Four Spheres of Physical Geography

blue marble apollo 17
Earth, the “Blue Marble”, taken by Apollo 17 astronauts.

In 1972, during the final mission to the Moon, Apollo 17 astronauts looked back at the Earth and took one of the most iconic photographs ever taken. This photograph, known as the “Blue Marble,” was until recently the only such photo in which the Earth is not partially covered in shadow.

What’s immediately apparent from the “Blue Marble” photo is that the Earth is much more than a blue marble; it’s awash in a variety of colors. Encircling the globe are large swirls of bright-white clouds that blend in perfectly with the antarctic ice. Africa and the Arabian peninsula are clearly visible with deserts appearing tan and vast grasslands and tropical forests reflecting a deep green hue.

Home to millions of species of plants, animals, and other organisms, and undergoing constant geological evolution through the movement of tectonic plates, volcanic activity, and weathering, the Earth is a dynamic and ever-changing system. Although the many processes, cycles, and flows that drive the Earth system are often varied and complex, geologists like to classify these components into four main categories or “spheres:” the geosphere, hydrosphere, atmosphere, and biosphere.

Earth's four spheres
The Earth’s 4 spheres: atmosphere (air), hydrosphere (water), geosphere (land) and biosphere (life).

Geosphere (or Lithosphere)

The geosphere is the physical Earth: rocks, soils, minerals – essentially everything under your feet that isn’t built by man or living. The geosphere is by far the most massive of the four Earth systems, extending from the surface to the very center of the Earth, some 6,400 kilometers (3,977 miles) underground. For perspective, humans have only managed to drill about 12.3 km (7.6 miles) into the Earth at the Kola Superdeep Borehole in Russia. The Geosphere is composed of four main layers: crust, mantle, outer core, and inner core.

The crust is the solid, outer shell of the planet – the part of the geosphere we all inhabit. Compared with the other layers of the geosphere, the crust is incredibly thin. Oceanic crust is typically 5 km (3 miles) to 10 km (6 miles) thick, while continental crust is anywhere from 35 to 70 km (22 to 43 miles) thick. Continental crust is significantly less dense than oceanic crust and therefore tends to stay on the surface.

The crust is not one large piece of rock; it is broken up into dozens of enormous lithospheric plates that move across the surface of the Earth at the rate of centimeters per year. At divergent boundaries the crust grows and moves apart, and at convergent boundaries one plate may subduct (moves beneath) forming a trench, or they may ram into one another forming a mountain chain like the Himalayas. When oceanic and continental crust meet at a convergent boundary, the denser oceanic crust is always subducted under the lighter continental crust, explaining why oceanic crust is much younger than continental crust.

The mantle comprises about 82 percent of the Earth’s volume, and extends from the crust-mantle boundary down to the outer core about 2,900 km (1,800 miles) deep within the planet. The mantle is composed of mostly solid rock, though the upper portion, called the lithosphere, is more rigid than the aesthenosphere just below and the lower mantle below that. Partial melting of rock occurs at the lithosphere-aesthenosphere boundary, which allows the lithosphere and crust (100 km thick on average) above to detach from the lower mantle and move laterally. Beginning at a depth of 660 km, the lower mantle is composed of solid rock packed tightly together by tremendous pressure. The rocks are hot enough, however, to flow slowly.

The Earth’s geosphere is composed of several distinct layers.

The Earth’s core is metallic; believed to be composed of a iron-nickel alloy. The incredible pressure at this depth causes the material in the core to have a density about 14 times that of water. The outer core, which is found at a depth of 2,900 to 5,150 km, is liquid and flows readily around the solid inner core. An enormous metallic ball, the inner core has a radius of 1,216 km (754 miles); not much smaller than the Moon. Although the metal in the inner core is extremely hot, it remains a solid due to the confining pressure of the surrounding planet.

Hydrosphere

Streams, such as the Missouri River pictured here, make up a tiny fraction of the Earth’s hydrosphere, but is one of the most important sources of water for people.

The hydrosphere includes all the water on Earth. Covering some 71 percent of the Earth’s surface, the oceans constitute – by a wide margin – the largest reserve of water on the planet. With an average depth of 3,800 m (12,500 ft), the oceans contain about 97 percent of all the water on Earth, about 1.3 billion cubic km (310 million cubic miles). Of the remaining 3 percent, 2 percent of global water supplies are frozen in glaciers, about 0.6 percent is groundwater, and less than 0.1 percent is found in all the lakes, rivers and streams. Clearly, there is much less freshwater than saltwater on the planet, and most of it is in the form of polar ice.

Water does not stay in any one ocean, lake, stream, or underground reservoir indefinitely; water is constantly moving about the planet in an endless hydrological cycle. Due to energy provided by the Sun, water readily evaporates off the surface of the planet, where it is transported by the atmosphere and eventually returns to the surface as some type of precipitation (snow, ice, rain). If the precipitation falls as a liquid over land, it eventually finds its way back to the ocean under the influence of gravity or evaporates back into the atmosphere. Although some water seeps deep into the ground or becomes frozen in enormous polar ice sheets, none of it remains in one place indefinitely. Even the water absorbed by plants eventually evaporates back into the atmosphere in a process known as transpiration.

Atmosphere

When staring up into the sky, or when flying in a commercial aircraft, you may get the impression that the atmosphere – the layer of gases the encircle the Earth – is really quite thick. Compared with the geosphere below, however, the atmosphere is a very thin layer; about 50 percent of the gas molecules that comprise the atmosphere are found within 5.6 kilometers (3 miles) of the Earth’s surface and over 90 percent are found within 16 kilometers (10 miles). The atmosphere – like everything else on the planet – is held to the Earth by gravity.

Earth’s atmosphere has more major layers. Credit: NASA.

The gases that comprise most of the atmosphere include nitrogen (78 percent), oxygen (21 percent),  and trace amounts of argon, carbon dioxide, helium and hydrogen. The atmosphere is absolutely critical to life on the planet, not only because it provides the gases we need to breath, but also because it absorbs and reflects deadly solar radiation. A layer of ozone in the stratosphere absorbs about 97-99 percent of the ultraviolet radiation from the Sun.

The atmosphere can be divided into four layers based primarily on temperature changes: troposphere, stratosphere, mesosphere and thermosphere. The troposphere is the layer closest to the ground, the layer in which most of us will spend all or most of our lives. It is about 12 km (7.4 miles) thick, and is distinguished by a reduction in temperature with increasing altitude. The temperature at the lower boundary of the stratosphere is about -60 °C. The stratosphere, which rises to altitude of about 50 km, exhibits an overall increase in temperature with increasing altitude, up to about 0 °C. In the mesosphere, the temperature of the diffuse gas molecules begins to fall yet again, reaching the lowest temperatures in the atmosphere, about -90 °C. Temperatures again begin to rise within the outer layer of the atmosphere, the thermosphere, eventually reaching about 1000 °C. The atmosphere is so thin at this altitude, and the gas molecules so diffuse, however, that the amount of heat energy is relatively low; probably not enough to burn your skin.

Biosphere

The biosphere is the sum total of all life on Earth. Much of the life on this planet survives within a vary narrow layer just above or below the surface. But life flourishes within that later, having seemingly colonized every last corner of the planet, including extreme environments like boiling springs, deep underground caverns,  and frozen ice shelves. Life has proven to be exceedingly adaptive and robust, making it more likely that life has survived elsewhere in the universe. As far as we know, however, life is unique to the Earth. There is growing evidence that life, as integral part of the Earth system, plays a role in maintaining the balance of conditions necessary to sustain itself on this planet.

Life on Earth survives even in the most unlikely of places, including boiling hot springs. Pictured here is Grand Prismatic Spring in Yellowstone National Park. Credit: Anthony Stone, Flickr.

 

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