The Spheres of Earth

Planet Earth is a complex system, made up of sub-systems called ’spheres’, as they too are round like Earth.

It is possible to divide the Earth into at least 17 spheres (and possibly more), beginning with its core and mantle and ending with the farthest range of its atmosphere (the exosphere). We can simplify these 17 to just four if we consider only the most basic components of Earth, these being land, air, water, and life.

These four spheres are the lithosphere, the atmosphere, the hydrosphere, and the biosphere respectively, and together create a fifth called the pedosphere. This is the soil-containing upper layer of the Earth’s crust, and is arguably a sub-sphere of both the lithosphere and biosphere. The interactions of the four major spheres which form the pedosphere are collectively known as pedogenesis.

The Lithosphere

From the Ancient Greek words λίθος, líthos, ‘stone’; and σφαῖρα, sphaira, ‘sphere’.

The lithosphere is all the rocks of the hard, rigid outer layer of Earth, and sits on the weaker, hotter, deeper, and more fluid asthenosphere (from the Ancient Greek words ἀσθενός, asthenos, ‘without strength, weak’; and σφαῖρα, sphaira, ‘sphere’) (Fig. 1).

Some of these rocks are ancient — over 4 billion years old compared to Earth itself at about 4.5 billion years old.

The lithosphere appears static to us, but there is actually a very slow rock cycle in progress, whereby rocks sink into the asthenosphere, melt, become magma, rise, and re-enter the lithosphere. Magma solidifies in the lithosphere into igneous rocks, which either remain as such, or transform into metamorphic or sedimentary rocks. All three types may erode, and/or sink back into the asthenosphere to repeat the cycle.

The Atmosphere

From the Ancient Greek words ἀτμός, atmós, ‘vapour, stream’; and σφαῖρα, sphaira, ‘sphere’.

Also known as ‘air’, Earth’s dry atmosphere by volume is 78% nitrogen (N₂), 21% oxygen (O₂), 0.9% argon (Ar), and 0.04% carbon dioxide (CO₂), with the remaining 0.06% comprised of helium, neon and krypton.

Water vapour concentration can range from 0 to 3% by volume depending on location, but makes up 0.25% of the mass of the overall atmosphere.

There are five sub-spheres, or layers: the troposphere (0 to 12 km above the surface), the stratosphere (12 to 50 km), the mesosphere (50 to 80 km), the thermosphere (80 to 700 km), and the exosphere (700 to 10, 000 km).

The troposphere, or inner layer, is 80% of the total mass, and where most of Earth’s weather occurs. Its name is from the Greek word τρόπος, tropos, ‘turn’, referencing the vertical mixing it undergoes, driven by a temperature gradient along this direction.

The Hydrosphere

From the Ancient Greek words ὕδωρ, hydōr, ‘water’; and σφαῖρα, sphaira, ‘sphere’.

Earth’s hydrosphere is about 4 billion years old, covers 71% of the Earth’s surface, and comprises some 1,386 million cubic kilometres of water in the form of atmospheric vapour, oceans, rivers, lakes, streams, groundwater, soil moisture, snow, and ice. Only 2.5% of the hydrosphere is fresh water, and only 0.3% of that is accessible as rivers, lakes, streams and dams.

The Biosphere

From the Ancient Greek words βίος, bíos, ‘life’; and σφαῖρα, sphaira, ‘sphere’.

The biosphere is all life on Earth, which first arose at least 3.5 billion years ago. Life can be found in all extremes of habitats and temperatures, from hot, dry, rocky deserts, to frozen polar caps and thermal springs. Life exists in the deepest oceans, the highest mountains, inside rocks, and has been found over 5 km into the Earth’s crust.

The Pedosphere

The pedosphere (from the Ancient Greek words πέδον, pedon, ‘soil’; and σφαῖρα, sphaira, ‘sphere’) is the upper layer of the Earth’s crust containing soil, and in which soil-forming processes take place.

Pedogenesis

Pedogenesis is the process of soil formation (from the Ancient Greek words πέδον, pedon, ‘soil’; and γένεσις, génesis, ‘origin, birth’ — ‘soil birth’).

Soil, very simply, is rock that has broken down to its mineral components, but the type of soil which forms — and the minerals it contains — is very much determined by the composition of the original bedrock. For example, limestone bedrock forms high pH (alkaline) soils rich in calcium carbonate (CaCO₃), while granite bedrock leads to low pH (acidic) soils rich in silicon dioxide (SiO₂), also known as silica or quartz and a major component of sand.

Interactions of the Spheres in Pedogenesis

The breakdown of underlying bedrock into its mineral components is called weathering, of which there are three types: physical, chemical, and biological. These weathering processes arise from interactions between the lithosphere, atmosphere, hydrosphere, and biosphere.

Physical Weathering

Physical weathering is the break up of rocks without physical change, whereby rocks fragment into smaller and smaller particles until they eventually become the mineral component of soil. Wind, temperature, water and pressure are behind this type of weathering.

Wind is abrasive and has a sand-blasting effect. Temperature fluctuations over days, seasons and years cause rock to expand and contract until it cracks. Water continually freezing and thawing in holes or cracks also breaks rocks over time. These are all examples of the atmosphere and hydrosphere interacting with the lithosphere.

Plant roots in cracks can exert pressure and push rocks apart as they grow and thicken, and this is an example of the biosphere interacting with the lithosphere.

Chemical Weathering

Chemical weathering is the breakdown of rock through chemical changes. Water and oxygen are the major forces here. One example is water acting on granite: here the water reacts with feldspar crystals in the granite to create clay minerals, which weaken the rock and increase the likelihood of it fragmenting. This is the hydrosphere interacting with the lithosphere.

An example of the atmosphere interacting with the lithosphere is when oxygen reacts with iron to create iron oxides. These oxides are more fragile than iron and compromise the rock’s structure, which fragments.

Biological Weathering

Biological weathering is a form of chemical weathering — life is chemistry (biochemistry).

Some plants and microorganisms release acids that break down rocks and mineral compounds. Lichens (which are not single organisms but rather fungi and algae or fungi and cyanobacteria living together in beneficial relationships) produce a weak acid capable of dissolving rock. Some bacteria, known as lithotrophs [from the Ancient Greek λίθος, líthos, ‘stone’; and τροφή, trophḗ, ‘nourishment’: ‘nourishment from rock’ (inorganic materials)] ‘eat’ rocks by breaking chemical bonds to release energy.

While it is clear that the biosphere is interacting with the lithosphere here, the hydrosphere also has a role, as without water there is neither a means to drive these reactions, nor a biosphere for that matter.

Rate of Pedogenesis

The many and varied interactions between the spheres determine the rate of, and the type of, weathering which occurs. These in turn determine the rate of, and the type of, soil which forms.

Thus pedogenesis is as much a function of time as well as the climate, environment, organisms and rock type specific to a region, and perhaps we could argue the existence of a fifth major sphere, the chronosphere!

These variables are the reason soils vary so much nationally and globally. Bedrocks rich in quartz, such as sandstone, will form sandy soils while bedrocks poor in quartz, such as shale or basalt, will not. And bedrocks in hot, dry deserts will break down more slowly, and no doubt differently, to the same rocks on a wet, temperate coast, and form different soils as a result.

Each soil is different, and understanding the processes that formed each one helps us understand its unique properties and work with it all the better.