3. Origin of the Oceans

Early Oceans

The early atmosphere was rich in water vapor from volcanic eruptions and comets. When Earth was cool enough, water vapor condensed and rain began to fall. The water cycle began. Over millions of years enough precipitation collected that the first oceans could have formed as early as 4.2 to 4.4 billion years ago. Dissolved minerals carried by stream runoff made the early oceans salty. What geological evidence could there be for the presence of an early ocean? Marine sedimentary rocks can be dated back about 4 billion years.

By the Archean, the planet was covered with oceans and the atmosphere was full of water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.

  

 


Scientists hypothesize that the water that makes up the global oceans came from meteorites, comets, and volcanism. The early atmosphere was rich in these gases. It is believed that both comets and meteorites released some of their water content to Earth when they've collided with our planet. Volcanoes release water vapor as a main constituent of their eruptions. It is hypothesized that our planet was once much hotter and underwent a period of intense volcanic activity, which released a huge quantity of water vapor into the atmosphere.


The ocean basins are continuously changing today. Geological processes associated with the movement of Earth's tectonic plates are responsible for changing the size and shape of the world's ocean basins over time. The process of seafloor spreading is creating new basaltic ocean crust at the Mid-Atlantic Ridge in the Atlantic Ocean basin. Therefore, the Atlantic Ocean is slowly getting bigger. When new lithosphere is created at one point, it is simultaneously being destroyed at another location. This is happening at subduction zones, located at deep ocean trenches in the Pacific Ocean. Therefore, the Pacific Ocean basin is slowly shrinking over time. 

Approximately 250 million years ago, during the time of Pangaea, there was one global ocean that surrounded the supercontinent. This ocean has been called the Panthalassic Ocean.  By 180 million years ago, Pangaea began to break up. When Pangaea broke into the continents of Laurasia and Gondwanaland, the Panthalassic Ocean basin also changed. It was split into two oceans, the Pacific Ocean in the west and the Tethys Ocean (composed of today's Atlantic and Indian Oceans) in the east. Even today, the waters of all the oceans are connected. 

Source: Eary Atmosphere and Oceans. Retrieved from http://www.ck12.org/book/CK-12-Earth-Science-Concepts-For-High-School/r16/section/11.15/ on September 1, 2013. and USOE OER Resources. Retrieved from http://www.slcschools.org/departments/curriculum/science/documents/4-Earth-Science.pdf on September 1, 2013.