Convection is an important driver of circulation in many earth
systems. Here are a couple major earth systems that are powered by
convection.
First, circulation of fluids in the atmosphere is driven by
convection. Radiation from the sun heats up air on the earth,
however, it does not do so evenly because of the angle that the
surface of the Earth has with the sun's incoming radiation. Air
closer to equator will receive more thermal radiation than air
closer to the poles, so air closer to the equator will receive more
heat. This causes the hotter air to rise and the cooler air to be
pulled down from the poles by the lower pressure. If the Earth's
atmospheric movement was only influenced by convection, then a
simple circulation cycle of cooler air coming down from the poles
closer to the surface and warmer air rising and heading toward the
poles is what would exist. However, the Earth's atmosphere is also
influenced by its rotation, large landmasses, and tilted rotation
axis. This leads to three different convection cells: a low-latitude
Hadley Cell that moves air near the surface toward the equator and
air higher in the atmosphere toward the poles, a mid-latitude Ferrel
Cell that moves air near the surface pole-ward and air higher in the
atmosphere toward the equator, and a high latitude Polar Cell that
moves air near the surface toward the equator and air higher in the
atmosphere toward the poles.
Air circulation in circulatory cells on Earth.
By Kaidor - Own work based on File:NASA depiction of earth global
atmospheric circulation.jpg, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=23902538
Another way that convection interacts with the earth is in the
formation of sea and land breezes. Sea breezes are winds that come
from the ocean (or another large water body) during the day, and
land breezes are winds that come from the land toward the ocean
during the night. These winds are caused by differences in
temperature between air on land versus air over the ocean that drive
convective movement of air. During the day, the air over the land
will heat up more than the air over the ocean because of the
difference in thermal properties between land and water. This leads
to hotter air over land to rise, which leads to a low pressure area
over the land, which causes air from the ocean to move in and
creates the sea breeze. A land breeze is the same as a sea breeze,
but it just occurs by going from the land to sea during the night
when the air over the water is warm relative to the air over land.
This movement of air due to different temperatures in different
areas is a perfect example of convection in earth systems on a
smaller scale.
Diagram of how a sea breeze forms.
By Theonlysilentbob at English Wikipedia, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=93712026
A last way that convection works in Earth systems is in the mantle.
The mantle is mostly internally heated by processes such as
radioactive decay, with only a small about of its heat coming from
the Earth's core. So, convection in the mantle seems to be primarily
driven by the subduction of colder lithosphere plates into the
mantle, rather than heating from the base of the mantle. As a plate
subducts into the mantle, an area of relatively low pressure behind
it is created that is immediately filled by feeding more plate into
the subduction zone. Eventually, a ridge is formed at the boundary
between plates that are spreading apart, where warmer magma fills in
behind to form new plate. This creates a convection cycle very
similar to what is seen in the atmosphere or even with the sea
breezes. A misconception is that plate tectonics is a driver or a
result of mantle convection, plate tectonics is part of the cycle of
mantle convection. A mantle convection cycle isn't the only
convection going on in the mantle. In addition, there is convection
occurring due to the mantle being heated by the Earth's core.
Subducted lithosphere and mantle near the core form a relatively low
viscosity and hot layer that rise through the mantle as plumes,
which reach the lithosphere to become hot spots. "Plate tectonics
thus appears to be the method by which a largely internally heated
and viscous mantle cools by convection, whereas plumes essentially
cool the core..." (Winter 2010). Convectional processes are not the
only processes that are occurring below the surface of the Earth,
but they are a major process in mantle systems.
Plate tectonic and plume convection systems in the Earth's mantle
(Image from Winter 2010).
