IGNEOUS ROCKS

Formed by the cooling and solidification of magma

Classification of Igneous Rocks
COMPOSITION

TEXTURE Felsic Intermediate Mafic Ultramafic

Phaneritic Granite Diorite Gabbro Peridotite
Aphanitic Rhyolite Andesite Basalt

Vesicular Pumice Scoria Scoria

Glassy Obsidian

**Classification of Igneous Rocks**
	COMPOSITION
TEXTURE	Felsic	Intermediate	Mafic	Ultramafic
Phaneritic	Granite	Diorite	Gabbro	Peridotite
Aphanitic	Rhyolite	Andesite	Basalt
Vesicular	Pumice	Scoria	Scoria
Glassy	Obsidian

PART I - Volcanic (Extrusive) Rocks

BASALT

Basalt is an extrusive mafic volcanic rock. It is commonly erupted as passive lava flows due to its low viscosity (resulting from high magmatic temperatures and low silica contents). A dominant component of oceanic crust, basalt is the most abundant volcanic rock.

Texture: aphanitic

Composition: mafic (enriched in iron and magnesium)

Color: black, dark gray or dark brown

Cooling Rate: fast, extrusive

Intrusive Equivalent: gabbro

Other Characteristics: weathers to red-brown, hematite/limonite/clay mixture

ANDESITE

Andesite is an extrusive volcanic rock with a composition that is intermediate between basalt and rhyolite. Andesite magma is characterized by intermediate temperatures and viscosities resulting in both passive (lava) and violent (pyroclastic) eruptive styles.

Texture: aphanitic

Composition: intermediate

Color: gray

Cooling Rate: fast, extrusive

Intrusive Equivalent: diorite

Other Characteristics: found as both lava and pyroclastic tuff

RHYOLITE

Rhyolite is an extrusive felsic (silicic) volcanic rock. The high silica content and lower temperatures of rhyolite magma result in high viscosities and violent eruptive styles. Rhyolite is rarely found as lava. Instead rhyolite tuff and ash falls are more common.

Texture: aphanitic

Composition: felsic (silicic), enriched in silica and depleted in iron and magnesium

Color: light gray or pink

Cooling Rate: fast, extrusive

Intrusive Equivalent: granite

Other Characteristics: tuffs commonly contain pumice and rock fragments

OBSIDIAN

Obsidian is an extrusive volcanic rock formed by the rapid cooling of high viscosity magma. It is recognized by its glassy texture. Silicic magmatic compositions and fast cooling rates prevent the formation of crystals even at the microscopic scale.

Texture: aphanitic, glassy

Composition: intermediate (andesitic) to felsic (rhyolitic)

Color: black or dark brown

Cooling Rate: rapid, extrusive

Intrusive Equivalent: granite

Other Characteristics: conchoidal (curved, glass-like) fractures

PUMICE

Pumice is a low-density, light-colored felsic volcanic rock. It commonly forms as a frothy upper surface on silicic lavas.

Texture: aphanitic and vesicular (contains abundant small gas cavities)

Composition: felsic (rhyolitic)

Color: white to light-gray or light-tan

Cooling Rate: rapid, extrusive

Intrusive Equivalent: granite

Other Characteristics: very light and will float on water

SCORIA

Scoria is a dark-colored extrusive volcanic rock with abundant large vesicles. It commonly forms as a bubble-rich upper surface on many basalt and andesite lavas.

Texture: aphanitic and vesicular (contains abundant large gas cavities)

Composition: intermediate (andesitic) to mafic (basaltic)

Color: black or dark brown

Cooling Rate: rapid, extrusive

Intrusive Equivalent: diorite or gabbro

Other Characteristics: vesicular like pumice, but denser and darker with larger vesicles

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