Taking a Closer Look at the Earth's Three Divisions
The biosphere is the outermost part of the planet’s shell which includes the air, land, surface rocks and water within which life occurs, and wherein biotic processes in turn alter or transform. From the broadest geophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere (rocks), hydrosphere (water), and atmosphere (air). Our planet Earth is the only place where life is known to exist. This biosphere is postulated to have evolved, beginning through a process of biogenesis or biopoesis, at least some 3.5 billion years ago.
Biomass accounts for about 3.7 kg carbon per square meter of the earth’s surface averaged over land and sea, making a total of about 1900 gigatonnes of carbon.
What characterizes the lithosphere (from the Greek for “rocky” sphere) is being the solid outermost shell of a rocky planet. On the Earth, the lithosphere includes the crust and the uppermost layer of the mantle (the upper mantle or lower lithosphere), which is joined to the crust.
At the cooling surface layer of the Earth’s convection system, the lithosphere thickens over time. It is fragmented into relatively strong pieces, called tectonic plates, which move independently relative to one another. This movement of lithospheric plates is described as plate tectonics.The distinguishing characteristic of the lithosphere is not composition, but its flow properties. Under the influence of the low-intensity, long-term stresses that drive plate tectonics motions, the lithosphere responds essentially as a rigid shell and thus deforms primarily through brittle failure, whereas the asthenosphere (the layer of the mantle below the lithosphere) is heat-softened and accommodates strain through plastic deformation. Both the crust and upper mantle float on the more plastic asthenosphere. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity.
There are two types of lithosphere:
ð oceanic lithosphere
ð continental lithosphere
Oceanic lithosphere is about 70 km thick (but can be as thin as 1.6 km at the mid-ocean ridges), while continental lithosphere is about 150 km thick (and can be considerably thicker at continental collision zones). Oceanic lithosphere consists mainly of mafic and ultramafic rocks and is denser than continental lithosphere, which consists predominantly of felsic rocks. This higher density has the effect that at subduction zones the oceanic plate will invariably sink underneath the continental plate. New oceanic lithosphere is constantly being produced at mid-ocean ridges from mantle material and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 200 million years old. As oceanic lithosphere grows older, it gets cooler and denser, with the result that if two oceanic plates converge, the older one will subduct below the younger one.
Layers of the Earth
The Earth is made up of four layers. The thin outer layer is called the crust, followed by the hot, partly molten rock, the mantle. Beneath it is the outer core, which is a layer of liquid metal. In the center of the Earth is ball of very hot solid metal called the inner core.
The crust accounts for only about 0.6% of the planet’s volume. It varies in thickness from 248 miles (40 km) beneath the parts of the continents to only 3.1 miles (5 km) under parts of the ocean floor. It is made up of lighter rock than the outer layers. The temperature of the rocks increases by about 86 degrees Fahrenheit (30 degrees Celsius) for every kilometer under the surface. New crust is being made all the time as molten rock bubbles up through the huge cracks between plates in the ocean floor. There are two kinds of earth’s crust: oceanic crust between the seas and the continental crust beneath the land. The oceanic crust has an average thickness of 3.6 miles (5-9 km) and varies comparatively little throughout the world. Its rocks are highly varied, including volcanic lava flows, huge blocks of granite, and sediments laid down in shallow water when parts of the continents were inundated by the sea. Despite the diversity of materials, the average composition is roughly that of rock granite, and the two most common elements in addition to oxygen are silicon and aluminum. The oceanic crust has the much higher average thickness of 18.5 – 25 miles (30-40 km) and varies much more. It is much more of uniform composition and apart from a thin covering of sediment; it is composed largely of the rock basalt. Aside from oxygen, the most common elements in the oceanic crust are the silicon and aluminum with traces of magnesium.
The mantle accounts for about 82% of the Earth’s volume and extends from the base of the crust to a depth of about 1,800 miles (2,900 km). The sharp boundary between the crust and the mantle is called Mohorovicic discontinuity after the Yugoslav seismologist who discovered it in 1909.
Aside from oxygen, it is thought to consist largely of peridotite, a rock that contains high proportions of the elements iron, magnesium and silicon. The mantle is inaccessible and although it is mostly solid, it contains a partially molten layer.
The core accounts for about 17% of the Earth’s volume and extends from the base of the mantle of the earth’s center. The discontinuity between the mantle and the core called the core-mantle boundary or Gutenberg discontinuity, after Beno Gutenberg, a German-American seismologist who discovered it. The core is composed of two distinct parts: the outer core and the inner core. The outer core extends down to a depth of about 3,200 miles (5,155 km) and is liquid. The inner core measures about 1,500 miles (2,400 km). The main component of the core is iron and contain some lighter elements possibly carbon, hydrogen, oxygen, silicon, and sulfur.
Trivia about the Earth
The Earth is the place where we all live and it is interesting to note some facts about it. Here they are:
- The Earth’s total surface area is 196,950,000 sq. miles
(510,100,000 sq. km.).
- The total land area of the Earth is 57,400,000 sq. miles
(148,800,000 sq. km.).
- The distance between the Earth and the moon is 238,884 miles
- The average speed of the Earth in orbit is 18.5 miles (29.8)/sec.
- The Earth’s density is 5,517 times the density of water.
- The Earth tilts on its axis at an angle of 23.5 degrees.
- The distance between the Earth and the Moon is 238,884 miles (384,365 km).
- Light from the Sun takes about 8 minutes to reach the Earth.
- The Earth’s nearest star, apart from the Sun is Proxima Centauri.
- Light from Proxima Centauri takes more than four years to reach Earth.
- What is Earth Made Of? | Composition of Earth
The structure of the Earth is like an onion; it has multiple layers. Earth's layers consist of the crust, mantle and core. Several elements make up the composition of Earth.
- 10(h) Structure of the Earth
- Plate tectonics
The Continental Drift Theory, The Seafloor Spreading Theory and The Plate Tectonics Theory are the three important theories taking place in our planet.
Layers of the Earth
The Composition of the Earth
Questions for Review
1. Describe the layers of the Earth.
2. What is the difference between the oceanic and continental lithosphere?