Ring dike
Aring dikeorring dykeis anintrusiveigneousbody that is circular, oval or arcuate in plan and has steep contacts.[1]While the widths of ring dikes differ, they can be up to several thousand meters.[2]The most commonly accepted method of ring dike formation is directly related to collapsecalderas.[3]
Caldera collapse and ring dike formation
[edit]
Collapse calderas form due to the emptying of amagma chamber.[4]Effusive eruptions that take place on the flanks of the associatedvolcanoand afissuresystem that directmagmaaway from the chamber are both mechanisms that can empty a magma chamber. As pressure in the magma chamber changes, an increase in tensile stresses create tension fractures at the surface of thevolcano.The geometry of the top of the magma chamber dictates the location and magnitude of the tension fractures. In addition, it was found that the higher the radius to depth ratio of the magma chamber, the higher the probability of forming a collapsecaldera.[3]
Once a tension threshold is approached, the roof of the magma chamber collapses in on itself, and is known as cauldron subsidence.[5]Tension fractures extend deeper in the profile and shear fractures ordip-slip faultsform in a circular pattern around the caldera and are known asring faults.Ring faults can be either vertical or steeplydippingfaults.[6]When they are inward dipping, they are known asnormal faultsand when they are outward dipping they are known asreverse faults.Ring faults then allow magma to rise through thefractures,forming a ring dike.[7]These dikes can form as a direct result of collapse caldera formation, or through many injections around the ring fault over time.[8]The magma of a ring dike is typically composed of acidic or intermediate composition due to the less dense melt that exists at the top of the magma chamber.[9]
Another mechanism of ring dike formation
[edit]It has been hypothesized that ring dikes may form when inclined sheets are captured within a ring fault system, which cause them to act as feeder dikes. The deflection of the sheets may be caused by the difference in material properties between and within thefaultzone.[10]
Implications
[edit]Whether or not a caldera ring fault dips inward or outward from the center of subsidence is a highly contentious issue. Ring faults near the surface are subject toerosionandmass wasting,changing the morphology of the caldera walls and making it difficult to tell the dip of the fault at formation.[10]
Well known examples
[edit]Ossipee ring-dike complex
[edit]About 36 ring dikes have been found in theOssipee MountainsinNew Hampshire.Calderasubsidence appears to be the mechanism leading to the formation of some of the ring dikes, but not all of them. The composition ranges frommonzoniteto quartzsyenite.[11]
Loch Bà ring dike
[edit]TheLoch Bàring dike, found on theIsle of MullinScotland,serves as a classic example of a well formed ring dike.[12]Thisintrusivebody forms an oval and its diameter can be measured at roughly 5.8 km by 8.5 km. The width of the dike varies throughout the profile, with a maximum width of approximately 300 meters. The composition varies fromrhyolitetofelsite,withphenocrystsofalkali feldsparandmaficminerals.[13]
See also
[edit]References
[edit]- ^Billings, Marland P. "Section of Geology and Mineralogy: Ring-dikes and Their Origin."Transactions of the New York Academy of Sciences5.6 Series II (1943): 131–44
- ^Johnson, Scott E., S. R. Paterson, and M. C. Tate. "Structure and emplacement history of a multiple-center, cone-sheet–bearing ring complex: The Zarza Intrusive Complex, Baja California, Mexico."Geological Society of America Bulletin111.4 (1999): 607–19).
- ^abGudmundsson, Agust. "Formation of collapse calderas."Geology16.9 (1988): 808–10.
- ^Troll, V. R.; Walter, T. R.; Schmincke, H.-U. (2002-02-01)."Cyclic caldera collapse: Piston or piecemeal subsidence? Field and experimental evidence".Geology.30(2): 135–138.Bibcode:2002Geo....30..135T.doi:10.1130/0091-7613(2002)030<0135:CCCPOP>2.0.CO;2.ISSN0091-7613.
- ^Blatt, Harvey, Robert Tracy, and Brent Owens.Petrology: igneous, sedimentary, and metamorphic.Macmillan, 2006.[ISBN missing][page needed]
- ^Walter, Thomas R.; Troll, Valentin R. (2001-06-01)."Formation of caldera periphery faults: an experimental study".Bulletin of Volcanology.63(2): 191.Bibcode:2001BVol...63..191W.doi:10.1007/s004450100135.ISSN1432-0819.S2CID59140680.
- ^Emeleus, C. Henry; Troll, Valentin R.; Chew, David M.; Meade, Fiona C. (March 2012)."Lateral versus vertical emplacement in shallow-level intrusions? The Slieve Gullion Ring-complex revisited".Journal of the Geological Society.169(2): 157–171.Bibcode:2012JGSoc.169..157E.doi:10.1144/0016-76492011-044.ISSN0016-7649.S2CID130108022.
- ^Gudmundsson, Agust, Joan Marti, and Elisenda Turon. "Stress fields generating ring faults in volcanoes."Geophysical Research Letters24.13 (1997): 1559–62
- ^O'keefe, John A., Paul D. Lowman, andWinifred S. Cameron."Lunar Ring Dikes from Orbiter I."Science155.3758 (1967): 77–79.
- ^abBrowning J. & Gudmundsson A. (2015)."Caldera faults capture and deflect inclined sheets: An alternative mechanism of ring-dike formation"(PDF).Bulletin of Volcanology.77(4): 4.Bibcode:2015BVol...77....4B.doi:10.1007/s00445-014-0889-4.S2CID54022484.
- ^Modell, David. "Ring-Dike Complex of the Belknap Mountains. New Hampshire."Geological Society of American Bulletin47.12 (1936): 1885–1932.
- ^Young, Davis A.,Mind Over Magma: The Story of Igneous Petrology, Princeton University Press, 2003, pp. 341–42,ISBN978-0691102795
- ^Lockwood, John P. and Richard W. Hazlett,Volcanoes:Global Perspectives,Wiley-Blackwell (2010).[ISBN missing][page needed]