Igneous Rocks

 MiraCosta College

Igneous rocks are classified in a variety of ways:

1) by their origin: the place where molten material (magma or lava) crystallized below, or at or near the surface.
  • Intrusive igneous rocks crystallize slowly in magma chambers or plutons deep underground and have a crystalline texture (visible crystals).
  • Extrusive igneous rocks when lava cools and crystallizes quickly at or near the surface.
2) by their mineral components: based on ultramafic, mafic, intermediate, or felsic mineral composition.
3) by their appearance (color shades and rock textures) if the mineral crystals are too small to idendify, and if they preserve characteristics unique to how they formed.
4) a mix of all of the above. Some volcanic rocks contain phenocrysts which are crystals that crystallize deep below and are carried to the surface and imbedded in extrusive igneous rocks, such as a porphyry.

Note there are a variety of igneous rock classifications that are much more technical than the simplified version presented here.

Click on images for a larger view.

Intrusive Igneous Rocks
two green crystaline rocks with a black surface coating of basaltic lava
Peridotite 21		 a gray rock with a crystaline texture
Chromite peridotite 10		 a black rock with glistening crystal surfaces of horneblende and pyroxene
Pyroxenite 22		 Pyroxenite harzburgite
Pyroxenite gabbro 38		 dark gray crystaline rocks with mall white and black crystals
Gabbro 32
dark gray crystaline rocks with mall white and black crystals
Gabbro 13		 a gray crystalline rock with small light gray and black crystals
Diorite 35
Diorite (layered) 40		 Granodiorite beach cobble.
Granodiorite 37		 a crystalline rock displaying smal white, gray, and black crystals like salt and pepper
Tonalite 27
a light colored crystalline rock display small to medium sized crystals of pink, white, gray and black.
Granite 15		 Syenite granite pegmatite
Syenitic granite 45		 Quartzolite
Quartzolite (granitoid) 43		 a white vein of fine white crystalline quartz cutting throug a darker crystalline rock
Aplite 33		 a chunk of a white crystalline milky quartz
Alaskite 16
a rock composed of large black and white mineral crystals of hornblende and feldspar
Gabbro pegmatite 12		 a chunk of crystalline rock with large pink, white, and gray crystals
Granite pegmatite 14		 a chunk of crystalline rock with large pink, white, and black crystals
Tourmaline pegmatite 18
 a chunk of crystalline rock with large pink and green, and white crystals
Tourmaline pegmatite 19		 a chunk of crystalline rock with large pink, white, and silver-gray crystals
Pegmatite 20

Extrusive Igneous Rocks
a dark gray, rugged rock with no visible crystals.
Basalt 9		 a dark rock with now visible crystals, but is full of bubbles (vescicles)
Basalt scoria 8		 a rock made of black angular chunks of basalt surrounded by a white bubbly lava matrix.
Basalt tuff/scoria 4		 a dark basalt rock with lots of bubbles and small green peridot crystal
Basalt porphyry 28		 a black rock with a glassy surface appearance
Obsidian 17
a smooth, uniform gray rock with no visible crystals
Andesite 7		 a purplish brown rock displaying flow banding like stretched taffy
Andesite 5		 a greenish gray rock displaying a mix of smaller rock fragments. No visible crystals.
Andesite tuff 2		 a gray round cobble of volcanic rock with vissible pale blue, gray, and pink phenocrysts (crustals)
Andesite porphyry 6		 a rugged chunk of brown lava rock made up of angular fragments stuck together
Welded tuff 29
a medium gray lava rock with no visible crystals, with some flow banding
Dacite 11		 a gray rock with a frothy (bubbly) surface texture like a sponge
Dacite pumice 24		 an orange/brown smooth rock with concoidal surface fracture, no visible crystals.
Rhyolite 26		 a cobble with small white and black crystals (phenocrysts) imbedded in a pinkish lava rock
Dacite porphyry 14		 a frothy red lava rock with lots of bubbles
Rhyolite scoria 25
Rhyolite porphyry, red rock with white sprinkles of quartz and feldspars
Rhyolite porphyry 36
Dacite (flow banded) 39
Rhyolite tuff
Rhyolite tuff 41		 pahoehoe lava rock
Pahoehoe (lava rock) 42
Sulfur vent deposit 1
See Introduction to Physical Geology: Chapter 7 - Igneous Rocks and Processes More rocks may be added as they become available.
https://gotbooks.miracosta.edu/rocks/igneous_rocks.html 5/2/2022