горячие точки

Горячие Точки Земли

 

Гавайская Горячая Точка
Гавайская Горячая Точка

В 1963 году Вилсон предположил [ Wilson, J.T., 1963, A possible origin of the Hawaiian Islands: Canadian Journal of Physics, v. 41, p. 863–870.], что развивающийся во времени вулканизм вдоль Гавайской цепи вулканических островов можно объяснить перемещением литосферы над восходящим мантийным потоком лавы, расположенным под Гавайскими островами.
Вулканические острова появились благодаря термальным аномалиям, что и явилось причиной почему их назвали горячими точками. Было сделано предположение, что основным, определяющим параметром является абсолютная температура. Магма служит средством для переноса мантийной температуры и эти представления широко используется в настоящее время. Обогащенность мантии, магматические очаги, неоднородности, разломы и литосферные напряжения могли также локализовать плавление, но термальные горячие точки также хорошо образуются и это понятие может быть использовано в этих условиях, даже при отсутствии свидетельств о высокой температуре или высоком тепловом потоке. Горячие точки так же называются внутриплитным вулканизмом, аномальным плавлением и плюмами. Все это вводит в заблуждение, так как аномалия предполагается на фоне однородных и мантийного состава, и температуры, а также на фоне постоянного напряжения, что несовместимо с самой тектоникой плит. Большинство горячих точек не являются “срединноплитными”. Горячие точки, как и другие вулканические образования, встречаются в обширных областях литосферы, либо на границах плит, либо внутри плит. Много горячих точек находятся в пределах срединноокеанических рифтовых зон. Много внутриплитных вулканов начинали свою жизнь в рифтовых зонах и с тех пор переместились относительно аномалии или отнесены. Некоторые из них могут быть выражением зарождения или будущих границ плиты.
Происхождение мантийных горячих точек – спорная тема. Только семь (“первичных”) из 49 горячих точек удовлетворяют критериям, цель которых – выявление очень глубинного происхождения горячих точек (три в Тихом океане, четыре в Индо – Атлантическом полушарии). В каждом полушарии горячие точки двигаются медленно, в то время как раннее, до 50 млн лет назад, движение было 50 мм/год относительно двух полушарий. Это коррелирует с широтными сдвигами в Гавайской и Реюнионской горячих точках, а также с изменением в положении полюсов (a change in true polar wander). Ряд исследователей предполагает, что горячие точки могут поступать из различных мантийных пограничных слоев и что “первичные” горячие точки трассируют смещения квадрупольной конвекции в нижней мантии.
Несмотря на то, что теория тектоники плит успешно применяется для объяснения таких явлений, как глобальный вулканизм, сейсмичность, было отмечено, что происхождение значительного число вулканов, часто удаленных от границ плит, не может быть объяснено с позиций тектоники плит. Морган предположил, что глубинные мантийные плюмы, созданные в нижней части мантии, являются источником таких горячих точек. Горячая точка обозначена на поверхности Земли линейной цепочкой потухших вулканов, которая, как предполагается, сформирована на литосферной плите, дрейфующей над плюмом. Гаваи и связанная с ним Гавае – Императорская цепочка подводных гор – наиболее характерный пример. В дальнейшем было отмечено, что вулканы горячих точек могли бы быть созданы и разломами литосферы, в то же время Морганом было введено понятие второго типа островных горячих точек. Ряд исследователей внимательно рассмотрели всевозможные аспекты первоначальной версии вторичных горячих точек. Совсем недавно Андерсон пришел к заключению, модель глубинного плюма Моргана может быть опровергнута. С его точки зрения весь вулканизм, не связанный с границами плит может быть объяснен неглубинными, связанными с плитами стрессами, которые разрушают литосферу и являются причиной вулканизма вдоль этих разломов, активизирую, например, вторичную, связанную с перемещением плит, конвекцию в верхней мантии. То, что такие различные точки зрения по-прежнему имеют место, свидетельствует о том, что горячие точки имеют различные источники в мантии.

Ниже перечислим пять особенностей, которые могут охарактеризовать вулканизм горячих точек, продуцированный плюмами, зародившимися в глубокой мантии. Эти пять критериев используются для сортировки каталогов горячих точек. Вот эти критерии:
1. Наличие линейных вулканических цепочек с монотонной изменением возраста. Всплывающий поток определяющий движение вещества из мантии является причиной топографического поднятия, ассоциируемого с горячей точкой.
2. Наличие покровных базальтов в начале следа.
3. Большой восходящий поток / высокая плавучесть. Всплывающий поток определяющий движение вещества из мантии является причиной топографического поднятия, ассоциируемого с горячей точкой. Детальные численные расчеты показали, что плюмы поднимающиеся от подошвы мантии и имеющие плавучесть менее, чем 103 kg s-1 могут быть охлаждены настолько, чтобы они не расплавлялись под зрелой литосферой. Такие слабые плюмы могут быть захвачены мантийным потоком до того, как они достигнут литосферы. Если же значение плавучести, превышающее 103 kg s-1, то подобная высокая плавучесть проявится на поверхности в форме классической горячей точки.
4. Согласованно высокое отношение третьего изотопа гелия к четвертому изотопу гелия (3He/4He ). Высокие значения соотношения 3He/4He или 21Ne/22Ne в лаве горячей точки часто приписывается подъему вещества из длительное время изолированного и наиболее примитивного резервуара. Если неглубокие резервуары, как правило, связываются со срединно – океаническими рифтами, то примитивные резервуары, часто считается, расположены глубоко в мантии в пределах транзитной зоны верхней мантии или даже глубже, в нижней мантии.
5. Значительное снижение скорости поперечных волн (VS) в подстилающей мантии. Горячая точка по определения горячая. Поэтому  мантия под горячими точками характеризуется аномально низкими скоростями поперечных волн (VS). Такие низкие скорости на глубине будут указывать на существование пониженной плотности, возможно, более горячего вещества из плюма.

Источники:
– Anderson, D.L. and Schramm, K.A., 2005, Global Hotspot Maps, in Plates, Plumes & Paradigms, Foulger, G.R., Natland, J.H., Presnall, D.C, and Anderson, D.L., eds., Boulder, CO, Geological Society of America, Special Paper 388, pp. 19-29.
– Courtillot V.,Davaille A., Besse J., and J. Stock Three distinct types of hotspots in the Earth’s mantle. Earth and Planetary Science Letters 205 (2003) 295-308.
– Morgan W.J., (1972) Plate motions and deep convection, Geol. Soc. Am. Mem. 132 7-22.
– Morgan W.J., (1981) Hotspot tracks and the opening of the Atlantic and Indian oceans, in: C. Emiliani (Ed.), The Sea, Vol. 7, Wiley Interscience, New York, , pp. 443-487.
– Wilson J.T., (1965) Evidence from oceanic islands suggesting movement in the Earth, Philos. Trans. R. Soc. London Ser. A 258 145-167.

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