Download Chapter 4 – Igneous Rocks: Solids from Melts

Igneous Rocks: Solids from Melts
Igneous Rocks are formed from liquid or molten rock
Means to identify igneous rocks: Texture and Minerals
Texture
Are there visible crystals? Are there large visible crystals? Coarse vs. Fine Grained
Where were they formed?
Intrusive Rocks – Plutonic Rocks
1) Molten rock (magma) cooling below the Earth’s surface forms crystalline
rocks that cool slowly, we look for large mineral crystals
2) Country rock – is the existing rock that magma intrudes
Extrusive Rocks – Volcanic Rocks
1) Molten rock (lava) cools at the Earth’s surface forming smooth rocks with
few crystals visible with the unaided eye
2) Types of material from Volcanic Rocks
a) Lava – is the erupted molten rock
b) Pyroclasts – broken shards expelled by violent eruptions
c) Volcanic Ash – the finest Pyroclastic material
d) Tuff – Pyroclastic layers lithified by heat from cooling particles
e) Pumice – Frothy volcanic glass with a high content of vesicles (holes)
f) Obsidian – smooth volcanic glass, often black, dense
g) Porphyry – large visible crystals, visible with the unaided eye, and set
into a fine crystalline matrix
h) Phenocrysts – large visible crystals that formed just prior to the
molten rock material being extruded
Chemical composition and Minerals
Plutonic and Volcanic rock composition
Felsic vs. Mafic
Felsic – feldspar plus silica – light in color
Alkali feldspar – K-spar
Plagioclase feldspar – Ca,Na Feldspar
Muscovite
Mafic – Magnesium and F for Fe Iron – dark in color
Olivine
Pyroxene
Amphibole
Biotite
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Discuss Magma, Composition, Temperature and Rock Type Chart
Magma Chamber - is an open area in the lithosphere where magma sits. We know
magma exists as liquid in these chambers from seismic work done below volcanoes.
Partial Melt – this has been conducted in labs to show that all minerals do not melt
at the same temperature (therefore they didn’t crystallize at the same temperature.)
Only a portion of the magma forms crystals while the rest remains in a liquid state.
Places of Magma Activity
1- Ocean Island Arcs (Mafic and Intermediate – Volcanic)
2- Magma Chamber (Mafic and Intermediate – Plutonic)
3- Mid-Oceanic Ridges (Basaltic – Volcanic)
4- Continental Volcanic Arc (Mafic to Silicic – Plutonic) & 2
Mantle Plumes – are thin pencil-like streams of lava that rise from great depths
allowing hot basaltic magma to rise and erupt far from plate boundaries.
Basaltic Magmas come from :
1) upper mantle beneath mid-ocean ridges
2) mantle plumes
Magmatic differentiation – a uniform parent magma develops a variety of
composition. This occurs because different minerals crystallize at different
temperatures.
Fractional Crystallization – Is where the first crystals in a magma are separated
from the remaining magma.
1) crystals settle to the magma chamber floor
2) crystals are pressed into an area by deformation
The Palisades Sill – This shows fractional crystallization and lead geologists to
examine how magmas can differentiate. Triassic seds above and below, a chilled
zone above and below, plagioclase (no olivine), plagioclase and pyroxene (no
olivine), olivine.
Magmatic Intrusions
Magma chamber – is a large subterranean “room” where magma intrudes and
crystallizes.
Pluton – large igneous body formed at depth
1) wedges open overlying rock
2) breaks off large existing blocks of rock
3) melts surrounding rock
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Granitization – is the melting and recrystallization of rocks to form granites
Batholiths – huge (100 square Km) plutons (magma chambers) of coarse-grained
igneous rocks that cut across rock layers
Stock – are small plutons that cut across rock layers
Dikes – are tabular igneous intrusions that cut across rock layers
Sills – are tabular igneous intrusions that follow along rock layers
Discordant intrusions – they cut across the layers of country rock that they intrude
Batholith, Stock, Dike
Concordant intrusions – follow along the layers of country rock that they intrude
Sill
Veins – deposits of minerals found within a rock fracture that are foreign to the host
rock (different composition).
Pegamtite – veins of extremely coarse-grained granite that cut across a much finer
grained country rock – they are from late stage water-rich magma. They often
contain the exotic mineral suites.
Plate Tectonics
Batholiths form cores for many mountain ranges (like the Sierra Nevada
Mountains) that are formed by converging plates (Pacific Plate and the North
American Plate)
Mention Juan de Fuca plate and the Cascade Range
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