IGNEOUS ROCKS
Igneous rocks are composed of interlocking mineral grains (and often
glass particles and rock fragments) which crystallized from magmas and
lavas.
They are described and classified on the basis of the textures (sizes
and shapes) and the compositions of these components.
IGNEOUS ROCK TEXTURES
Phaneritic textures:
- mineral grains are coarse (> 1 mm) and clearly visible to the unaided
eye
- pegmatitic: mineral grains are extremely coarse (> 1 cm)
- indicative of slow cooling of magma
- common to plutonic rocks
Aphanitic textures:
- mineral grains are too fine (< 1 mm) to be seen with the unaided
eye
- indicates rapid cooling of magma
- common to volcanic rocks and the chilled margins of intrusive rocks
Porphyritic textures:
- phaneritic phenocrysts embedded in a groundmass of aphanitic
grains
- indicative of multistage cooling history:
- a. long period of slow partial cooling in a pluton phenocrysts....
- b. sudden extrusion of the remaining magma and these phenocrysts.....
- c. rapid crystallization of remaining magma groundmass
Pyroclastic textures:
- consist of mineral grains, volcanic glass, and rock fragments hurled
into air by explosive volcanic eruptions
- grains settle to the ground or sea floor downwind from volcanic source
ROCK COMPOSITIONS
Four classes of igneous rocks can be defined on the basis of mineral
composition and color:
- sialic (or felsic)
- intermediate
- mafic
- ultramafic
Each class contains several rock types that are differentiated on the
basis of their textures.
IGNEOUS ROCK TYPES
Sialic rocks:
- rich in quartz, K-spars and Na-plagioclase,
- as well as micas and amphiboles
- typically white, gray, red, and pink
- most common varieties:
- granite: phaneritic and pegmatitic
- rhyolite: aphanitic and porphyritic
Intermediate rocks:
- rich in pyroxenes, Na-Ca plagioclases, and amphiboles,
- typically gray, green, and brown
- most common varieties:
- diorite: phaneritic
- andesite: aphanitic, porphyritic, glassy
Mafic rocks:
- rich in Ca-plagioclases, olivines, and pyroxenes
- typically gray, green, brown, and black
- most common varieties:
- gabbro: phaneritic
- basalt: aphanitic, porphyritic, glassy, and vesicular
Ultramafic rocks:
- rich in olivines and pyroxenes
- typically green in color
- most common variety:
- peridotite: phaneritic
- no aphanitic equivalent
OTHER IGNEOUS ROCKS
Obsidian:
- composed of massive volcanic glass
- black, with red, yellow, brown streaks
- glassy luster and conchoidal breakage
Pumice and scoria:
- porous vesicular volcanic rocks
- pumice is light colored and rhyolitic and
- andesitic in composition
- scoria is dark colored and mafic
Volcanic breccia:
- composed of coarse pyroclastic grains (blocks, bombs, and cinders)
- brown and grayish-brown in color
Volcanic tuff:
- composed of poorly consolidated ash
- glassy in composition
- light weight, soft, friable rocks
- white, gray and yellow in color
IGNEOUS ROCK BODIES
The three types of igneous rock bodies are intrusive plutons
and surface volcanoes and lava plateaus.
Plutons are differentiated on the basis of their size, shape, and relationship
to their country rocks:
- concordant: pluton intrudes parallel to structure of country
rocks
- discordant: pluton cuts across structure
Volcanoes and lava plateaus are differentiated on the basis of their
morphology (shape).
PLUTONS
Batholiths and stocks:
- the largest of all plutons:
- batholiths: surface area > 100 sq km
- stocks: surface area < 100 sq km
- generally discordant
- typically composed of granite
- often form massive domes
- example: Enchanted Rock, Texas
Smaller plutons:
1. dikes:
- tabular shapes
- discordant
2. sills
- tabular shapes
- concordant
3. laccoliths:
- mushroom shapes
- flat bases and domed tops
VOLCANOES
Volcanoes are mountains that are composed of extrusive rocks, including
both lava flows and pyroclastic rocks.
There are three morphologic types:
- shield volcanoes
- cinder cones
- stratovolcanoes (or composite cones)
LAVA PLATEAUS
Lava plateaus:
- broad flat-lying plains underlain by many stacked layers of flood
basalts
- basalt flows extruded from linear fissures
example: the Columbia Plateau, which extends over 200,000 sq km and
reaches a maximum thickness of 3000 m
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