UA geology exam 2 (Igneous and volcanos)
glassy texture
- Very rapid cooling of lava - Resulting rock called obsidian
dike geology
- tabular, discordant pluton
pegmatitic texture
-Exceptionally coarse grained (> 1 cm) -Form in late stages of crystallization of magmas
pyroclastic texture
-Fragmental appearance produced by violent volcanic eruptions
Batholith
-Largest intrusive body - Surface exposure > 100+ km2 (smaller bodies termed stocks) - Cores of mountains
The explosivity of a volcano is determined by _________.
1) Composition (% silica) 3) Temperature 3) Dissolved gases (volatiles)
what are the three components of magma
1) Liquid portion = melt 2) Solids, if present, silicate minerals 3) Volatiles = dissolved gases in melt, e.g. water vapor (H2O), carbon dioxide (CO2)
Evolution of magma
1. magmatic differentiation 2. assimilation 3. magma mixing
Igneous rocks that cool fast tend to exhibit ________ texture.
A. Aphanitic - not visible crystals
______ igneous rocks cool relatively slow exhibiting ________ texture
A. Intrusive; Phaneritic
lava tubes
Active lava tube, lava flows faster in insulated tube 50 km/hr
Higher silica (SiO2 ) content in magma can be associated with:
B. Increased viscosity
Bowen's Reaction Series
Both branches in operation as magma cools One mineral changes to another over specific temp. At biotite & Na-rich plag. point, remaining magma depleted in Fe, Mg, Ca, & Na remaining magma enriched in K, Al, & Si
Which two felsic rock types have the same composition but formed differently (i.e. intrusive / extrusive)?
C. Andesite/ diorite
What is an igneous rock?
Form as molten rock cools and solidifies Magma
Assimilation
Incorporation of surrounding rock bodies into magma
columnar joints
Lava contracts as cools, open up fractures Form polygonal (6- sided) fractures Polygonal fractures extend downward into flow
Laccolith
Lens shaped mass (not large as batholith) - Arches overlying strata upward
magma mixing
Mixing of 2 chemically distinct magmas
Aphanitic
Rapid cooling- mineral nuclei form quicker than rate of growth-- results in fine grained rock extrusive
magmatic differentiation
Separation of melt from earlier formed crystals (solid)
What is the most common type of rock forming mineral group?
Silicates (i.e. Quartz, Augite)
Most of the rocks making up the Earth's crust are composed dominantly of what elements?
Silicon and Oxygen
Phaneritic
Slow cooling- rate of growth exceeds rate of nuclei formation—results in coarse-grained rock intrusive
Stock (geology)
a smaller, remnant of a batholith
phaneritic texture
coarse grained Slow cooling -Large, visible crystals
Mafic Composition
dark in color (even more iron and magnesium) - Ferromagnesian silicates & Ca-rich feldspar - Silica poor (45-52%) - magnesium and ferrum, for iron - Comprise ocean floor & volcanic islands
What is lava?
extrusive rocks or volcanic rocks
slow rate of cooling
fewer but larger crystals
aphanitic texture
fine grained Rapid rate of cooling -Microscopic crystals
low viscosity
flows easily (water)
high viscosity
flows slowly (honey)
very fast rate of cooling
forms glass
factors generating magma from solid rock
heat pressure volatiles
felsic magmas
high silica content and highly viscous (i.e. honey)
What is magma?
intrusive rocks or plutonic rocks
Felsic Composition
light in color - No ferromagnesian silicates , feldspar and silica - Silica rich (SiO2 ) (>65%) - Major constituent of continental crust
mafic magma
lower silica content & more fluid-like behavior (less viscous) (i.e. water)
fast rate of cooling
many small crystals
intermediate composition
medium in color (silica rich & iron poor) - No ferromagnesian silicates , feldspar and silica - Silica rich (SiO2 ) (>65%) - Major constituent of continental crust
viscocity
resistance to flow
What is texture?
size, shape, and arrangement of minerals in a rock.
sill
tabular, concordant pluton (e.g., Palisades Sill in New York)
what is the most important factor in crystal size
the rate of cooling
Ultramafic composition
very dark in color most iron/magnesium) - Rare, high in Mg and Fe - Entirely ferromagnesian silicates - <45% silica
Pahoehoe lava
(ropey texture) Low viscosity
aalava
(rough, blocky texture) Higher viscosity
dissolved gasses
- Gases expand near Earth's surface due to decreasing pressure - Explosivity of eruption related to how easily gases escape!! **More Silica = gases don't escape as easily!! Pressure buildup! Boom!
porphyritic texture
- Indicates 2 stages of cooling -Large crystals (phenocrysts) embedded in matrix of smaller crystals (groundmass)