Continental crustal fragments, partly
synonymous with microcontinents,[1] are pieces of
continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin.[2]
Causes
Continental fragments and microcontinent crustal compositions are very similar to those of regular
continental crust. The rifting process that caused the continental fragments to form most likely impacts their layers and overall thickness along with the addition of
mafic intrusions to the crust. Studies have determined that the average crustal thickness of continental fragments is approximately 24.8 ± 5.7 kilometres (15.4 ± 3.5 mi).[3] The
sedimentary layer of continental fragments can be up to 5 kilometres (3.1 mi) thick and can overlay two to three crustal layers. Continental fragments have an average crustal density of 2.81 g/cm3 (0.102 lb/cu in) which is very similar to that of typical continental crust.
Strike-slipfault zones cause the fragmentation of microcontinents. The zones link the extensional zones where continental pieces are already isolated through the remaining continental bridges. Additionally, they facilitate quick
crustal thinning across narrow zones and near-vertical strike-slip-dominated
faults. They develop
fault-block patterns that slice the portion of continent into detachable slivers. The continental fragments are located at various angles from their
transform faults.[4]
History
Some microcontinents are fragments of
Gondwana or other ancient
cratonic continents; examples include
Madagascar; the northern
Mascarene Plateau, which includes the
Seychelles Microcontinent; and the island of
Timor.[5] Other islands, such as several in the
Caribbean Sea, are composed largely of granitic rock as well, but all continents contain both granitic and basaltic crust, and there is no clear dividing line between islands and microcontinents under such a definition. The
Kerguelen Plateau is a
large igneous province formed by a volcanic
hotspot; however, it was associated with the breakup of
Gondwana and was for a time above water, so it is considered a microcontinent, though not a continental fragment.[6][7] Other hotspot islands such as the
Hawaiian Islands and
Iceland are considered neither microcontinents nor continental fragments. Not all islands can be considered microcontinents:
Borneo, the
British Isles,
Newfoundland, and
Sri Lanka, for example, are each within the continental shelf of an adjacent continent, separated from the mainland by
inland seas flooding its margins.[8]
Several islands in the eastern
Indonesian Archipelago are considered continental fragments, although this designation is controversial. The
archipelago is home to numerous microcontinents with complex geology and tectonics. This makes it complicated to classify landmasses and determine causation for the formation of the landmass.[9] These include southern
Bacan,
Banggai-
Sulu Islands (
Sulawesi), the
Buru-
Seram-
Ambon complex (
Maluku),
Obi,
Sumba, and
Timor (
Nusa Tenggara)[10]
List of continental fragments and microcontinents
Continental fragments (pieces of Pangaea smaller than Australia)
Ajan, a continent that will form in 3 to 20 million years time because of its breakoff with mainland
Africa.[13]
References
^"Microcontinent" was initially the broader term, because it was defined morphologically rather than genetically (in terms of genesis or origin). Scrutton, Roger A. (1976) "Microcontinents and Their Significance" pp. 177–189 In Drake, Charles L. (1976) (editor) Geodynamics: Progress and Prospects American Geophysical Union, Washington, D.C.,
ISBN978-0-87590-203-6. But, using Scrutton's definition, "microcontinent" is a narrower term, excluding aseismic ridges of continental material, such as the
Lomonosov Ridge and the
Jan Mayen Ridge, which could still be considered "continental fragments".
^Monk, K.A.; Fretes, Y.; Reksodiharjo-Lilley, G. (1996). The Ecology of Nusa Tenggara and Maluku. Hong Kong: Periplus Editions Ltd. pp. 41–43.
ISBN978-962-593-076-3.
^Monk, K.A.; Fretes, Y.; Reksodiharjo-Lilley, G. (1996). The Ecology of Nusa Tenggara and Maluku. Hong Kong: Periplus Editions Ltd. pp. 27–29.
ISBN978-962-593-076-3.
^Monk, K.A.; Fretes, Y.; Reksodiharjo-Lilley, G. (1996). The Ecology of Nusa Tenggara and Maluku. Hong Kong: Periplus Editions Ltd. p. 41.
ISBN978-962-593-076-3.