Page 51 - (DK) Ocean - The Definitive Visual Guide

TECTONICS AND THE OCEAN FLOOR 49

Earthquakes and Tsunamis DISCOVERY

Earthquakes are associated with all plate boundaries, but they are particularly TSUNAMI ALERTS
frequent at convergent boundaries, such as subduction zones. Stress builds up
at faults in the crust until it overcomes the strength of the rock and the fault Tsunamis can be very destructive,
slips. When this happens, a huge amount of energy can be released in a short so systems have been established
to look out for their distinctive
time. The earthquake that produced the 2011 Tohoku Tsunami in Japan signs and give warning of their
released 600 million times more energy than the Hiroshima atomic bomb. approach. These systems use
A tsunami may be triggered if an earthquake results in the uplift or networks of seismic stations
subsidence of part of the seafloor. The water above suddenly rises or sinks, to detect earthquakes, and
then flows to regain equilibrium. Surface waves radiate out at 310–497 mph automated deep-sea buoys with
(500–800 kph) and can quickly cross an entire ocean basin. seafloor pressure sensors to
confirm whether a tsunami has
been generated. The prototype
surface waves buoy pictured at right is destined
spread out at for seismic monitoring off the
high speed
waves spread Caribbean coast of Grenada.
in opposite
directions
waves of moderate SEISMIC SEA WAVES
water suddenly size in deep ocean Tsunami waves increase in size as they encounter
elevated
above fault shallow water near the shore. They can grow from
10 ft (3 m) in the open ocean up to 100 ft (30 m) in
extreme cases at the coast.
waves become
tall and destructive
in shallow water
shockwaves
spread out few buildings
from the can survive the
earthquake in onslaught of a
all directions movement large tsunami
along fault
causes uplift
of seafloor

Hotspots and Island Chains
The seafloor between plate boundaries is far from
featureless. Volcanic island chains are found far from any plate
boundary due to the presence of hotspots (deep-seated and
long-lived zones of volcanic activity) in the mantle. Some hotspots,
such as the one beneath Iceland, are associated with divergent plate
boundaries, while others lie in the middle of oceanic or continental plates.
Chains of volcanoes often trail away from mid-ocean hotspots, with the oldest
volcanoes, long extinct, now lying far away from the hotspot. These hotspot tracks
are aligned along the direction of motion of the overlying plate. They change seawater moves
direction when the plate motion changes and may be interrupted when a new in circular motions
beneath each wave
spreading ridge opens up, as it has between India and the Réunion hotspot. as it passes

Iceland
Yellowstone
Azores
Bermuda
Canary Is. Hoggar
Hawaii Is. Caroline
Cape Is.
Verde Is. VOLCANIC ISLAND
Cameroon Molokini Island is the tip of an extinct volcanic
Galapagos Ascension Is. crater, part of the Hawaiian–Emperor chain of
Samoa St Helena islands and seamounts that stretches across
Marquesas
the north Pacific.
Réunion
Easter Is.
Tristan da
Cunha Crozet Is.
Kerguelen Is.
Bouvet Is.
HOT SPOTS
Some hot spot tracks link to areas KEY Plate boundaries INTRODUCTION
where huge amounts of basalt flood basalts
flooded from the hotspot onto convergent
the surface long ago. The Tristan hotspot tracks transform
da Cunha hotspot is linked to
flood basalts on both sides of hotspot divergent
the south Atlantic Ocean. uncertain