Page 24 - Oceans

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22 Titles:E.Explore_Ocean (ED594) 1 022 000 18/04/07 1:30 NT1-5 Titles:E.Explore_Ocean (ED594) 1 023 000 18/04/07 1:30 NT1-5
Hotspots and seamounts Crust
Dome pushed up
Not all oceanic crust is formed at the spreading rifts of by mantle plume
midocean ridges. The hot mantle of the Earth is dotted
Mantle
with extra-hot regions known as mantle plumes. Many plume rises
from near core
of these lie well away from the boundaries between the
plates of the Earth’s crust. They form stationary hotspots Mantle
that burn through the crust, erupting molten lava that

builds up into volcanoes. As each volcano is carried away
from the hotspot by the moving crust it becomes extinct, Core
and another volcano erupts over the hotspot. This process

has created chains of volcanic islands and thousands of
≤ Mantle pluMes
submerged volcanoes known as seamounts. The mantle plumes that cause hotspots probably rise from
deep in the mantle near the Earth’s core. Each plume pushes
up a broad dome in the oceanic crust— 600 miles (1,000 km)
across and 1 mile (1.6 km) high in the case of Hawaii—and an
even higher volcano erupts from the center of the dome.

< Volcanic cHains
The hotspot burns a hole through the Earth’s crust,
creating a volcano (A). But the mobile crust carries the
a b a c b a volcano away from the hotspot, so it becomes extinct and
starts sinking. A new one erupts (B), but that is also slowly
carried off the hotspot, too. By the time a third volcano
appears (C), the oldest may have sunk below the waves.
1 First volcano 2 Volcano drifts off hotspot 3 Extinct volcanoes
erupts over hotspot and becomes extinct slowly sink as another
to create an island while a new one erupts volcano erupts

hotspots

≤ Hawaii
The islands of Hawaii, seen here from space, have been created by
the Pacific Ocean plate slipping northwest over a hotspot at about
3 in (9 cm) a year. The hotspot is currently beneath the highly active
volcano on the biggest southeastern island. Meanwhile, the extinct
volcanoes to the north are slowly sinking, and will become progressively
smaller until they dwindle to a long chain of submerged seamounts.

< fire fountains
The volcanoes of Hawaii are huge domes that rise from the ocean floor.
The largest, Mauna Loa, is higher from base to summit than Mount
Everest in the Himalayas. Kilauea, on the flanks of Mauna Loa, is the
most active volcano on Earth, constantly erupting fire fountains of
fluid basalt lava that flow down to the sea in rivers of molten rock.