathering

            Creates rock fragments with no change in composition

            Wind, water, relief, temperature, and plant cover are key factors

Chemical Weathering

            chemical alteration of minerals to form new minerals

 

Erosion

Erosion is the process of removing unconsolidated Earth material and transporting it to other depositional sites (mainly the oceans)

Wind, water, ice, and gravity are transport agents

Erosion rates depend on climate, relief, and degree of weathering

 

Products of Weathering and Erosion

Solid material

            Rock fragments (gravel, sand, silt, clay)

            New minerals from chemical weathering

            Chemical precipitates

            Biological materials (shell, bones, organics)

Dissolved material

            Gases from atmosphere and chemical reactions

            Dissolved solids - inorganic and organic

 

Water as a Geologic Agent

Water Abrasion (weathering)

            Water and particles carried in water actively weather (physically) earth           materials

            Chemical ions in water also chemically weather earth materials

Water Transport (erosion)

Deposition by Water (sedimentation)

            Clastic sedimentary rocks

            Chemical sedimentary rocks

 

 

 

Wind as a Geologic Agent

Wind Abrasion (weathering)

            Causes deflation of unconsolidated sediment

            Causes physical erosion of resistant material

Wind may carry sediment 1000s of km (erosion)

Sediment Deposition

            Loess sediments

            Dunes

 

Continental Sites of Deposition

All solid materials are moved toward local depressions, like lakes and valleys, where they are deposited

Enroute, the particles are sorted by size - coarsest materials stay closest to source

Eventually most particles reach the oceans - but their movement may involve several steps by different processes over long periods of time