Geoscience Reference
In-Depth Information
Eurasia
Africa
PACIFIC
OCEAN
North
America
South
America
Australia
Ancient Landmass
Modern Landmass
Subduction Zone (triangles point in the
direction of subduction)
Antarctica
Sea Floor Spreading Ridge
Figure 11.5
The next Pangaea, 'Pangaea Ultima', may form 250 Ma in the future as a result of the subduction of the North and
South Atlantic ocean floor beneath eastern North and South America, and closure of the Mediterranean and Middle Eastern
seaways. This supercontinent may have a small ocean basin trapped at the centre.
Source: Scotese, PALEOMAP
Abyssal plains
occupy most of the deep ocean floor
between the ridges and trenches, covering 42 per cent of
the total area, at an average depth of 5-6 km. They are
floored by cool, older oceanic crust which has subsided
into the lithosphere beyond the spreading ridges and are
the flattest places on Earth, broken only by submarine
plateaux and
seamounts
. The latter are distributed
randomly and form away from ridges, although they may
be associated with their positive thermal anomalies.
Elsewhere, seamount chains form as ocean crust migrates
over fixed hot spots. Islands such as Hawaii appear where
they break surface but most are submarine
guyots
. These
are summits levelled by marine planation or post-eruptive
subsidence and many provide attachment sites for coral
reefs in shallow, clear-water tropical seas. A thick carpet
of
pelagic sediments
provides the stark contrast between
seamounts, sloping at 15-25
, and abyssal plains at less
than 0ยท1
Ocean architecture
The dynamic architecture of mid-ocean ridges and
trenches, which drive the supercontinental cycle, covers
approximately one-third of global ocean area. Ridges
account for 95 per cent of this and form Earth's principal
continuous 'mountains' with broadly symmetrical,
parallel crest-and-trough structures, 10
2-3
km wide and
2-4 km high. They are offset along transform faults and
slope away from the thermal rise at their central axis, at
angles proportional to their spreading rate. Topographic
symmetry is matched by a geomagnetic 'bar code' of
bands showing normal and reversed magnetic polarity.
Magnetic minerals in basalt extruded at the central rift
assume Earth's magnetic polarity before cooling and
record its reversals over 10
4-6
years. Paired bands, of
similar palaeomagnetic direction and radiometric age,
either side of the axis underpin our reconstruction and
timing of ocean evolution (see
Figure 10.8
).
Lying beneath
an average ocean depth of 3 km, ridge axes are also
important minerogenic centres where hydrothermal pro-
cesses exchange minerals at the ocean-lithosphere
interface (see
Chapter 12).
. They are derived from minero-biogenic
sediment sources by rain-out or solid precipitation and
infill bedrock depressions.
Ocean-continent boundaries have a fluctuating
coastline and characteristic offshore slope system covering
over 20 per cent of ocean area. The
hypsometric curve
of
