Lecture 4- Igneous Rocks and Processes

Geology in the news

3.7 Ga fossil found in Greenland
Possibly the oldest life form ever found

Magma

Magma
Molten lava
Underground
Lava
Above ground
Melting process
How to melt Rock
1- temperature
Partial melting
At any given temperature on component will melt and the others will remain solid
B/c different minerals w/ different melting points
Wet melting
If a lot of water is added to the reaction, the water will lower the melting point
Fills up cracks and fractures and crystal structures
Permeate the entire body
Decompression Melting
If pressure is decreased, the melting point is lower
Magma Composition
Gases
SiO2
3 main types
Basaltic Magma
Most common type of magma
Dry magma
Very little water content
SiO2 content 50% (defining component)*
>1100° C (hottest type of magma)
1800° F
Andesitic Magma
Andesite Line aka Ring of Fire
More rare
Gets generated in subduction zones
Magma gets other types of minerals added as it flows to surface, changing the composition
SiO2 60%
Rhyolitic Magma
700°-800° C (lowest)
Shallow
Only found in continental crust
Wet Magma
Higher water content
SiO2- 70% (highest)

Study Hint

Understand how all those properties are linked instead of memorizing

Freezing of Magma
Crystallization
Taking the liquid and turning it into a solid (freezing)
Partial Freezing
Some components are frozen before others
Two things could occur
Equilibrium Crystallization
There’s no change in the chemistry as the magma is frozen
Nothing is added or subtracted
More rare
Fractional Crystallization
Some fraction of the composition changes as it is frozen
Adding or subtracting something
No longer chemically identical to original
More common
Minerals form in a specific order as magma cools
Bowens Reaction Series
Discontinuous Branch
Left hand branch and the stem
As the temperature decreases, a mineral is formed and then stopped and another mineral is formed and then stopped as the temperature changes
Ex: cars
2015 models, then discontinued and in 2016 make 2016 models
Continuous branch
Right hand side
Wide temperature range
Plagioclase is formed throughout the process but the composition is changing
As the temperature changes, substitutions are added
Still same mineral but different composition (changes the ratio)
Continuous reaction
Both discontinuous and continuous branch can happen at same time

Igneous Rocks

Two broad types
Plutonic (Intrusive)
Forming below the surface of the earth
Volcanic (extrusive)
Form above the surface
Two properties needed for ID:
Texture
Size of the mineral crystals
Composition
Which minerals are present
Plutonic Rocks
Magma flows toward surface but doesn’t make it (blocked or cools down)
Plutons
Any large body of plutonic rock
Types
Dikes and Sill
Long pipe-like
Fault like or crack
Dike are vertically oriented
Cut through layers
Sills are Horizontally oriented
In between layers
Laccolith
Magma get stuck as it works it way up
Pimped shaped
Cause rocks above it to warp but never makes way through
May be able to see they exposed at surface later on due to weathering or other
Batholith
Blobby, no distinct shape
Largest bodies
Texture of Intrusive Rocks
Pegmatitic (Pegmatite)
Crystals are very large
Ex: larger than fingernail
Phaneritic
One size notch down from pegmatitic
1cm and smaller
Large still
Composition
Two ways
Identify each mineral w/in rock
Color Index
% of light and dark minerals w/in rock
It’s a shortcut than can fail
Works about 90% of the time
Felsic
Dominated by light color minerals
Intermediate
Grey composition
Mafic
Dominated by brown and black minerals
Ultramafic
Green and yellow minerals
Volcanic Rocks
ID with the same traits- texture and composition
Same conditions apply
Texture (Extrusive Rocks)
Porphyritic
Some grains that are large enough to see and some that are too small to see
Phenocrysts
Large crystals
Below ground
Ground mass
The little stuff that looks homogenous
Above ground
Cooled slowly crating the big crystals and then some lava came up and had to cool fast creating little stuff
Aphanitic
All the crystals are too small to see w/o magnification
Formed rapidly
Glassy
All of the specimens look like they are made of glass
Large reflective surface
Vesicular
Having bubbles or pockets throughout the structure
Tiny craters
A lot of the gas trapped in lava until the last stage of cooling
Has to be many pockets
Volcanoes
Misconception
The US doesn’t have to worry about volcanoes
Japan and Indonesia are the only countries with more volcanoes than the US
Active Volcanoes in US
Hawaii= 7
Alaska= 41
Contiguous 48 States= 20
Case Study
Krakatoa
Indonesian volcano
Aug 26, 1883
200 million tons TNT= 13,000x the yield of the Hiroshima A-bomb (VEI 6)
25 cubic km of ejecta
Heard>3,000 mi away
Air pressure waves circled the globe for 5 days, caused waves in the English channel
Over 30,000 dead, several languages went extinct
2/3 of island destroyed, new volcano built up
Volcano Explosively Index (VEI)
Categorize how explosive a volcanic eruption was
Explosive v Non-explosive
Magma Properties
Viscosity
The ability to flow
Resistance to flow= high viscosity ex: peanut butter
Controlled by two main things
1- Temp
The higher the temp the lower the viscosity, the lower the silica content
2- Silica Content
High silica content -> high viscosity
Controls gas content
High pressure= more dissolved gas
Non-Explosive features
Pahoehoe
Lava is starting to crust over at surface but still soft and squishy right underneath
Aa
Almost finished cooling down
Close to being a solid
Rocky appearance
Vesicles
Tiny holes where magma is escaping at last minute
How fast does lava actually flow
16 km/hr or 10 mph
Explosive Eruptions
Several hazards beyond lava
Lahar
A mud flow (mudslide) that’s associated with a volcano that is erupting or about to erupt
Pyroclasts
All the solid debris that get blown out of volcano
Chunks of rock
Crystals and minerals
Divided by size
Bombs >64mm
Lapilli 64-2mm
Ash <2mm
Pyroclastic Flow
All the solid debris gets trapped inside cloud of volcanic gases that rises and then falls
Can move very quickly
Different types of volcanoes based on type of eruption and erupted material
Shield Volcano
Captain America shield shape
Most common
Basaltic magma
Gives shield shape, so fluid (low viscosity) it can extend over a long distance
Tephra (cinder) cone
Steeper
Small most of the time
Pyriclastic (solid debris) debris
Nothing to help fuse into solid rock and therefore can only make so fall w/o falling or rolling down
Can form around or in the surrounding area of another volcano
Statocones
Composite volcanoes or stratovolcano
Very tall, steep, and mountainous looking
What we think of volcanoes
Higher viscosity
Pyroclastic material that can be fused together by the lava to build up the volcano
Supervolcanic Eruptions
Large enough to have an impact on a global scale
Very few
Tambora (1815) 100 km^3 eject
Upset global weather patterns into the next year
Yellowstone Huckleberry Ridge eruption
2 Ma, 2500 km^3 ejecta
Hot Spots
The Hawaiian Problem
An area below the lithosphere where there is a lot of magma continuously making its way to the surface over time
As the plate moves, it opens up a “vent” for the hotspot that’s rooted deep in the earth