Presence of water in abundant amount on the surface of Earth. But do you know how oceans were formed? If no, this article precisely describes that.
The oceans started forming billions of years ago due to the continual 'degassing' of the Earth's interior. The water was present in the form of gas until the Earth cooled down below degrees Fahrenheit.
Let us have a look at the two possible ways that led to the formation of oceans on the surface of Earth. Around 4. The Earth was so hot that the rocks were molten they were liquid.
When volcanic eruptions took place, one of the gases came out as steam water vapour. As the Earth cooled off, the water vapour condensed and rained back down onto the surface of the Earth for millions of years.
Some scientists think that is how some of the water came on the surface of Earth. Comets are huge chunks of ice and dust that were left after the formation of stars and other planets. Asteroids are large rocks that are known as planetoids or minor planets.
However, the timing of this transition - which has implications for when surface conditions necessary for the development were established - is poorly known. Simply put, not much is preserved in the rock record for this time.
It is the 'not much' part of the rock record that hold the clues however. So what does the rock record tell us? The oldest known rocks on Earth are almost exactly 4 billion years old and are comprised of metamorphosed and deformed granitoids from northwestern Canada collectively called the Acasta gneiss.
Direct radiometric dating using the U-Pb method on zircons has demonstrated that these rocks crystallized million years ago. However, these 'oldest rocks' do not record information on surface conditions at the time of their formation.
The oldest direct evidence for the presence of surface waters are slightly younger ca. The very existence of the Isua BIF requires the presence of stable surface water, at least locally for the chemical deposition of the sedimentary components at ca. These rocks were deposited in a somewhat analogous way to how limestones or cherts are deposited directly from seawater in modern marine environments.
The oldest known Earth materials are actually not rocks. Sand grains comprised of the mineral zircon ZrSiO4 have been discovered that are almost million years older than the oldest rocks in the rock record. Zircon is a very useful mineral that is mechanically resistant to erosion, chemically resistant to fluids, and can be 'dated' with the U-Pb method owing to the ubiquitous presence of trace amounts of radioactive U and Th that are incorporated in most zircons at the time of crystallization.
The very existence of these ancient zircons demonstrates that igneous rock e. But the evidence of oceans preserved in these grains comes in a different form. Oxygen isotopes in geologic materials are affected by temperatures present during the formation and alteration of rocks and minerals.
Analysis of oxygen isotope ratio in zircon can address the nature of the reservoir of oxygen in the magma that is adopted by the zircon during crystallization. Because we can also 'date' the zircon grains, we can place these conditions in a temporal context. Just after Ma the story changes.
In other words, surface waters were present by at least Ma. Hypothesis 1: Oceans first formed at ca. Most of the salts and dissolved elements in the ocean were probably outgassed along with the water vapor, so the ocean has probably always been about as salty as it is now.
Therefore, the rate of input of new material must be balanced by the rate of removal; in other words, the oceans are in a steady state in regards to salinity.
There are multiple pathways through which dissolved ions enter the ocean; runoff from streams and rivers, volcanic activity, hydrothermal vents see section 4.
Ions are removed from seawater as they are incorporated by living organisms for example in shell production or sediments , sea spray, percolation of water into the crust, or when sea water gets isolated from the ocean and evaporates. The relationship between the input and removal of an ion can be examined through the concept of residence time , which is the average length of time a single atom of an element remains in the ocean before being removed.
Residence time is calculated as:. There is great variation in residence times for different substances Table 5. Generally speaking, substances that are readily used in biological processes have short residence times, as they are used up as they become available. Substances with longer residence times are less reactive, and may be a part of long-scale geological cycles.
So what about lakes? One reason is that compared to the oceans, lakes and ponds are relatively temporary phenomena, so they do not last long enough to accumulate the same levels of ions as the oceans.
Furthermore, lakes often have rivers flowing both into and out of them, so many ions are removed through the outflow, eventually finding their way to the oceans. The oceans only receive river input; there are no rivers flowing out of the ocean to remove these materials, so they are found in greater abundance in sea water. It should be noted that there are some lakes that contain water whose salt content may rival or exceed that of the ocean; these lakes usually lack river outflow.
Skip to content So how did the oceans form in the first place?
0コメント