Early Earth and its Composition

The Earth is what is known as a terrestrial planet, contrary to the planets in the outer part of the solar system known as Jovian planets. Terrestrial planets (also including Mercury, Venus, and Mars) are fairly small and dense. They have rocky surfaces with metals packed in their interiors. They have very few moons and no rings, and in addition comprise of hydrogen, helium, and hydrogen compounds (water for example).

But how did Earth come to be a terrestrial planet? Since hydrogen and helium originally made up 98% of the solar nebula (rock, metal, and hydrogen compounds making up the remaining portion) and had not condensed yet, the solar nebula was almost entirely gaseous. The rock, metal, and hydrogen compounds could only condense into a solid where the temperature permitted. Because Earth was fairly close the sun, most types of rock and metal were able to condense. However, the hydrogen compounds could only condense into ice beyond the frost line, located between Mars and Jupiter. Thus, the hydrogen compounds remained gaseous at Earth’s location, explaining why Earth was not made up of any ice and, instead, became a rocky, terrestrial planet.

The Earth’s interior contains three layers. The first is the core which has the highest density and is made up of primarily nickel and iron. The outermost part of the core is liquid. However, unlike conventional wisdom, geologists believe the innermost part of the core is solid. (1) Although the temperature gets increasingly hotter toward the center (up to almost 13,000 degrees Fahrenheit), the rock still remains solid because of the enormous pressure. It should also be noted that scientists claim the currents flowing in the core are responsible for Earth’s magnetic field.

The second layer is the mantle, having a moderate density and consisting mostly of minerals like silicon and oxygen. (1) It lies about 2,000 miles beneath the crust and slowly flows. The slow motions of the rock in the mantle drag the crust and cause continents to move, as well as earthquakes, volcanoes, and the formation of mountains.

The last layer is the crust, the outermost layer containing the lowest-density rock like granite and basalt. Materials rise and sink near the Earth’s surface. This is called convection. The hot rock in the mantle flows upward while the cooler rock near the surface sinks. The crust breaks apart into numerous slabs of tectonic plates which move, as noted above, due to the flowing of the Earth’s mantle. (1) The first large masses of continental crust are believed by geologists to have formed about 3.5 billion years ago, with plate tectonics active the last two billion years.

During Earth’s early formation, it began as a waterless mass of rock surrounded by a cloud of gas. The radioactive materials in the rock and the increased pressure in the Earth’s interior produced the necessary heat to melt the interior of the Earth. The heavy materials (iron and nickel for example) sank to the high density core while the lighter materials (like silicon rock) rose to the surface forming the Earth’s crust. (1) The heating of the interior caused chemicals from within the Earth to come to the surface, some of them forming water and others forming the gases in the atmosphere. Eventually, water would collect in low places of the crust to form oceans.

(1) Scientists theorize that Earth’s crust and oceans were created about 200 million years after the Earth had taken shape. This is due to the discovery of crystals of the mineral zircon that is believed to be over four billion years old. Zircon is made up of silicon, oxygen, and zirconium and is a hard mineral that is not affected by erosion and weathering (allowing it to be long lasting). Thus, scientists declare liquid water to have existed when the crystals formed and base their prediction upon that hypothesis.

References:

1.) Dunbar, Brian. “Nasa-Earth.” 2007. Nasa.gov. 2 Apr. 2009.