Earth: A Terrestrial Planet

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Figure 1: Earth is the third planet from the sun, with Mercury and Venus in locations 1 and 2, and Mars just beyond the Earth, 4th from the sun. In the above depiction of our solar system, there are 9 planets, including Pluto, which is farthest from the sun. In 2006, scientists demoted Pluto from a planet to a “dwarf planet”, leaving the 8 planets that are most widely recognized as comprising our solar system.1

To understand how Earth obtained the minerals and elements that ultimately gave rise to and support all life, we will look briefly at how Earth was formed. Earth is among the eight planets of our solar system that orbit around the sun. Earth is positioned third from the Sun (Figure 1). This location gives it certain properties and characteristics, such as having a hospitable temperature, being terrestrial or rocky, and having water in solid, liquid, and gaseous forms. All of these characteristics of Earth and its location in our solar system allow it to be the only planet known to support life.

Earth's Dynamic Nature

Closer Look

Seismology studies help us discover Earth’s layered structure.

Earth formed when a single nebula (an interstellar cloud) collapsed and spewed debris and energy throughout its region in space. As the debris particles collided with one another, matter gathered together and eventually formed separate bodies of mass having their own gravitational attractions that led to the beginnings of the 8 planets. On Earth, the energy from these collisions diffused throughout the planet’s mass as heat, causing the planet’s surface to grow very hot and its interior to grow even hotter. At that time, 4.5 billion years ago, Earth was a body of hot molten liquid minerals. Through the process of differentiation, denser materials, such as metals, were drawn toward the Earth’s core by gravitational pull, and lighter materials (such as silicon, aluminum, magnesium, and calcium combined with oxygen) remained closer to the surface. This differentiation among particle size and density of Earth’s materials gave way to the planet’s layered structure.

Earth has four primary layers (Figure 2): the core (outer and inner), the mantle (upper and lower), the crust, and the Earth’s atmosphere. Each layer has a different chemical and physical composition.

Figure 2: Cross Section of Earth's Layers.1

 

Closer Look

Check out this short film demonstrating plate tectonics.

Earth’s layered structure is not static. Heat from the Earth’s inner core flows outward, causing matter to move from the interior of the Earth to its surface. Volcanoes, earthquakes, and plate tectonics are evidence of this dynamic behavior. This is important because when the Earth’s inner core flows toward the surface, minerals and heavy metals are redistributed, making them more accessible to human exploitation. In the lower mantle, cooler, denser matter sinks and displaces warmer, lighter matter. This is accompanied by a flow of heat called convection. Convection from the hot molten in Earth’s core forces relatively hard, shell-like structures of the  upper mantle and crust, known as tectonic plates, to slowly move on Earth’s surface and collide with one another.

Closer Look

Read about the development of instruments designed to measure the gradual movements of Earth’s tectonic plates in this article from Earth Magazine.

Tectonic plate movement is so slow that it is generally not detectable to humans without highly sensitive instruments. As these large pieces of the Earth’s crust move and collide with one another, the collision sites result in the development of mountains, where one land mass buckles under another, forcing the plate on top upward. For example, 40-50 million years ago the land mass that we now call India was located south of the equator near Australia. It began moving north until it collided with what is now called Tibet, forming the Himalayan Mountains (Figure 3).

Figure 3: Movement of Indian land mass over millions of years. 1

The force of the Indian land mass collision was so great that India is still moving northward today, such that the Himalayan Mountains continue to rise at an average rate of 2cm per year. To learn more about this amazing phenomenon, go to “two continents collide”. Likewise, where tectonic land masses pull away from one another, the development of valleys and deep ocean trenches occurs.

Figure 4. The Ring of Fire is the subduction zone at the edges of the Pacific Ocean tectonic plate where it adjoins with land mass plates. Where these plates meet, the Pacific Ocean plate typically gets wedged under the edge of the land plates, causing buckling, severe tension and pressure at the margins. 1

The edges of tectonic plates, where they push and pull against one another, are often areas where the greatest volcanic and earthquake activity is experienced. The majority of the world’s most powerful volcanoes and earthquakes come from the Ring of Fire, a seam between the Pacific Oceanic plate and the land mass tectonic plates that surround it stretching from New Zealand north to Japan, across to Alaska, and southward along the west coast of North and South America (Figure 4).

Volcanic eruptions bring to the Earth’s surface many elements which were trapped deep within the Earth at the point of Earth’s formation, including silicon, aluminum, iron, magnesium, calcium, sodium, potassium, phosphorus, titanium, sulfur, and many other metals and non-metals. The Ring of Fire has produced the most violent volcanoes, earthquakes, and tsunamis in our history. Take a look at this news story about The Really Big One, an earthquake and tsunami anticipated to hit the Pacific Northwest of North America within the next 50 years.

Earth’s Spheres

At the crust and the surface of Earth, matter is organized in four primary spheres: the atmosphere, the hydrosphere, the biosphere, and the lithosphere (Figure 5). All of the available natural resources occur within these spheres.

Figure 5: Earth’s four spheres1

Looking Ahead

When some people observe the incredible beauty and complexity of Earth's four spheres they feel they are experiencing something 'sacred'. We will discuss this phenomenon is the Spirituality Section of this chapter.

The atmosphere is a relatively thin layer of gaseous matter that surrounds the Earth’s surface. The hydrosphere defines the location and movements of water on and under the surface of Earth, as well as the water vapor within the atmosphere. The lithosphere comprises the solid soils, sediments, and rocks of the Earth’s crust and upper mantle. The biosphere is made up of all ecosystems and zones of life that occur within the other three spheres of Earth. Life exists nearly everywhere, in all hospitable spaces on Earth and also in the less hospitable habitats including the extremely hot, high-pressure environments of the deep ocean’s hydrothermal vents, in clouds in the sky  (where some microorganisms reside), and inside frozen sandstone in the Antarctic, where specialized algae and bacteria thrive (see Figure 6).

Figure 6: Biospheric life forms occur in the Atmosphere, Hydrosphere, and Lithosphere1

While the resources that Earth provides come from the four primary spheres, some resources are also present at the interface of these spheres. For example, soil formation occurs at the interface of all four spheres where organic matter and living organisms (from the biosphere), air (from the atmosphere), water (from the hydrosphere), and weathered rock and mineral particles (from the lithosphere) combine. Figure 7 provides a flow chart of our basic natural resources (materials and energy) depicting the spheres that contribute to those resources.

Figure 7: General categories of Earth’s resources (in black boxes) and their relationship to the four spheres.1

  • 1.

    adapted from Fig 1.6 Earth Resources and the Environment, Fourth Edition, Pearson Education Inc., 2011m Craig, James R., David J. Vaughan, Brian J. Skinner

All of our natural resources, in solid, liquid, and gaseous forms, are found within these four spheres. Various types of matter move within and across the four spheres by means of biogeochemical cycles, through which elements change chemical form. This cycling of matter contrasts with the unidirectional flow of energy through Earth’s system. Energy from the Sun, which flows unidirectionally toward Earth, is transformed by different processes in the Earth’s spheres, and then flows out of Earth, usually in the form of dissipating heat.

Looking Ahead

The hydrologic cycle, or the biogeo-chemical cycle of water, is detailed in the Water Chapter.

Matter, on the other hand, moves through biogeochemical cycles where it changes form, but does not dissipate. Energy flows unidirectionally, and matter flows in cycles. Transformations of matter are kept in balance and remain repeatable through the natural biogeochemical cycles, and this is crucial in order to provide a continuous source of elements that are required for the maintenance of life. Before describing the biogeochemical cycles we will first look at the elements that exist on Earth, along with their properties.