Extrusive rock refers to the mode of
igneous volcanic rock formation in which hot
magma from inside the
Earth flows out (extrudes) onto the surface as
lava or explodes violently into the
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
to fall back as
pyroclastics or
tuff. In contrast,
intrusive rock
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: Marc ...
refers to rocks formed by magma which cools below the surface.
[Jain, Sreepat (2014). ''Fundamentals of Physical Geology''. New Delhi, India: Springer. .]
The main effect of extrusion is that the magma can cool much more quickly in the open air or under
seawater, and there is little time for the growth of
crystals.
Sometimes, a residual portion of the
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
fails to
crystallize
Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely depo ...
at all, instead becoming a natural glass or
obsidian.
If the magma contains abundant
volatile components which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in
pumice,
scoria
Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) '' ...
, or
vesicular basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
. Other examples of extrusive rocks are
rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...
and
andesite.
Texture
The texture of extrusive rocks is characterized by fine-grained crystals indistinguishable to the human eye, described as
aphantic. Crystals in aphantic rocks are small in size due to their rapid formation during eruption.
Any larger crystals visible to the human eye, called
phenocrysts, form earlier while slowly cooling in the magma reservoir.
When igneous rocks contain two distinct grain sizes, the texture is
porphyritic, and the finer crystals are called the
groundmass
The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded.
The matrix of an igneous rock consists of finer-grained, often microscopic, crystals in which larger crystals, ...
.
The extrusive rocks scoria and pumice have a vesicular, bubble-like, texture due to the presence of vapor bubbles trapped in the magma.
Extrusive bodies and rock types
Shield volcanoes are large, slow forming volcanoes
that erupt fluid basaltic magma that cools to form the extrusive rock
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
. Basalt is composed of minerals readily available in the planet's crust, including
feldspars and
pyroxenes.
Fissure volcanoes pour out low viscosity basaltic magma from
fissure vent
A fissure vent, also known as a volcanic fissure, eruption fissure or simply a fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few metres wide and may be many kilo ...
s to form the extrusive rock basalt.
Composite or
stratovolcanoes
A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and peri ...
often have andesitic magma and typically form the extrusive rock andesite. Andesitic magma is composed of many gases and melted
mantle
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
rocks.
Cinder or scoria cones violently expel lava with high gas content,
and due to the vapor bubbles in this
mafic lava, the extrusive basalt scoria is formed.
Lava domes are formed by high viscosity lava that piles up, forming a dome shape. Domes typically solidify to form the rich in silica extrusive rock
obsidian and sometimes
dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained ( aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyo ...
domes form the extrusive rock dacite, like in the case of
Mount St. Helens
Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United ...
.
Caldera
A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber i ...
s are volcanic depressions formed after an erupted volcano collapses. Resurgent calderas can refill with an eruption of rhyolitic magma to form the extrusive rock rhyolite like the
Yellowstone Caldera
The Yellowstone Caldera, sometimes referred to as the Yellowstone Supervolcano, is a volcanic caldera and supervolcano in Yellowstone National Park in the Western United States. The caldera and most of the park are located in the northwest corne ...
.
Submarine volcano
Submarine volcanoes are underwater vents or fissures in the Earth's surface from which magma can erupt. Many submarine volcanoes are located near areas of tectonic plate formation, known as mid-ocean ridges. The volcanoes at mid-ocean ri ...
es erupt on the ocean floor and produce the extrusive rock pumice.
Pumice is a light-weight glass with a vesicular texture that differs from scoria in its silicic composition and therefore floats.
See also
*
Intrusive suite
An intrusive suite is a group of plutons related in time and space.Glazner, Allen F, Stock, Greg M. (2010). ''Geology Underfoot in Yosemite''. Mountain Press, p. 45. . All rocks in an intrusive suite result from the same magma-producing event.The ...
*
Seamount
A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abr ...
References
External links
Igneous and volcanic textures - images
Igneous petrology
Igneous rocks
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