L7: Igneous Rocks Flashcards
Define rock
A cohesive aggregate on one or more minerals, which are held firmly together in a solid mass
Intrusive
- Underground; has time to form big crystals
- Magma cools very slowly bc its covered by insulated rocks
Extrusive
- Cools above ground
- Does not have time to form crystals
In what 2 ways can rocks form?
- texture/crystal structure
2. Composition
Phenocryst
Crystals have enough time to form and grow into large grains (aka plutonic or intrusive)
Igneous = __________, sedimentary = ___________
I: Crystals
S: Grains
How are igneous rocks formed?
- Through the crystallization of a cooling magma
- Classified according to texture (intrusive, extrusive)
Define dikes
- Cut across layers of country rock
- Cross-cut the bedding
- Formed when magma forces its way through open fractures
Define sills
- Run parallel to country rocks
- Think of 2 L’s being parallel to one another
- Tubular, sheetlike body formed
- Injection of magma between parallel layers of pre-existing sedimentary rock
Define veins
- Deposits of minerals found within a rock fracture
- Big crystals
Define country rock.
Any rock into which the sill/dike has gone into
Examples of pyroclasts
- Volcanic ash
- Volcanic bomb
- Pumice
Examples of mafic rocks
- Basalt
- Gabbro
- Both differ in crystal size
Examples of felsic rocks
Rhyolite: extrusive
Granite: intrusive
What are batholiths?
- The largest forms of plutons
- They’re intrusive rocks that take on enormous sizes often exceeding thousands of km
Define lava
- Magma that flows quickly to the Earth’s surface still partially or wholly molten
- Cools quickly therefore there is less time for crystallization to take place
- Very fine grained
- Only a glass may be formed due to quick cooling
Volcanic/extrusive rock
Given to an igneous rock formed close to, or at, the Earth’s surface
What is a pyroclast?
- Any volcanic rock ejected into the air
- Viscous, siliceous rhyolitic and andesitic lavas; typically trap more gas
What does pyroclastic mean?
Erupts violently, very quick
Volcanic tufts
Rocks resulting from smaller fragment
Volcanic breccias
Rocks resulting from larger fragments
How are tephra rocks formed?
Pyroclasts that eventually fall and are often lithified together upon cooling
Define lithification
The process by which sediment and soil become rock
What is a porphyry?
- When a melt may occasionally begin to slowly crystallize at depth and grow some large crystals
- May after be subjected to rapid cooling following a volcanic eruption