Rock Formation and the Rock Cycle

• How are the various rocks formed?

Minerals

• All rocks consist of minerals, which are solid.
• They are composed of one or more chemical elements.
• Properties include crystal form, density, hardness, gloss and color.
• Quartz is the most common in the earth's crust.
• Talc is the softest mineral.
• Diamond is the hardest mineral.

Rocks

• Rocks originate and change through various processes, starting with solidified lava.
• Weathering breaks rocks into small pieces called sediments.
• Water, ice, and wind transport these pieces, known as erosion and sedimentation.
• Loose pieces are compressed into new rock, called diagenesis.
• Cementation (gluing together) and compaction (compressing) play a role.
• Metamorphosis occurs when rock changes due to high pressure and temperature.
• Rocks can melt into magma.
• This entire process repeats, and is called the rock cycle.
• Rocks can be categorized based on their genesis.

Magmatic Rocks

• Magmatic rocks originate from the cooling and solidification of magma.
• They occur at various depths and have different subtypes.
• They can be extrusive or intrusive.

Extrusive Rocks

• An alternate term is effusive rocks.
• They originate on the earth's surface.
• Rapid lava solidification prevents crystal formation.
• Gas bubbles in solidifying magma create cavities in the rock.
• Basalt is fine-grained and dark.
• Obsidian is glassy.
• Rhyolite is light colored and contains a lot of silica (grains).

Pyroclastic Rocks

• Pyroclastic rocks consists of fragments from the volcano itself.
• These include fine, large, and small fragments.
• During an eruption, fragments are flung through the air and land on the earth's surface.
• Volcanic ash consists of particles smaller than 2 mm.
• Lapilli consist of particles between 2-64 mm.
• Volcanic bombs consist of particles larger than 64 mm.

Intrusive Rocks

• These rocks form in the earth's crust.
• Because the earth's internal temperature is so high, magma solidifies very slowly.
• This allows larger crystals to form.
• Larger crystals indicate slower solidification at a deeper level.
• Granite, a plutonic rock, has large crystals.
• Porphyry, a dike rock, has smaller crystals.

Sedimentary Rocks

• Another term is deposits.
• They are classified based on the mode of formation.

Detrital Rock

• Detrital rocks originate from the weathering and erosion of other rocks.
• Carried substances in water crystallize between grains.
• Individual particles can cement together to form solid rocks.
• Loose detrital rocks include gravel which is larger than 2 mm.
• Sand consists of particles ranging between 2 mm and 63 mm.
• Loam consists of particles that measure 63-2mm.
• Clay consists of particles smaller than 2 mm .
• Solid detrital rocks include conglomerate, which contains large rock fragments in a matrix.
• Sandstone grains are palpable and visible.
• Siltstone is hard to distinguish from shale.
• Shale grains are not visible to the naked eye.

Chemical and Biogenic/ Organic Rocks

• These are made up of other chemical elements.
• Chemical rocks are formed by crystallization of dissolved substances in water, such as stalactites in caves.
• They are formed from plant or animal remains.
• Carbonates (CaCO3) such as Limestone are made up of skeletons of sea lilies, corals.
• Dolomite can appear grainy by processing.
• Chalk is made up of coccolithophore skeletons and is very soft.
• Silex (SIO2) is made up of skeletons of radiolarian sponges spicules.
• Peat-lignite-coal is made up of plant material, especially marsh vegetation.

Chemical Rocks

• Evaporites
• Halite (rock salt) components precipitate directly in water upon evaporation, tastes salty.
• Gypsum components precipitate directly in water.

Metamorphic Rocks

• Metamorphic rocks arise from rock that changes due to increased pressure and temperature.
• This process is called metamorphism.
• For example, Gneiss forms from granite while bands are visible.