Editing Laccolith (section)

==Description==
A laccolith is a type of [[igneous intrusion]], formed when [[magma]] forces its way upwards through the [[Earth's crust]] but cools and solidifies before reaching the surface. Laccoliths are distinguished from other igneous intrusions by their dome-shaped upper surface and level base. They are assumed to be fed by a conduit from below, though this is rarely exposed.<ref name=PhilpottsAgue2009_93_94>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |pages=93–94}}</ref><ref>{{Cite book|last=Leong|first=Goh Cheng|url=https://books.google.com/books?id=XhJ4RAAACAAJ&q=certificate+physical+and+human+geography|title=Certificate Physics And Human Geography; Indian Edition|date=1995-10-27|publisher=Oxford University Press|isbn=978-0-19-562816-6|pages=20|language=en}}</ref> When the host rock is volcanic, the laccolith is referred to as a [[cryptodome]].<ref>{{Cite journal|last1=Burchardt|first1=Steffi|last2=Mattsson|first2=Tobias|last3=Palma|first3=J. Octavio|last4=Galland|first4=Olivier|last5=Almqvist|first5=Bjarne|last6=Mair|first6=Karen|last7=Jerram|first7=Dougal A.|last8=Hammer|first8=Øyvind|last9=Sun|first9=Yang|date=2019-08-14|title=Progressive Growth of the Cerro Bayo Cryptodome, Chachahuén Volcano, Argentina—Implications for Viscous Magma Emplacement|journal=Journal of Geophysical Research: Solid Earth|volume=124|issue=8|pages=7934–7961|doi=10.1029/2019JB017543|bibcode=2019JGRB..124.7934B|issn=2169-9313|doi-access=free}}</ref> Laccoliths form only at relatively shallow depth in the crust,{{sfn|Philpotts|Ague|2009|p=95}} usually from [[intermediate composition]] magma, though laccoliths of all compositions from [[silica]]-poor [[basalt]] to silica-rich [[rhyolite]] are known.{{sfn|Philpotts|Ague|2009|p=94}}

A laccolith forms after an initial [[sheet intrusion|sheet-like intrusion]] has been injected between layers of [[sedimentary rock]]. If the intrusion remains limited in size, it forms a [[Sill (geology)|sill]], in which the strata above and below the intrusion remain parallel to each other and the intrusion remains sheetlike. The intrusion begins to lift and dome the overlying strata only if the radius of the intrusion exceeds a critical radius, which is roughly:{{sfn|Philpotts|Ague|2009|loc=equation 4.4, p.94}}

:<math>r \ge \frac{2T\tau}{P_m-P_l}</math>

where <math>P_m</math> is the pressure of the magma, <math>P_l</math> is the [[lithostatic pressure]] (weight of the overlying rock), <math>T</math> is the thickness of the overlying rocks, and <math>\tau</math> is the shear strength of the overlying rock. For example, in the [[Henry Mountains]] of [[Utah]], US, the geologist [[Grove Karl Gilbert]] found in 1877 that sills were always less than {{convert|1|km2|sigfig=1|sp=us}} in area while laccoliths were always greater than 1 square kilometer in area. From this, Gilbert concluded that sills were forerunners of laccoliths. Laccoliths formed from sills only when they became large enough for the pressure of the [[magma]] to force the overlying strata to dome upwards. Gilbert also determined that larger laccoliths formed at greater depth.<ref name=PhilpottsAgue2009_93_94/> Both laccoliths and sills are classified as ''concordant'' intrusions, since the bulk of the intrusion does not cut across host rock strata, but intrudes between strata.{{sfn|Philpotts|Ague|2009|pp=79-80}}

More recent study of laccoliths has confirmed Gilbert's basic conclusions, while refining the details. Both sills and laccoliths have blunt rather than wedgelike edges, and sills of the Henry Mountains are typically up to {{convert|10|m||sp=us}} thick while laccoliths are up to {{convert|200|m||sp=us}} thick.<ref>{{cite journal |last1=Johnson |first1=Arvid M. |last2=Pollard |first2=David D. |title=Mechanics of growth of some laccolithic intrusions in the Henry mountains, Utah, I |journal=Tectonophysics |date=July 1973 |volume=18 |issue=3–4 |pages=261–309 |doi=10.1016/0040-1951(73)90050-4}}</ref> The periphery of a laccolith may be smooth, but it may also have fingerlike projections consistent with [[Rayleigh-Taylor instability]] of the magma pushing along the strata.<ref>{{cite journal |last1=Pollard |first1=David D. |last2=Muller |first2=Otto H. |last3=Dockstader |first3=David R. |title=The Form and Growth of Fingered Sheet Intrusions |journal=GSA Bulletin |date=1 March 1975 |volume=86 |issue=3 |pages=351–363 |doi=10.1130/0016-7606(1975)86<351:TFAGOF>2.0.CO;2|bibcode=1975GSAB...86..351P }}</ref> An example of a fingered laccolith is the [[Shonkin Sag]] laccolith in [[Montana]], US.{{sfn|Philpotts|Ague|2009|p=95}} The critical radius for the sill to laccolith transition is now thought to be affected the viscosity of the magma (being greater for less viscous magma) as well as the strength of the host rock. A modern formula for the shape of a laccolith is:
[[File:Laccolith shape.jpg|thumb|Idealized laccolith shape]]

:<math>z = \frac{3(P_m-\rho_cgT)}{16BT^3}(r_0^2-r^2)^2</math>

where <math>z</math> is the height of the laccolith roof, <math>g</math> is the acceleration of gravity, <math>B</math> is the elastic modulus of the host rock, <math>r</math> is the horizontal distance from the center of the laccolith, and <math>r_0</math> is the outer radius of the laccolith.{{sfn|Philpotts|Ague|2009|p=95}} Because of their greater thickness, which slows the cooling rate, the rock of laccoliths is usually coarser-grained than the rock of sills.{{sfn|Philpotts|Ague|2009|p=94}}

The growth of laccoliths can take as little as a few months when  associated with a single magma injection event,<ref>{{Cite journal|last1=Castro|first1=Jonathan M.|last2=Cordonnier|first2=Benoit|last3=Schipper|first3=C. Ian|last4=Tuffen|first4=Hugh|last5=Baumann|first5=Tobias S.|last6=Feisel|first6=Yves|date=December 2016|title=Rapid laccolith intrusion driven by explosive volcanic eruption|journal=Nature Communications|volume=7|issue=1|pages=13585|doi=10.1038/ncomms13585|issn=2041-1723|pmc=5123016|pmid=27876800|bibcode=2016NatCo...713585C}}</ref><ref>{{Cite journal|last1=Mattsson|first1=Tobias|last2=Burchardt|first2=Steffi|last3=Almqvist|first3=Bjarne S. G.|last4=Ronchin|first4=Erika|date=2018-02-05|title=Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards|journal=Frontiers in Earth Science|volume=6|pages=5|doi=10.3389/feart.2018.00005|bibcode=2018FrEaS...6....5M|issn=2296-6463|doi-access=free|hdl=10023/20984|hdl-access=free}}</ref> or up to hundreds or thousands of years by multiple magmatic pulses stacking [[Sill (geology)|sills]] on top of each other and deforming the host rock incrementally.<ref>{{Cite journal|last1=Morgan|first1=Sven|last2=Stanik|first2=Amy|last3=Horsman|first3=Eric|last4=Tikoff|first4=Basil|last5=de Saint Blanquat|first5=Michel|last6=Habert|first6=Guillaume|date=April 2008|title=Emplacement of multiple magma sheets and wall rock deformation: Trachyte Mesa intrusion, Henry Mountains, Utah|journal=Journal of Structural Geology|volume=30|issue=4|pages=491–512|doi=10.1016/j.jsg.2008.01.005|bibcode=2008JSG....30..491M}}</ref>

Over time, erosion can form small hills and even mountains around a central peak since the intrusive rock is usually more resistant to weathering than the host rock.<ref name=Fillmore2010_28_289>{{cite book |last1=Fillmore |first1=Robert |title=Geological evolution of the Colorado Plateau of eastern Utah and western Colorado, including the San Juan River, Natural Bridges, Canyonlands, Arches, and the Book Cliffs |date=2010 |publisher=University of Utah Press |location=Salt Lake City |isbn=9781607810049 |pages=288–289}}</ref> Because the emplacement of the laccolith domes up the overlying  beds, local [[topographic relief]] is increased and erosion is accelerated, so that the overlying beds are eroded away to expose the intrusive cores.{{sfn|Fillmore|2010|pp=292-293}}