英語聽力文摘 夏威夷超級火山
Look at a map of Hawaii and you’ll see volcanic islands strung out like pearls on a necklace. The biggest–and the youngest–is Hawaii, in the south-east corner.
As the islands stretch away to the north-west they become older and more worn. While Hawaii is about a million years old, the extinctvolcano of Kauai on the north-west end is five times older.
The Hawaiian islands were formed by what geologists call “hot spot” volcanos. These occur where hot magma from the Earth’s liquid mantle is steadily forced through the Earth’s crust.
Hot spots can last for millions of years, staying in the same place while the plates of the Earth’s crust move gradually overhead.
The plates of the Earth’s crust move at uneven speeds, so when the plate moves slowly, this allows a tremendous volcano to build up. That’s why we get a string of separate volcanos, rather than a continuous line.
This is exactly what’s happened with the Hawaiian islands. The hot spot stayed in the same place, pushing up a string of volcanos as the Earth’s crust moved over it.
Because of this, the volcano on north-west Kauai is old and extinct, while the volcanos on Hawaii’s south-east edge are the most active in the world.
The Hawaiian hot spot is now located about seventeen miles off the south-east coast of Hawaii. There’s no new island there, but there is a huge, underwater volcano, rising almost three miles above the ocean floor.
This new volcano will break the ocean’s surface in about fifty thousand years, giving us our next Hawaiian island.
下一個夏威夷超級火山
隨便翻看一張夏威夷的地圖，你會發(fā)現(xiàn)夏威夷的火山島分布得就像串在一條項鏈上的一顆顆珍珠一樣。其中威力最大——也是最年輕的火山，當屬夏威夷東南角的夏威夷火山。
隨著島嶼不斷地向西北方向延伸，各大火山變得越來越陳舊，破敗。雖然說夏威夷已歷經了一百萬年的歲月，但是，在西北端考艾島的死火山卻已是其五倍了。
夏威夷群島是由地質學家們所稱的“火山熱點”組成。“火山熱點”處在由地幔(又稱軟流層，很可能是巖漿的發(fā)源地)產生的熾熱的巖漿不斷地被噴出地殼的地方。
“火山熱點”能持續(xù)活動數(shù)百萬年。就算是地殼板塊活動而漸漸居于其上，“火山熱點”也一直都不會改變地方。
地殼板塊不是以均勻速度移動的，因而，當板塊運動較慢時，一個巨大的火山就可“趁機”形成。這就是為什么火山是一個個單獨的火山，而不是一片連續(xù)不斷的火山了。
在夏威夷群島上也就是如此。“火山熱點”一直在某個地方，在地殼板塊對其作用時形成了一座又一座的火山。
因此，西北端考艾島上的火山都是很老的死火山，然而，位于夏威夷東南邊的火山卻是世界上活動最頻繁的活火山。
而今，夏威夷的“火山熱點”位于夏威夷東海岸約17英里的地方。目前，那里還沒有新的島嶼形成，但是那里有一座很大很大的海底火山，高出海底足有三英里高。
在未來的五萬年里，這座新火山將會使海底地殼斷裂，創(chuàng)造出下一個夏威夷火山。
