As magma cools underground, it will begin to solidify within Earth's crust. Once completely crystallized, these masses of igneous intrusive rock are referred to as intrusions (because they form from magma that intruded into the crust) or as plutons (after Pluto, Greek god of the underworld). Both terms are used interchangeably by geologists, so take your pick.
dikes - are relatively small igneous intrusions formed as magma penetrates into rock fractures and solidifies into thin, sheet-like bodies. Where dikes are very abundant they are referred to as dike swarms.
three-inch thick dike formed within a larger mass of granite (a batholith), in Joshua Tree National Park; Scott Hooker for scale
dike swarm in ancient metamorphic rocks in Canyon Sin Nombre, Anza Borrego State Park, California; note geology student for scale
swarm of felsic dikes formed within metamorphic rock; Death Valley National Park; Diane Kawahata provides scale
large felsic dike contorted by tectonic forces after the intrusion formed; San Gabriel Mountains, Bridge to Nowhere trail
felsic dike (composed of the igneous intrusive rock granite), cutting across older diorite; San Gabriel Mountains
close view of felsic dike formed within diorite; San Gabriel Mountains
closer view of photo above showing sharp contact between the granite dike and the diorite country rock; San Gabriel Mountains
thick felsic dike; cooling of the intrusion was from the outside in, with pink potassium feldspar crystals forming from the body of magma first, and finally gray quartz crystals forming in the center of the dike as the last bit of magma crystallized - referred to as zonation; San Gabriel Mountains, southern California
felsic (light) and mafic (dark) dikes cutting across granite of the San Gabriel Mountains; Diane Kawahata for scale
sills - are also sheet-like intrusions, like dikes, but they tend to be much thicker, and form as magma intrudes in between layers of sedimentary rock, then cools. Sills are not as common as dikes.
dark-colored, mafic sill formed in between the lighter-colored limestone rocks, Salt River Canyon, Arizona; by Larry Fellows, Arizona Geological Survey
volcanic necks - form as an episode of volcanic activity comes to an end. Magma still trapped within a volcano crystallizes, forming a body of gabbro, diorite or granite that retains the internal shape of the volcano's throat or neck. Given time, the volcano will erode, exposing the intrusive rock in the form of a volcanic neck. These plutons can be very dramatic in their appearance, forming steep, pointed peaks that rise vertically 1000's of feet above the surrounding landscape.
Mt. Thielsen, Oregon is an example of a volcanic neck. The original stratovolcano has eroded away exposing a relatively fine-grained diorite that comprises the neck.
Diane Kawahata approaching the summit of Mt. Thielsen.
Bruce Perry reaches the summit of Mt. Thielsen, with Diamond Lake and the Cascade Mountains in the background.
Shiprock Mountain, New Mexico is a very large volcanic neck. Even from a distance of over 20 miles, it is still an impressive sight.
batholiths - are large plutons lacking a specific shape. They form as huge quantities of magma intrude into country rock, forcefully remove (stope) fragments of country rock and melt their way towards Earth's surface. Some of the magma may work its way to the surface as a volcanic eruption, but most of it remains underground where it slowly cools over a period of hundreds to thousands of years. Large batholiths can be over 100 miles across.
Yosemite Valley is carved into the Sierra Batholith, containing plutonic rocks ranging from granite, to granodiorite, to diorite.
large granitic dome, part of the Sierra Batholith exposed in Sequoyah National Park; note forest observation tower on top of dome
view of some of the California Batholith, exposed in Joshua Tree National Park
view of San Gabriel Mountains granite, part of the California Batholith
Dave Schaar on the high ridge of Mt. Williams, San Gabriel Mountains, California Batholith
origin of batholiths Geologists think that such large bodies of magma are associated with subduction of oceanic crust beneath continental crust. As the oceanic crust subducts, it begins to melt releasing great quantities of intermediate magma that slowly rise toward Earth's surface, intruding into overlying continental crust. During this process, the magma begins to cool and differentiate (see discussion of Bowen's Reaction Series on the Introduction to Igneous Rocks page), and incorporates continental crust elements as well. This forms felsic magma that eventually crystallizes into large bodies of granite in the form of batholiths. The Sierra Nevada Mountains of California are composed of a number of batholiths emplaced during a period of many millions of years, collectively called the Sierra Batholith. The same is true for much of southern California, where the California Batholith extends from the Mojave Desert southward to near the tip of Baja California! This massive series of intergrown batholiths is referred to as the California Batholith. The presence of both the Sierra and California batholiths tell geologists that subduction was occurring from what is now central California all the way down to the tip of what is now Baja California from roughly 150 to 80 million years ago - a grand tale of plate tectonics and igneous intrusive activity!
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During the intrusive process, hot magma is forced into cracks under great pressure. Sometimes, fragments of the original rock present before the intrusive process began, called country rock, are broken off from the country rock and incorporated into the magma. These foreign rocks within the magma, called xenoliths (xeno = foreign, and lith = stone), can either melt into the magma, or remain as solid fragments within the crystallized pluton.
xenoliths within granite; San Gabriel Mountains; triangular xenolith is six inches across
closer view of triangular xenolith from photo above
close view of another xenolith, showing evidence of melting and intrusion of surrounding felsic magma, before magma crystallized into granite
Big Rock climbing wall, Lake Perris, California Batholith; black splotches on granite face are xenoliths
This is a closer view of Big Rock, with climber for scale, near xenoliths which were broken (stoped) from interior of magma chamber as magma intruded into continental crust roughly 100 million years ago. Large fragments broke into smaller fragments, which descended downward into magma, freezing into place before melting or settling to the bottom of the magma body.
This unusual boulder is a glacial erratic (carried by a glacier then dropped far from where it originated) in the high country of Yosemite National Park. Note the rock hammer for scale, the two streamlined xenoliths, and the small dike that cuts across the boulder. This rock has had a very interesting geologic history!
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