Page 50 - (DK) Ocean - The Definitive Visual Guide

48 OCEAN GEOLOGY

Tectonics and the Ocean Floor

THE THEORY OF PLATE TECTONICS HAS REVOLUTIONIZED geology over the last half century,
explaining many of the Earth’s physical features. Tectonic plates are huge fragments of the
Earth’s lithosphere, which consists of the crust fused with the top layer of the upper mantle.
They move over a more deformable layer of the mantle called the
asthenosphere. Plate motion builds mountain ranges, but plate- BASALT
The ocean floor is largely made of
tectonic processes are perhaps most clearly seen on the basalt, a fine-grained igneous rock
derived from the upper mantle.
ocean floor, where most plate boundaries are found. It is a dense rock due to a high
proportion of iron and magnesium.

hotspot produces
volcanic activity Recycling Ocean Crust
The oldest rocks on the ocean floor are 180 million years old. This is
mid-ocean ridge AGE OF THE OCEAN FLOOR
young compared with the oldest continental rocks, which date from
about 3.8 billion years ago. While the continental crust has been steadily The age of the ocean floor
lithospheric plate increases away from the
pushed away accumulating throughout the Earth’s history, it seems the oceanic crust spreading ridges where new
from ridge is created and destroyed rather quickly. It is created at the mid-ocean crust is forming. The map
ridges from hot material rising in the mantle, and then spreads away below shows the East Pacific
from the ridges, before eventually being recycled into the mantle at Rise to be the fastest-
rising mantle spreading ridge, since it
plume forms subduction zones. Continental crust is always less dense and more
hotspot at buoyant than oceanic crust, so where they meet, it is the oceanic is flanked by the broadest
surface spread of young rock (shaded
crust that gives way, sinking (subducting) back into the mantle. red and orange).
incipient
mantle
plume

convection
cell

oceanic
lithosphere
descends at
subduction
zone
MANTLE CONVECTION
Convection cells in the Earth’s mantle are
the driving force behind plate tectonics.
The cycle of hot material rising, cooling,
spreading out, and sinking pushes and KEY 144 89 54.8 24 1.8
pulls the lithospheric plates around. ocean ridges at divergent age
plate boundaries (millions
transform of years) 154 127 65 33.5 5 0 undated
plate boundary
direction of plate
movement
Plate Boundaries
DIVERGENT AND TRANSFORM BOUNDARIES
The boundaries of a tectonic plate may At divergent boundaries, parallel ridges emerge
be divergent, convergent, or transform. At as new ocean floor spreads out either side of an
divergent boundaries, the crust is extended, ocean ridge. A transform boundary arises when
thinned, and fractured by the upwelling of hot sections of the ridge are offset from each other.
mantle material. The crust buoys up, producing
a mid-ocean ridge, and lava is extruded through
continent compressed, oceanic movement
a central rift valley to create new oceanic crust. forming volcanic trench of oceanic
Seamount volcanoes may also arise (see p.174). mountains lithosphere
magma
Plates collide at convergent boundaries. rises from
Where oceanic lithosphere meets continental mantle
lithosphere, the crust on the continental side may movement movement of
INTRODUCTION lithosphere, forming an ocean trench (see p.183), and volcanic CONVERGENT BOUNDARIES magma oceanic lithosphere oceanic
be compressed and thickened, resulting in mountain-building.
continental
of oceanic
The oceanic lithosphere sinks beneath the lighter continental
lithosphere
lithosphere
activity occurs above the descending plate. Where slabs
of oceanic lithosphere converge, the oldest, most dense is
Ocean lithosphere is destroyed
subducted and an arc of volcanic islands is formed parallel
by subduction at convergent
boundaries. The subducting
to the trench. Transform boundaries arise where plates are
plate carries water with it,
moving past each other. No plate is created or destroyed.
which allows the surrounding
forms
They can occur where segments of a divergent boundary
mantle to melt, forming
subducted beneath
as plate
are offset, and extensive fracture zones can result.
explosive volcanoes above.
crust
continental lithosphere
descends