Igneous Rocks Flashcards
Silicic minerals
Quartz
Orthoclase
Plagioclase
Muscovite
Mafic minerals
Biotite
Hornblende
Augite
Olivine
What is Acid/silicic magma?
> 66% SiO
C-O
most explosive
What is Intermediate magma?
55-65% SiO
O-O
What is Basic/Mafic magma?
45-55% SO
Mid-plate got spits and oceanic ridges
What is Ultrabasic/Ultramafic magma?
<44% SiO
Asthenosphere
Least explosive
Grain size
Coarse- >5.0mm
Medium- 0.1-5.0mm
Fine- <0.1mm
Example of a silicic rock?
Granite
Examples of mafic/basic rocks?
Gabbro
Dolerite
Basalt
Example of an Ultra mafic/ Ultrabasic rock?
Peridotite
What is holocrystalline?
Rocks that are wholly crystalline I.e entirely composed of silicate crystals
What is glassy?
Rocks that are composed of natural glass such as obsidian. This texture develops when lava cools very quickly and is unable to form crystals
What is porphyritic?
The rock contains large crystals called phenocrysts set in a matrix of finer crystals.
Phenocrysts form first at higher temperatures where they were unrestricted.
The matrix formed lager when magma had cooled at a lower temperature
What is euhedral?
Crystals that show well developed crystal faces. They crystallise first at higher temperatures and were unrestricted. Usually phenocrysts.
What is anhedral?
Crystals that show poorly developed crystal faces. They formed at the late stage at lower temperatures so their development was restricted
What is groundmass?
Mosaic of small crystals that form in the spaces left behind by earlier formed crystals
What is granular?
Rocks composed of similar sized silicate crystals. This texture is formed when all the crystals begin to form at the same temperature and grew at the same rate
What is vesicular?
Holes seen in lava rocks that are created by escaping gas bubbles
What is fragmental tuff?
Volcanic rocks composed of angular rock, mineral and ash that has been blasted out of a volcano following an eruption
Granite
Crystalline Coarse Plagioclase Feldspar showing cleavage Anhedral Biotite Euhedral crystals of Orthoclase Feldspar Quartz
Gabbro
Coarse grained >5mm Very slow cooling Basic magma Magma chamber
Dolerite
Medium grained 0.1- 5mm Moderately slow grained Basic magma Small and shallow intrusions Dyke or sill
Basalt
Fine grained
<0.1mm
Rapid cooling
Surface lava
Pillow lava
Ball shaped masses Eruption of lava in water 1m in size Kidney bean shaped Upper convex surface Lower concave surface
Columnar jointing
Six sided columns of igneous rocks created by contracting magma or lava during cooling
Amygdales
Secondary minerals that infill vesicles
Pahoehoe
Ropey structure seen on the surface of lava flow caused by compression of surface lava in ridges as it is slowing more slowly that the lava below
Aa
A block structure seen on the surface of lava. Formed by the rapid loss of gas from the lava as it erupts
Dyke
Sheet of igneous rock that is formed when magma is injected into a crack
It is discordant Few cms to a few kms Creates baked margins Medium grained Fine grained chilled margins
Will
Magma is injected along a bedding plane
Concordant
