The Evolution of Earth’s Fluid Spheres

الأحد مايو 17, 2015 4:52 pm

The Evolution of Earth’s Fluid Spheres

Earth’s interior has been releasing gases since its formation 4.6 billion

years ago. These gases were trapped within the solid rock particles that

came together (accreted) to form Earth. The accretion process began

under the influence of gravity. Over time, more and more rocky debris

was pulled in from the solar system. As Earth’s mass increased, its gravity

grew. The pressure increased on its interior. With an increase in pressure,

its interior heated and melted. Meteorites bombarded the surface of

the young Earth. This caused its primitive crust to melt and rift. As this

occurred, large amounts of gases were released through fissures in Earth’s

surface. The young atmosphere grew in mass and volume through volcanic

activity. Earth’s gravity kept these gases from being stripped away by the

solar wind and blowing off into space. The process by which huge volcanic

eruptions transfer matter from the mantle to the atmosphere is called

outgassing.

Outgassing early in Earth’s history produced a primordial atmosphere of

largely water vapor and carbon dioxide. There were also lesser amounts

of carbon monoxide, hydrogen, and hydrogen chloride. It is possible that

there were traces of methane and ammonia as well. In contrast to the

atmosphere today, oxygen was mostly absent. The early atmosphere was

toxic to life on Earth. The atmospheric pressure may have been about 250

times that of today. During the early Hadean, Earth’s surface temperature

was extremely hot. It was too hot for water to exist as a liquid at the

surface. As a result, there were no oceans, lakes, rivers, or groundwater.

This atmosphere was similar to that on Titan, one of Saturn’s moons.

As the young Earth cooled, outgassing formed a new atmosphere. It

consisted of methane, hydrogen, nitrogen, and water vapor, with smaller

amounts of noble gases and carbon dioxide. (Helium is an example of a

noble gas.) Hydrogen, the lightest component, escaped into space.

Earth’s cooler surface radiated less heat into the atmosphere. Further

cooling enabled water vapor to condense. It fell to the surface as rain.

Much of the first rains would have fallen on hot volcanic rock and

evaporated. As the crust cooled more, rain began to collect in low-lying

areas. This formed bodies of water on the surface. Eventually, enough

water was released from Earth and condensed to form the world’s

oceans. Greenland contains Earth’s oldest marine sedimentary rocks. They

are about 3.8 billion years old. They suggest that it took about a billion

years for the oceans to form. This also means that the atmosphere was

the first part of the fluid spheres to form. The hydrosphere arrived as a

product of the atmosphere.

On the primitive continents, Earth’s first river networks formed. Drainage

basins developed. The rivers flowed and transported rock particles worn

from the continents. These sediments built up in the oceans. They formed

early marine sedimentary rocks. Eventually, these rocks would be recycled

through young subduction zones. They would produce lava with a greater

silica content than that which formed the early basaltic crust.

The outgassing of carbon dioxide

produced rainfall and oceans

that were more acidic than

today. This is because carbon

dioxide, when combined with

water, forms carbonic acid. The

water reacted with rocks through

weathering processes. This added

new chemicals to the water. The

oceans become salty quite quickly.

Scientists have calculated a pH of

about 5.8 for the early Hadean

ocean. By the late Hadean, the

ocean’s pH was closer to neutral

(a pH of 7). Some of the dissolved

chemicals would later become

limestone and the shells of marine

organisms. Today, the oldest

organic marine limestones are

found in western Australia. They

are in rocks 3.5 billion years old.

Some scientists believe that the process of outgassing did not form the

atmosphere and oceans. They argue that the young Earth’s gravity pulled

in chemicals released by the Sun. This material then formed Earth’s early

atmosphere. Other scientists think that Earth’s water came from gigantic

comets several kilometers long, passing by Earth. They suggest that

Earth’s gravity pulled in ice and rocky material from these comets. Using

mathematics, they have calculated that as few as four of these comets

would have been enough to provide water to fill the oceans. Comets do

pass by Earth today. However, they do not do so at a rate fast enough to

fill the oceans with water. If the oceans had formed at the current rate, it

would have taken tens of trillions of years. Because the oceans formed in

about a billion years, scientists argue that the rate at which comets came

in contact with Earth in the past must have been much higher.

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The Evolution of Earth’s Fluid Spheres

The Evolution of Earth’s Fluid Spheres

رد: The Evolution of Earth’s Fluid Spheres