The Four Spheres of Earth and Their Influence
There are four interlocking puzzle pieces that create our environment. The puzzle includes biological (living) pieces and
physical (non-living) pieces. These four pieces are dependent upon each other. To make the study of the biological and
physical elements of the Earth easier to understand, scientists separate these biophysical elements into four spheres:
Atmosphere
Hydrosphere
Lithosphere
Biosphere
To better understand what these terms mean, it is important to understand that '-sphere' in this instance means 'to
surround or encompass'. The following prefixes help us understand the meaning of the four spheres:
1. atmo - steam; vapor
2. hydro – water
3. litho - rock; mineral
4. bio - life
The atmosphere
The atmosphere is the air that surrounds the Earth. It is always in motion and constantly changing, held in its place by
the Earth's gravity. There are about 14 different gases that make up the atmosphere. There are three gases which are
most prevalent and compose about 99 percent of the environment. Nitrogen (N2) = over 78 percent, oxygen (O2) = over
20 percent, and argon (Ar) = about 1 percent. Aside from these gases, the atmosphere is also made of moisture found
within the gases and solid particles, known as particular matter.
The atmosphere is responsible for the weather. All weather occurs within the lower atmosphere. Weather is the shortterm change within the atmosphere and is dependent upon several factors:
Temperature
Precipitation
Humidity
Air pressure (weight of the air)
Speed and direction of wind
The atmosphere is made of various 'layers' of air. The layers are separated by invisible 'lines'. These lines separate the
changes in temperature that occur with the increase in altitude in the atmosphere. The bottom layer of the atmosphere
is the troposphere. The troposphere is where weather happens. It is warmest near the Earth because of the heat rising
from the Earth's surface. It becomes colder with altitude. This layer is separated from the next by the tropopause. The
tropopause is the point where temperatures change and begin to increase with altitude.
Above the tropopause is the stratosphere. There is a large concentration of ozone gas in the stratosphere. The ozone
gases absorb most radiant solar energy, protecting the Earth from harmful ultra violet (UV) rays. The stratosphere
becomes warmer with increasing altitude, due to the high absorption of solar energy. Temperatures then start to
decrease around the imaginary line known as the stratopause and continue to decrease rapidly into the next layer.
The mesosphere is the coldest of the spheres. It is so cold that water vapours often freeze and create clouds made
purely of ice. The mesosphere is separated from the thermosphere by the mesopause. This is the point at which the
temperature changes again. The topmost layer is the thermosphere. Within this layer, many satellites circle the Earth.
Because of the thin air and proximity to the sun, the temperatures in the thermosphere increase and decrease rapidly.
Beyond the atmosphere, past the thermosphere, the exosphere stretches out into the vast regions of space.
The hydrosphere
Water is life's most important resource. Plants and animals rely on water for survival. Many animals live in the water.
The hydrosphere encompasses all forms of water in the Earth's environment. This includes the oceans; all water found
on the Earth's surface such as lakes, rivers, snow and glaciers; water under the Earth's surface; and water vapor found in
the atmosphere.
Much like the gases in the atmosphere, the water in the hydrosphere is always in motion. We can see the flow of rivers
and streams and the waves at the beach. The middles of oceans and lakes also move due to wind and temperature
changes, which create currents.
The hydrosphere formed in conjunction with the atmosphere. About 3.6 to 4.3 billion years ago, there were many hot
molten rocks containing water and gases. When the rocks cooled down and released hydrogen and water vapours into
the air, the atmosphere was formed. The water vapors condensed to form clouds. Precipitation fell to the Earth and the
hydrosphere was also formed. Oxygen was produced as a by-product of small organisms and plant life.
The lithosphere
The lithosphere, which evolved about 4.6 billion years ago, is composed of an outermost layer of hard rock. This outer
rock layer is made up of the crust and the first hard layer of mantle. The lithosphere includes various landforms such as
mountains and valleys, as well as rocks, minerals and soil. The lithosphere is constantly being shaped by external forces
such as sun, wind, ice, water and chemical changes.
There are many different types of rocks that compose the outer crust. These rocks can be grouped into three basic
categories based on how they are formed.
Igneous rocks are formed by the cooling of hot molten rock (magma). When the hot, flowing magma cools, it begins to
harden. Once completely cooled it is then an igneous rock.
Sedimentary rocks are formed from pre-existing rocks. Rocks erode, mix with other dirt, clay and particles and settle
together. This mix creates a sedimentary rock.
Metamorphic rocks are formed by heat and/or pressure from pre-existing rocks.
The lithosphere does not wrap perfectly around the Earth. It is in fact about 15 different pieces of crust that sit on top of
a softer layer called the asthenosphere. These tectonic plates drift around on top of the asthenosphere and are moving,
though too slowly for the human eye to notice. As a result of the plate movement, mountains become taller and valleys
grow wider. Scientists believe that the continents, which rest on the various plates, were at one point joined in a single
'super continent' known as Pangaea.
The Earth's surface is composed of the two types of lithospheres: the oceanic and the continental. The oceanic
lithosphere includes the uppermost layers of mantle topped with the thin but heavy oceanic crust. This is where the
oceans of the hydrosphere meet the lithosphere. The continental lithosphere includes the uppermost layers of mantle
topped with a thick, lighter continental crust. This is where the atmosphere, biosphere and the hydrosphere meet the
lithosphere.
The biosphere
The three previous spheres - the atmosphere, the hydrosphere, and the lithosphere - are the abiotic, or non-living, parts
of the biophysical environment. The fourth sphere, the biosphere, includes all biotic, or living, parts of the biophysical
environment. The biotic elements include all plants and animals.
All life exists in the biosphere. The biosphere cannot survive without elements from all the other spheres. Plants and
animals need water from the hydrosphere, minerals from the lithosphere and gases from the atmosphere. The air,
water, and land provide homes for all the various forms of life.
The environment contains all the factors that surround and influence the biotic and abiotic things within it. The
environment is our surroundings. Each living thing within the biosphere inhabits and interacts with the things that
surround them. This is their biophysical environment. It is in this environment that we find ecosystems. An ecosystem is
a smaller function within the environment. It is the unique interaction between the living and non-living elements. An
ecosystem is a community functioning together as one unit.