Will we ever run out of magma?

This map shows the boundaries of the Earth’s tectonic plates. Most earthquakes and volcanoes occur along plate boundaries.

Image courtesy NASA

Molten material beneath the crust of the earth is called magma. When magma leaves the Earth’s crust through volcanic activity, it’s called lava. Most volcanoes are on plate boundaries, the edges of the tectonic plates, which are like puzzle pieces that that make up the earth’s crust. A few volcanoes, like ones found on Hawaii, are instead located over magma hotspots.

A common misconception is that magma comes from the Earth’s molten core. It really comes from the mantle, the layer between the core and the crust. The mantle is solid, but it shifts and becomes fluid due to changes in temperature and pressure. The tectonic plates, which carry continents and ocean floors, rest on top of this mantle. As the plates move, the Earth’s continents slowly reorganize themselves.

(function () {
if (!HSW.utilities.isMobile() && HSW.utilities.hashArgs[‘video-ad’] !== ‘off’ && userData.adsActive) {
HSW.utilities.delayLoad(() => {
HSW.pq.add(() => {
if (userData.adsActive) {
HSW.utilities.loadScript(‘https://tg1.unrulyvideo.com/api/adserver/spt?AV_TAGID=642d0774bf980e733c0bdd08&AV_PUBLISHERID=63a310da17f8afa0e105a8f5’, true, () => {});
else {
console.log(‘Unruly player blocked’);
else {
let el = document.getElementById(‘inline-video-wrap’);

These plates move at about the same rate that your fingernails grow. Volcanic activity mostly takes place at convergent boundaries, where two plates are pressing together. At divergent boundaries, the plates are moving apart, creating a rift valley. Areas where two plates slide past each other are called transform boundaries. Magma rises up through volcanoes because of the pressure of the colliding plates. It rises up through rift valleys to fill the space that the rift creates. See this graphic for more information on these boundaries.

As lava explodes from volcanoes or seeps up to fill rifts, it can seem inexhaustible. In a way, it is. At convergent plate boundaries, one plate can slide under another plate. The lower, or subducting, plate’s crust disappears into the Earth’s mantle. There, it gets recycled, becoming part of the mantle. When a volcanic explosion is so massive that the magma underneath is completely depleted, it forms a caldera, or bowl-like depression.

All of this matter circulates on a very large scale, like an enormous pot of thick stew boiling on a stove. It’s both a cause and effect of the tiny movements of the tectonic plates that changed the Earth from a world with one continent to a world with several.

Lots More Information

Related HowStuffWorks Articles

  • How the Sun Works
  • How Volcanoes Work
  • How Earthquakes Work
  • How Evolution Works
  • How the Nature Conservancy Works
  • How the World Wildlife Fund Works
  • How Asteroids Work
  • How Stars Work
  • How Black Holes Work
  • How Compasses Work
  • How Electromagnets Work
  • How Landfills Work
  • How Telescopes Work
  • Where did the moon come from?
  • Can we plug the hole in the ozone layer?
  • Could reversing global warming start an ice age?
  • Why does the North Pole move?

More Great Links

  • 101 Amazing Facts About the Earth
  • Science Topics at USGS
  • NASA Earth Observatory
  • Climate Change at Exploratorium
  • Windows to the Universe


October 16 Birthday Astrology

The astrological symbol for Libra, the seventh sign of the year, is the Scale. Numerology [...]

Tiny nanopores can contribute to faster identification of diseases

Jørgen Kjems and collaborators have achieved a remarkable breakthrough in developing tiny nano-sized pores that [...]

More Carbon Dioxide in the Air Makes Some Crops Less Nutritious

Around three billion people worldwide depend on rice for their diet. But a new study [...]

Rare Meteor Event Inspired Walt Whitman

Feedloader (Clickability) Walt Whitman’s Leaves of Grass includes the poem “Year of Meteors, (1859-60)” in [...]

What in the World Are Tonsil Stones?

Tonsil stones are really no big deal, but they can cause a sore throat and [...]

What Animal is the Best Mother?

In April 2005, Zazi, one of the National Zoo’s female cheetahs, gave birth to six [...]

Pros and Cons of Smartphone Leasing

High prices make buying a smartphone difficult for some Smartphones are designed to make our [...]

Using carbon nanotubes to strengthen graphene-based membranes used for desalination

A large-area atomically thin graphene nanomesh membrane with excellent mechanical strength for ionic and molecular [...]