Laccolith
Encyclopedia
A laccolith is a sheet intrusion
(or concordant pluton
) that has been injected between two layers of sedimentary rock
. The pressure of the magma
is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.
Laccoliths tend to form at relatively shallow depths and are typically formed by relatively viscous magmas, such as those that crystallize to diorite
, granodiorite
, and granite
. Cooling underground takes place slowly, giving time for larger crystals to form in the cooling magma. The surface rock
above laccoliths often erodes away completely, leaving the core mound of igneous rock
. The term was first applied as laccolite by Grove Karl Gilbert
after his study of intrusions of diorite
in the Henry Mountains
of Utah
in about 1875.
It is often difficult to reconstruct shapes of intrusions. For instance, Devils Tower
in Wyoming
was thought to be a volcanic neck, but study has revealed it to be an eroded laccolith. The rock would have had to cool very slowly so as to form the slender pencil-shaped columns of phonolite
porphyry
seen today. However, erosion has stripped away the overlying and surrounding rock, and so it is impossible to reconstruct the original shape of the igneous intrusion; that rock may not be the remnant of a laccolith. At other localities, such as in the Henry Mountains
and other isolated mountain ranges of the Colorado Plateau
, some intrusions demonstrably have shapes of laccoliths. The small Barber Hill syenite
-stock laccolith in Charlotte, Vermont
, has several volcanic trachyte
dikes associated with it. Molybdenite
is also visible in outcrops on this exposed laccolith. In Big Bend Ranch State Park
, at the southwesternmost visible extent of the Ouachita orogeny
, lies the Solitario
. It consists of the eroded remains of a laccolith, presumably named for the sense of solitude
that observers within the structure might have, due to the partial illusion
of endless expanse in all directions.
There are many examples of possible laccoliths on the surface of the Moon
. These igneous features may be confused with impact crater
ing.
Sheet intrusion
A sheet intrusion is where a mass of molten magma takes advantage of a pre-existing linear feature in a host rock, such as a long rupture or fault, and forces its way into these spaces...
(or concordant pluton
Pluton
A pluton in geology is a body of intrusive igneous rock that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies...
) that has been injected between two layers of sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
. The pressure of the magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
, granodiorite
Granodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
, and granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
. Cooling underground takes place slowly, giving time for larger crystals to form in the cooling magma. The surface rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
above laccoliths often erodes away completely, leaving the core mound of igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
. The term was first applied as laccolite by Grove Karl Gilbert
Grove Karl Gilbert
Grove Karl Gilbert , known by the abbreviated name G. K. Gilbert in academic literature, was an American geologist....
after his study of intrusions of diorite
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
in the Henry Mountains
Henry Mountains
The Henry Mountains are located in the southeastern portion of the U.S. state of Utah and run in a generally north-south direction, extending over a distance of about 30 miles . They were named by John Wesley Powell in honour of Joseph Henry, the first secretary of the Smithsonian Institution. The...
of Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
in about 1875.
It is often difficult to reconstruct shapes of intrusions. For instance, Devils Tower
Devils Tower National Monument
Devils Tower is an igneous intrusion or laccolith located in the Black Hills near Hulett and Sundance in Crook County, northeastern Wyoming, above the Belle Fourche River...
in Wyoming
Wyoming
Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
was thought to be a volcanic neck, but study has revealed it to be an eroded laccolith. The rock would have had to cool very slowly so as to form the slender pencil-shaped columns of phonolite
Phonolite
Phonolite is a rare igneous, volcanic rock of intermediate composition, with aphanitic to porphyritic texture....
porphyry
Porphyry (geology)
Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are called phenocrysts...
seen today. However, erosion has stripped away the overlying and surrounding rock, and so it is impossible to reconstruct the original shape of the igneous intrusion; that rock may not be the remnant of a laccolith. At other localities, such as in the Henry Mountains
Henry Mountains
The Henry Mountains are located in the southeastern portion of the U.S. state of Utah and run in a generally north-south direction, extending over a distance of about 30 miles . They were named by John Wesley Powell in honour of Joseph Henry, the first secretary of the Smithsonian Institution. The...
and other isolated mountain ranges of the Colorado Plateau
Colorado Plateau
The Colorado Plateau, also called the Colorado Plateau Province, is a physiographic region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. The province covers an area of 337,000 km2 within western Colorado, northwestern New Mexico,...
, some intrusions demonstrably have shapes of laccoliths. The small Barber Hill syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
-stock laccolith in Charlotte, Vermont
Charlotte, Vermont
Charlotte is a town in Chittenden County, Vermont, United States. The town was named for Sofia Charlotte of Mecklenburg-Strelitz, youngest daughter of Duke Charles Louis Frederick of Mecklenburg-Strelitz.The population was 3,569 at the 2000 census....
, has several volcanic trachyte
Trachyte
Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture. The mineral assemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present....
dikes associated with it. Molybdenite
Molybdenite
Molybdenite is a mineral of molybdenum disulfide, MoS2. Similar in appearance and feel to graphite, molybdenite has a lubricating effect that is a consequence of its layered structure. The atomic structure consists of a sheet of molybdenum atoms sandwiched between sheets of sulfur atoms...
is also visible in outcrops on this exposed laccolith. In Big Bend Ranch State Park
Big Bend Ranch State Park (Texas)
Big Bend Ranch State Park is a state park located on the Rio Grande in Brewster County and Presidio County, Texas. It is the largest state park in Texas. The closest major town is Presidio, Texas, where the state park's head office is located.-Features:...
, at the southwesternmost visible extent of the Ouachita orogeny
Ouachita orogeny
The Ouachita orogeny was a mountain building event that resulted in the folding and faulting of strata currently exposed in the Ouachita Mountains...
, lies the Solitario
Solitario
The Solitario is a large geologic formation in Big Bend Ranch State Park in West Texas. Viewed from above, it suggests an impact crater. In fact, it is the eroded remains of a laccolith. The approximate center of the Solitario is located east southeast of Presidio, Texas, just west of the line...
. It consists of the eroded remains of a laccolith, presumably named for the sense of solitude
Solitude
Solitude is a state of seclusion or isolation, i.e., lack of contact with people. It may stem from bad relationships, deliberate choice, infectious disease, mental disorders, neurological disorders or circumstances of employment or situation .Short-term solitude is often valued as a time when one...
that observers within the structure might have, due to the partial illusion
Illusion
An illusion is a distortion of the senses, revealing how the brain normally organizes and interprets sensory stimulation. While illusions distort reality, they are generally shared by most people....
of endless expanse in all directions.
There are many examples of possible laccoliths on the surface of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
. These igneous features may be confused with impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
ing.
See also
- BatholithBatholithA batholith is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the Earth's crust...
- DikeDike (geology)A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
- HydrolaccolithPingoA pingo, also called a hydrolaccolith, is a mound of earth-covered ice found in the Arctic and subarctic that can reach up to in height and up to in diameter. The term originated as the Inuvialuktun word for a small hill. A pingo is a periglacial landform, which is defined as a nonglacial...
- LopolithLopolithA lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body...
- SillSill (geology)In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
- Stock