Earth's Inner Structure Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Planetary Hypothesis

In 1900, Forest Moulton and Thomas Chamberlin, both from the University of Chicago, added their spin on this early theory. They thought that our Sun was a larger star before the planets were formed. A roving star with a strong gravitational field passed by and pulled a chunk of solar material away. This material spun off and in time condensed to form planets.

When the Earth was first formed, it was made up of a molten mass of simmering rock and metals. An outer cloud of elements that included hydrogen, helium, and carbon slowly circulated with heavy metals sinking deep into the molten core, and lighter elements rising to the surface. In this way, a multilayer crust was formed. This thin crust floated on a sea of molten rock for about four billion years, spitting volcanic gases.

Gravity had a large effect on this early formation of a layered Earth. From the core with its dense elements to the atmosphere with its light elements, the Earth’s vertical differentiation was decided by gravity. Figure 3-1 shows the approximate densities of the Earth’s layers from the atmosphere to the core.

On the Inside

Fig. 3-1. Gravity had a big role in the vertical layering of the Earth.

The movement of continental landmasses affects the horizontal configuration of the Earth, but the development of the core, mantle, and crust came about as a result of gravitational forces.

The first atmosphere had little or no free oxygen. It was not user friendly and made today’s pollution problems look like child’s play. Some scientists think it wasn’t until after the first single-celled blue-green algae appeared on a global basis to metabolize toxic gases that breathable air was possible. Blue-green algae produce food by photosynthesis and as now, solar energy was plentiful. Oxygen was a side effect of the process. Remnants of these ancient organisms have been discovered in rocks over three billion years old.

After millions of years had passed, enough elemental oxygen was formed to provide an atmosphere for oxygen-breathing organisms. The Earth became a multilayered sphere of different temperatures and composition with a complex atmosphere and hydrosphere. Throughout this time, rain cooled the crust and collected in pools, rivers, lakes, seas, and oceans to provide good fishing, swimming, water skiing, and sailing millions of years later.


The crust of the Earth is a lot like the crust on bread, as far as the amount of crust compared to the rest of the loaf. The Earth’s crust is just a thin skin on the land and under the oceans compared to the larger whole.

The outer layer of the Earth, or lithosphere, is made mostly of a brittle, rocky crust with the lower crust/upper mantle made of slightly less firm, but denser rock. The crust is where all the land that we know and love is found. It is the easiest layer to study by everyone, from school children bringing home shiny rocks to mom and dad, to petroleum geologists looking for the best places to drill for oil.

If the crust is made of rock, then what is rock exactly? Some people might call it a hard piece of dirt or soil, some might think it is a smaller part of a boulder, like a branch is a smaller part of a limb. These descriptions work for everyday, but geologists, who want to find out everything about how, when, and where the Earth’s solid matter was formed, need to be more specific.

To a geologist, a rock is an individual mass of solid matter that makes up part of the planet.

The key to the geologist’s definition is that a rock is a mass of solid material. So then a handful of sand grains is not a rock because it is not cemented into a solid mass. If it was all one mass, like sandstone, then it would be considered a rock. A tree, though solid, is not a rock, but an ancient tree that has had all its organic material and water compressed out and replaced by minerals to become solid matter, is called petrified wood and is a rock.

The study of geology is about making simple observations. A lot of discoveries have been made by amateurs. But as instruments were developed that analyzed individual rock and mineral elements, as well as the Earth’s vibrations, even more information was gathered.

The crust is also the thinnest of the Earth’s layers making up only about 1/30th of the distance to the Earth’s core. The top part is made of fairly light, granite-like rocks made of silica (SiO 2 ) and aluminum. These were formed from melted rock that pushed up from the mantle and other parts of the crust to become new land and mountain ranges above and below sea level.

The continental crust is made up of a mixture of rock types, mostly granites that are lighter in color and high in silica minerals. In fact, the Earth’s crust is made up of over 70% silicon and oxygen. Table 3-1 shows the different elements found in the continental and oceanic crust.

Table 3-1 There is a variety of elements within the Earth’s crust.


Percent of the Earth’s crust


















Of the crust’s minerals, the silicate group is the largest. It is based on silicon and oxygen with a mixture of different elements thrown in for color. Continental crustal rocks are made up of mostly granitic rocks, while oceanic crust is mostly basaltic rocks. Figure 3-2 gives you an idea of the amounts of minerals like calcium, silica, magnesium, potassium, and others found in the continental and oceanic crusts.

On the Inside

Fig. 3-2. Buoyancy causes a mirror image between upper and lower crustal elevations.

The underlying structure of all silicates is the crystal tetrahedron shape of silica. It is formed by a single silicon cation (Si 4+ ) bonded to four oxygen anions (O 2– ). The different silicate mineral groups are separated by the way elements are bonded to the silicon tetrahedron. For example, in the silicate olivine , a single-tetrahedron silicate, the anion charges of oxygen are balanced by the two positive cations of iron (Fe 2+ ) or magnesium (Mg 2+ ). The formula then is (Fe or Mg) 2 SiO 4 . Another common silicate, garnet , is balanced by calcium, magnesium, aluminum, or iron in the formula (Ca, Mg, or Fe) 3 (Al or Fe) 2 Si 3 O 12 .

The oceanic crust is heavier with more metals like magnesium and iron. The ages of the continental crust and oceanic crust are widely different. The continental crust is about 650 million years old, while the oceanic crust is only about 60 million years old.

Geologists suspect this huge gap of time is due to the more active recycling of the oceanic crust. The recycling of the crust is known as subduction .

The crust is made of big, broken chunks of land from the original supercontinent, Pangea. This single giant landmass began breaking up about 200 million years ago. Figure 3-3 shows large land chunks (continents) scattered across the face of the Earth. So much of the Earth’s crust has changed since its formation that deciphering the mystery of its growth and change will go on for a long time.

On the Inside

Fig. 3-3. The layers of the Earth are so deep that geologists have only scratched the surface.

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