How Were The Seas Formed?

As the molten rock that formed the surface of the young earth began to cool and solidify, gases bubbled up out of the seething interior of the globe and escaped into the atmosphere that surrounded it. Here they collected together into great clouds of water vapor. When these clouds became heavy enough, the moisture condensed and began to fall back to earth in the form of rain. But the surface was still red-hot, and as the falling rain-drops approached it, they boiled away and returned into the upper atmosphere as vapor.

You can see how this happened by heating a griddle on a stove. When the griddle is hot, slowly drop water on it from an eye-dropper. You will see that as the drops of water touch the hot surface of the griddle, they immediately boil away as steam vapor.

So, for probably millions of years, the earth was surrounded by a heavy blanket of rain clouds that was many miles in thickness - endlessly condensing, falling as rain, and then being being boiled back up into the sky again.

Then, slowly, the earth's crust hardened and cooled. And at last the surface rocks became so cool that their heat would no longer boil water. And the rain that had been collecting for all this millions of years up in the thick blanket of clouds began to fall in a never-ending torrent.

For hundreds, perhaps thousands of years, the rains came pouring down in a solid cloudburst. They leveled off mountain ranges and cut great valleys in the earth. And when at last the deluge had slowed and stopped, the lowest levels of the earth's wrinkled and folded crust had been filled up with water. These were the first oceans.

Why is sea water salty? 

The rivers that flow across the face of the land carry millions of tons of silt and sediment down to the oceans each year. These dissolved materials contain nearly all the minerals that are found in the earth, including vast quantities of salt.

The heat of the sun evaporates, or dries up, some of the water on the sea's surface and sends it back into the air as water vapor. There it condenses into clouds and falls back to earth as rain or snow.

But in this process of evaporation, the minerals are left behind in the oceans. Some of them, like iron and calcium, are absorbed by the sea's animals and plants. But the salt is not used by either the animals or plants and so it continues to collect in the sea in ever-increasing quantities.

What causes the tides?

Anyone who has been to the seashore has seen the daily ebb and flow of the tides. At certain times of day the level of the water rises, usually ten or twenty feet. Then it recedes and leaves a long, empty stretch of beach behind it. This is caused by the gravitational pull of the sun and the moon.

During the periods of new moon and full moon, the sun, the earth and the moon are all in a straight line. And so both moon and sun work together to cause extremely high tides, known as "spring tides".

On the other hand, when the moon is in the first and third quarter, it is at right angles to the sun. Under these conditions, the pull of the sun and moon tend to offset each other, and thus the tides are lower. These are called "neap tides".

But there are other puzzling things about the tides that cannot be explained by the simple force of gravity.

Around most of the Atlantic, the tides come and go twice a day. But in parts of the Pacific and Indian Oceans, the tide comes in only once a day. At Nantucket Island, off the coast of Massachusetts, high tide changes the level of the water only about one foot. But in the Bay of Fundy, only a few hundred miles north of Nantucket, the water level changes as much as forty feet.

These differences are caused by the irregularities of the ocean floor.

The floor of the sea is not flat, but instead is composed of vast basins, some broader and deeper than others. In these basins the sea water goes back and forth like the water in a dishpan or in a bathtub when it is disturbed. But water in a bathtub reacts more violently than water in a shallow pan. And, in the same way, the disturbance created by the pull of the moon and sun on the ocean's waters, is always much greater where the ocean basin is deeper.