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
How can igneous rocks be classified?
- According to composition; more particularly silica content
- More siliceous
- Least siliceous
Give examples of more siliceous rock
- Granites
- Rhyolites
Give examples of least siliceous rock
- Gabbros
- Basalts
- This is because these rocks are higher in FeMg
What are found in Mafic and Felsic rocks?
MA: Magnesium Ferric
FEL: Feldspar Silica
Examples of convergent plate boundaries.
- Ocean-ocean
- Ocean-continent
- Continent-continent
Higher silica content + lower lava temperatures = ______________
More viscous lava therefore it flows slower
Lower silica content + higher lava temperatures = ______________
Less viscous lava therefore it flows faster
The more _______ lava claims the more _________ it erupts.
- Gas
- Violently
Basaltic (mafic) lavas
- High FeMg and low silica content (dark in color)
- Erupt at 1000°-1200°
- Highly fluid (can travel at speeds of 100km/hr)
Rhyolitic (felsic) lavas
- Low FeMg and high silica content
- Erupt at 800°-1000°
- Highly viscous, therefore speed is 10x slower than basaltic
- Viscosity causes it to resist flow and build up as thick, bulbous deposits
- Pumice = vesicular variety
Intermediate (andesitic) lavas
- Have an intermediate composition between basalt and rhyolite
- Intrusions of intermediate magma form diorite
List the 3 types of lavas.
- Basaltic (mafic)
- Rhyolitic (felsic)
- Intermediate
How do magmas form?
- Magma is derived from subducting rock, partially undergoes melting bc the minerals it is composed of undergo melting at different temperatures
- Water melts rock faster; it acts as a lubricant
Why can’t everything be magma?
Pressure henny!!!!
Density of melted rock = _______ than density of solid rock with the same _______
- Lower
- Composition
What happens to partial melts?
Rise upwards through pores, along grain boundaries and in any fractures in the overlying rocks
How are large pools of magma chambers formed?
Molten materials mixes with other liquids as it ascends
What makes lava more explosive?
More silica content
Rocks = _________, which means that . . .
- Buoyant
- Able to melt or move through
What happens when magma rises but the temperature decreases? This is known as . . .
- Solid rock may begin to melt without any additional heat
- Decompressional Melting
- Occurs at a certain point
- These produce the greatest volume of molten rock anywhere on earth
Define magmatic differentiation.
- Occurs when different minerals crystallize at different temperatures
- During the crystallization process, the composition of the parent magma changes as it becomes depleted of the chemical elements that comprise the crystals
- First to form cooling magma were last to melt
Why does magma composition change overtime?
- It’s constantly moving/changing, sinks, and so it starts over
- When it begins to cool is when we may be able to get mafic -> felsic and VV
What was Bowen’s reaction series?
- Studied the order of crystallization of the common silicate minerals to form a magma
- Found that they begin to cool and settle out, therefore begin to get cumulates
- Shows that the composition of the magma chamber is changing because crystals present are settling out over time
