The earth and the rest of the solar system are thought to have originated about 4.5 billion years ago from a cloud or clouds of dust. This dust was debris remaining from a huge cosmic explosion called the big bang, which astrophysicists estimate occurred about 15 billion years ago. The dust particles collided with each other,merging into larger particles. These larger particles collided in turn, joining into pebble-sized rocks that collided to form larger rocks, and so on. The process continued, eventually building up the earth and other planets.

So much heat was produced as the early earth formed that the planet was probably molten. This allowed materials to settle within the planet according to their density. Density is the weight, or more correctly, the mass, of a given volume of a substance. Obviously, a pound of styrofoam weighs more than an ounce of lead, but most people think of lead as “heavier” than styrofoam. This is because lead weighs more than styrofoam if equal volumes of the two are compared. In other words, lead is denser than styrofoam. The density of a substance is calculated by dividing its mass by its volume. If two substances are mixed, the denser material will tend to sink and the less dense will float.

During the time that the young earth was molten, the densest material tended to flow toward the center of the planet, while lighter materials floated toward the surface. The light surface material cooled to make a thin crust. Eventually, the atmosphere and oceans began to form. If the earth had settled into orbit only slightly closer to the sun, the planet would have been so hot that all the water would have evaporated into the atmosphere. With an orbit only slightly farther from the sun, all the water would be perpetually frozen. Fortunately for us, our planet orbits the sun in a narrow zone in which liquid water can exist. Without liquid water, there would be no life on earth.

Most scientists agree that the atmosphere and the oceans accumulated gradually over millions and millions of years with the continual 'degassing' of the Earth's interior.

According to this theory, the ocean formed from the escape of water vapor and other gases from the molten rocks of the Earth to the atmosphere surrounding the cooling planet.

After the Earth's surface had cooled to a temperature below the boiling point of water, rain began to fall—and continued to fall for centuries. As the water drained into the great hollows in the Earth's surface, the primeval ocean came into existence. The forces of gravity prevented the water from leaving the planet.