Tropic of Cancer to Antarctica between 30°east longitude and 120°east longi-
tude. The smallest ocean delineated is the Arctic and it is only 8 percent of the size
of the Pacific; this polar ocean presents an interesting counterpoint to the more
harsh regimes associated with the southernmost Pacific Ocean’s boundary with
Antarctica.
The various large undulations in the coasts of the continents have caused parts
of the oceans to be identified as “seas.” For instance, the largest of these is the
South China Sea, which is a tropical body of water bounded by southern China,
the Philippines, and Vietnam and is part of the Pacific Ocean. There are numerous
other seas on the planet. In a couple of Asiatic cases—the Caspian and Aral
Seas—the bodies of water are not connected to water and are actually saltwater
lakes. Bays are indentations in coastlines and Hudson Bay is the largest of these,
representing an Arctic Ocean incursion into the land mass of North America.
Gulfs are another major subdivision of oceans. The Gulf of Mexico near southern
North America is a notable example. There is no precise geographic definition
dividing the sizes and shapes of seas, bays, and gulfs.
The oceans are circumscribed by the land masses and have changed their shapes
and sizes over long amounts of Earth history because ofplate tectonics.For
instance, a quarter billion years ago the Atlantic Ocean did not exist and is still
in the inexorable process of widening through seafloor spreading. In other places,
ocean floors are subducting causing oceanic area to become less. Over much
shorter times the extent, volume and area can undergo appreciable changes. Since248 Oceans
Tides
The sweep of tides in and out from seacoasts is obvious to anyone who lives near the sea.
This motion is caused by gravity associated with the moon and, secondarily, the sun. The
moon’s attraction causes the oceans to bulge on the side toward the moon and the side away
from the moon. Earth’s rotation means the bulges progress around the Earth. Earth’s rotation
and the moon’s revolution are not synchronized, causing two high tides and two ebb tides
every 25 hours at each location. Additionally, the highest of high tides is called a spring tide
and occurs twice a month when Earth, moon, and sun are aligned. The height of tides varies
over the world’s oceans. In the deep ocean, tides tend to be about 1 m (3 ft) while along
coastlines the local depth of the sea bottom and the shape of the coastline play major roles.
Along shallow coastlines, tides sweep in and out over horizontal distances of many kilo-
meters. World record tides of 15 m (50 ft) are found in the Bay of Fundy in eastern Canada,
but other locations such as the British Isles, southeastern South America, and east central
Africa have tides well in excess of the world average of 2 m (7 ft). In human history, tides have
had considerable importance. Fisheries and boot moorage are intimately tied to the depth of
water in the coastal zone.