I’ve always been fascinated by the ocean: the endless rolling of the waves, the water’s changing moods, the limitless horizon. Or maybe it’s just because, coming from the dry prairies, I’m amazed that anything can be that big and wet.
How big and wet? The ocean covers 71 percent of the Earth’s surface: 361 million square kilometres, and holds roughly 1.34 billion cubic kilometres of water.
Close to continents, it’s relatively shallow, slowly sloping out, over a distance of many kilometres, to a depth of about 200 metres. This “continental shelf” gives way abruptly to the continental slope, which descends steeply to about 3500 metres; then there’s an area called the continental rise, which slopes more slowly down to the flat plains that make up most of the ocean’s floor.
In mid-ocean lie vast underwater mountain ranges, formed where the tectonic plates on which the continents float are pulling apart. Molten rock wells up here, adding new material to the crustal plates, which in other places are running into each other and diving back down into the Earth. These collision sites are where the ocean is deepest: the Mariana Trench, east of the Phillipines, plunges to 10.9 kilometres below sea level.
Half a kilometre or more of sediment covers the ocean floor. It’s a mixture of rock particles, carried out to sea by wind or by the action of the waves on the coastline, and organic particles, the remains of microscopic plants and animals, which drift like constant snow from the teeming surface waters down into the depths.
The salt in salt water is dissolved out of continental rocks. There are several different salts in ocean water, but the largest component is plain old sodium chloride–table salt. The ocean’s salinity varies from near zero in continental waters to 41 parts per 1000 in the Red Sea; the average is about 35 parts per 1000.
The ocean’s surface temperature varies, too, from 26 degrees Celsius in tropical water to -1.4 degrees (the freezing point of salt water) in the Arctic. Seasonal variations are much less drastic than they are on land, which is why it’s almost always too cold to swim comfortably off Canada’s coasts–the sun just can’t put out enough energy to heat up that much water. On the plus side, the ocean doesn’t freeze in winter, because it also takes a very long time to cool down. That’s why the coasts (i.e., Vancouver) are warmer than the interior (i.e., Regina).
In the deepest, darkest depths, the temperature is always just above freezing.
In some places, currents like the Gulf Stream bring warmer water from the south to the north. Currents, which have been clocked at up to nine kilometres an hour, are set in motion by the prevailing winds, but curve due to the rotation of the Earth. There are also deep-water currents, caused by the difference in density between masses of water of different temperature and salinity.
Tides aren’t caused by the wind, but by the gravitational pull of the moon, and, to a lesser degree, the sun. When the moon is directly over any given point on the surface of the Earth, its gravitational pull causes the water to form a hump on that side of the Earth.
Exactly opposite that spot on the Earth’s surface, another hump of water also forms. That’s because the Earth and Moon together form a system that circles around a point about one-quarter of the Earth’s diameter beneath surface. The system’s rotation around this point causes the water farthest away from the moon to be “thrown” outward. The Earth’s gravity keeps it from escaping.
As the Earth rotates and the moon orbits, the two humps move through the oceans, giving each location two high tides a day.
The sun produces tides, too, but the effect is less than half as strong because the sun is so far away. When the solar tide and the lunar tide line up with each other, during the new and full moon, you get a “spring tide,” much higher (and lower) than average. When the moon is in the first or third quarter, the solar tide is at right angles to it, and you get a “neap tide,” with less variation than normal.
Long and narrow, Canada’s Bay of Fundy has the greatest tidal variations in the world: an average of 18 metres.
Big, wet, and teeming with life, the ocean is the last true frontier left on the planet. Every time I look at it, I want to see what lies over the horizon–and under the waves.
