Page 49 - (DK) Ocean - The Definitive Visual Guide

THE EVOLUTION OF THE OCEANS 47

Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene Neogene Sea-level Change change of 100–200m (330–655ft) over a
Sea level has constantly changed through
few tens of thousands of years. The rate of
history, being up to 400m (1,300ft) higher
SEA LEVEL M/FT
22/72 in the past. One of the factors controlling sea-floor spreading also affects global sea
levels and has outweighed climatic factors at
sea level is the global climate. Thermal
20/68 PRESENT LEVEL AT 0 expansion of ocean water increases global some times. Faster-spreading ridges reduce
18/64 sea level by about 7.5cm (3in) for every 1˚C the volume of the ocean basins as the
16/61 (1.8˚F) increase in temperature. The transfer younger, hotter crust rises higher, causing
of water between ice caps and the oceans sea levels to rise (see p.88). Local changes
14/57 during glacial cycles accounts for a global also occur as a result of crustal movement.
300/980
12/54
200/660 MEDITERRANEAN BASIN HISTORY
10/50 100/330
TEMPERATURE ˚C/˚F 0/0

-100/-330
542 500 450 400 350 300 250 200 150 100 50 0
MILLIONS OF YEARS AGO

TEMPERATURE AND SEA LEVEL
Over the last 100 million years, climate has controlled sea levels,
and these (in blue on graph) have dropped as the climate has 1 The Mediterranean was isolated 2 21,000 years ago, sea levels were 3 100,000 years ago, water from melting
cooled (temperature in yellow). At other times, low sea levels from the Atlantic by the closure of the 120m (390ft) lower than they are today ice started to flood the continental
were due to reduced rates of sea-floor spreading. Strait of Gibraltar five million years due to water being locked up in ice caps shelves exposed during the glacial,
ago, and evaporated to a salty desert. at the height of the last glacial. leaving today’s familiar shoreline.
Sedimentary Basins

Most of the world’s sedimentary rocks were BASINS AND OILFIELDS
laid down in water over continental shelves Sedimentary basins are found on
or in inland seas. The movements of the the continental shelves and
continents and changes in sea level have adjacent ocean floor, but
also well inland where
determined where this deposition occurred areas were once PA CIFIC
at particular times, and many former marine covered with water.
OCEAN
sedimentary basins are now far inland.
PA CIFIC ATLANTIC OCEAN
Oil and gas deposits are found in marine KEY
sedimentary rocks, the result of animal and onshore OCEAN
sedimentary
plant remains decomposing and then being deposits INDIAN
buried and compressed. About 30 per cent
offshore OCEAN
of the world’s oil and gas production comes sedimentary deposits
from offshore fields, but many offshore basins SOUTHERN OCEAN
remain to be explored. Oil and gas deposits

GLACIAL COAST INTRODUCTION
During glacials, sea ice forms at lower
altitudes than it does today. This scene
may have been typical of the shores of
western Europe 21,000 years ago.