Rocks contain clues about the environments in which they formed.

2-1    The rock cycle:

• Igneous rock: form intrusion and extrusion of magma.

• Magma: molten materials inside the earth.

• Sedimentary rock: from the physical weathering of already formed rock, being transported and deposited, them lithified.

• Lithification: a process to convert sediments to tock, normally temperature and pressure are important.

• Metamorphic rock: previously formed rock undergoes recrystallization or structural rearrangement through metamorphism.

• Metamorphism: "Change form" change the type of minerals, the size of minerals, the texture, etc., without completely melting. Temperature and pressure are important.

• Basic rock cycle (as indicated by green arrows):

• Alternate path (as indicated by blue arrows): metamorphic rock derived from igneous rock, and weathering of sedimentary or metamorphic rocks.

2-2    Igneous rocks:

• Magma vs. lava

• Magma: molten materials beneath.

• Lava: magma that reaches the surface of the Earth.

• Volcanic or extrusive rock: rocks formed by solidification of lava, i.e. cooled on surface

• Plutonic or intrusive rock: rocks formed underneath the surface.

2-2-1    Crystallization process:

• Crystals: in which atoms have regular arrangement.

• Glass: atoms arrange themselves randomly in space.

• When cooling fast: no time for the atoms to arrange them orderly => glass.

• When cooling slow: atoms have time to arrange them into orderly fashion => crystals.

• Size of the crystals is a function of cooling time and available space.

2-2-2    Classification of igneous rocks: Based on

Texture: the size, shape and interparticle relationship of the minerals.

Different textures: 

• Fine-grained

• Coarse-grained

• Porphyritic: large crystals embedded in a matrix of smaller crystals 

• Glassy: quenched 

Mineral composition:

• Major minerals

• Minor minerals

• Trace minerals

Different minerals form at different temperature and pressure conditions

Seven common rock-forming minerals: Olivine, pyroxene, amphibole, biotite, mica, feldspar, and quartz.

Bowen's reaction series:

Igneous rock types v.s. common igneous rocks:

• Based on the composition and texture.

• Texture reflect the depth the rocks formed. e.g. coarse-grained formed deeply while glassy formed at surface.

• If we know the name of the rock we should be able to tell how it is formed and what it's approximate composition is.

2-3    Sedimentary rocks:

To change form sediments (loose particles) to a rock (solid piece) compaction and cementation are needed.

• Compaction: squeeze water out.

• Cementation glues particles together.

2-3-1    Classification:

Detrital sedimentary rocks:

• Rocks that are made of particles of previous rocks, sand, clay, gravels, etc.

• Based on the size of the particles. Fig 2.12

• 	Sediment name	Rock name
  > 2 mm	Gravel	Conglomerate, Breccia
  < 2 mm and > 1/16 mm	Sand	Sandstone
  < 1/16 mm	Mud (Silt, Clay)	Shale, Mudstone

• Difference between conglomerate and breccia: rounded particles for the former and angular for the latter.

Chemical sedimentary rocks:

based on the origin:

• Biochemical origin: e.g. limestone.

• Evaporites: due to evaporation of water and concentration > solubility, e.g. common salt like gypsum.

2-3-2    Lithification:

• Compaction: lose sediments compact tighter, volume reduction.

• Cementation: glue particles together.

2-3-3    Features of sedimentary rocks:

• Particle size: larger => close to source.

• Roundness: angular => close to source.

• Sorting: particle distribution, poor sorting => mixed particle size => close to source.

• Beds: on surface, like ripple, mark, mudcrack, fossils, etc.

2-4    Metamorphic rocks:

Rocks formed, then form changed as indicated by mineral composition and texture.

2-4-1    Classification of metamorphism:

Based on degree of metamorphism:

• Low-grade metamorphism

• High-grade metamorphism

Based on on the area that experienced

• regional metamorphism: larger area.

• contact metamorphism: limited to the area that is in contact with a magma.

2-4-2    Metamorphic agents: Fig 2.20-22

1. Heat: e.g. carbon to graphite.

2. Pressure: e.g. texture changes.

3. Chemical fluid: cementation and recrystallization.

2-4-3    Classification of metamorphic rocks: Fig 2.22

Foliated texture: show layers or bands.

• Slate => phyllite => schist => gneiss, increasing in metamorphism

Nonfoliated texture:

• limestone => marble, sandstone => quartzite, all due to recrystallization.

2-5    Resources from rocks and minerals:

2-5-1    Metallic ores:

formed by hydrothermal solution (hot water that dissolves significant amount of metals).

The solution flow along crack when deposits => form veins. Fig 2.26.

2-5-2    Nonmetallic minerals:

2-5-3    More names for metallic minerals and elements that can be extracted.

2-5-4    Common minerals associated with nonmetallic resources:

Minerals	Use
Apatite	P fertilizer
Calcite	Construction
Kaolinite	Ceramic
Quartz	Glass
Talc	Powder

This Page Was Last Modified On 07-25-02