What Are Convection Currents?

Convection occurs in the mantle of the Earth. Photo: Surachit via Wikimedia

Convection currents are the movement of fluid as a result of differential heating or convection. In the case of the Earth, convection currents refer to the motion of molten rock in the mantle as radioactive decay heats up magma, causing it to rise and driving the global scale flow of magma.

The Earth is made out of a number of different layers, and though¬†we live on the crust of the Earth there are miles of Earth beneath our feet. Yet most of the Earth beneath us isn’t solid, it is made out of semi-liquid molten rock that cycles and flows through convection currents in the mantle.

The convection currents that cycle through the molten rock of the Earth help keep the surface of the Earth hospitable for us and other creatures. How do convection currents influence the surface of the planet?

The Mechanism of Convection

All of the material and matter in the Earth’s mantle is subject to tremendous pressure. The pressure occurs because of the gravitational pull of the Earth. When the elements that make up the mantle are subjected to pressure they create heat, which drives convection.

Convection is how dense material in the Earth’s core sinks while lighter material rises. The lighter material is more buoyant, and because of the differences in buoyancy, vertical forces are created called buoyancy forces. The convection of the Earth’s mantle is driven by heat, meaning that the hotter material rises and the cooler material sinks towards the center of the mantle where it becomes hot.

There are three primary sources of heat within the mantle: primordial heat, radioactive heat, and friction heat. Primordial heat is the heat left over from the events that created the Earth’s core, radioactive heat is created as radioactive isotopes decay (it makes up most of the heat in the mantle, between 50-80%), and friction heat is the heat generated by tidal friction as the moon exerts its pull on the Earth.

The density of the material found in the mantle plays a role in driving convection. The less dense material moves towards the top because when molecules are heated up because the atoms deal with the added heat energy by spacing out and becoming less dense. There can be fluctuations in the heat and density of the mantle at any given point as pressure changes, so changes in pressure influence how the Earth’s convection moves and shifts material.

There are other factors which influence convection as well. Some of the Earth’s latent heat is lost to outer space, which means that as the Earth spins it cools over time. The matter that is closer to the surface of the Earth cools faster, causing it to sink back down into the core and the hotter, less dense material to take its place.

The Earth’s rotation also influences convection. There are many convection currents fluctuating as the Earth spins, and the spin of the Earth can influence where convection currents travel. The destination of convection currents naturally changes how much pressure and heat is found at any specific point in the mantle.

Temperatures at various depths of the Earth. Photo: Brews ohare via Wikimedia

Parts of the Earth’s Core:

  • Crust – The surface of the Earth. Oceanic crust makes up 71% of the surface, while the rest of the crust is continental.
  • Mantle – Split into the lithosphere (dense iron and nickel) and the asthenosphere (plastic and malleable iron and nickel).
  • Outer Core – Outer core is liquid materials.
  • Inner Core – Solid, due to all the pressure from the other layers.

Convection and Plate Tectonics

The three types of movement along a fault. Photo: American Museum of Natural History

Like the interior of the Earth, the surface of the Earth is also moving, though it moves very slowly. Plate tectonics is the force that shifts the Earth’s rigid plates around. The continents sit on top of these plates, and they move as the continental plates move. The motion of convection in the mantle is what breaks the plates in the lithosphere (the crust and upper mantle of the Earth) apart and moves them around.

A variety of things can happen as the continental plates move. They may move towards, nearby, or away from one another. The various ways that plates interact with each other are what creates much of the Earth’s varying topography.

The subduction process. Photo: Author(s): KDS4444
– Subduction-en.svg from Wikimedia Commons
– License: CC-BY-SA 4.0

When plates collide with each other, mountains are created. The upwelling of the mantle pushes the plates apart from each other. At these upwelling zones, mid-ocean ridges are created along with new sections of crust. When plates with oceanic crust converge on one another, one plate will be subducted or forced underneath the other plate. The subduction process pushes the crust of one plate down into the mantle where it becomes molten rock again and starts the process anew.

When plates move nearby one another the end result is the creation of fault lines. Fault lines, or fault systems, create many of the world’s earthquakes. Earthquakes occur when a fault ruptures and the built up tension is released all at once.

How Does Convection Impact The Climate Of The Earth’s Surface?

The convection that occurs deep in the Earth’s mantle also impacts the surface and the climate of the surface. Because the upwelling of the mantle at mid-ocean ridges is the primary way heat leaves the Earth’s center, the water near these heat vents is made hotter. The seawater itself then goes through a cycle based on hot and cold regions of water, as areas of the sea are heated up by thermal vents they circulate in a hydrothermal system.

Convection even influences the atmosphere via the movements of the ocean and continental plates. The atmosphere circulates massive amounts of air, and the position of the continents and basins in the ocean changes how air and weather move around the globe. The fluctuations of air currents and ocean currents allow precipitation to move to various areas of the globe.

Finally, the convection that happens in the mantle is responsible for creating the Earth’s magnetic field. The Earth’s magnetic field arises because the flow of liquid iron through the mantel creates electrical currents. These currents produce magnetic fields, and then the metals within the mantel move through this field and create more electrical currents, perpetuating the cycle.

If the Earth didn’t have a magnetic field created by convection, the solar wind from the sun would remove most of the ozone layer. The ozone layer protects the planet from the sun’s harmful ultraviolet rays, meaning that without convection the surface of the planet would be bombarded by intense ultraviolet rays, killing most life on it.

The various impacts convection has on the Earth all work together to make life possible. Convection influences plate tectonics, weather, and even how much solar radiation the planet receives.

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