Earth’s mantle is a 2,900-kilometer-thick layer of solid rock made primarily of oxygen, silicon, magnesium, and iron. It sits between the thin outer crust and the dense metallic core, and it accounts for about 84% of Earth’s total volume. Though it’s solid, the mantle behaves like an extremely slow-moving fluid over millions of years, which is what drives the movement of tectonic plates at the surface.
Chemical Makeup of the Mantle
The mantle’s chemistry is dominated by four elements. Oxygen is the most abundant by weight, locked into the crystal structures of minerals rather than existing as a gas. Silicon comes next at roughly 21.5%, a slight drop from the 27.7% found in the crust. Magnesium makes up about 22.8%, a dramatic jump from its mere 2.1% concentration in the crust. Iron rounds out the major players. Smaller amounts of calcium, aluminum, and other elements are present but play a much smaller role.
This chemical recipe produces a rock type called peridotite, which is denser and darker than the granite and basalt that make up most of the crust. If you’ve ever seen a chunk of green-tinted olivine crystal at a gem shop or on a Hawaiian beach, you’ve seen one of the mantle’s signature ingredients brought to the surface.
Minerals of the Upper Mantle
The upper mantle, stretching from just below the crust to about 410 kilometers deep, is built mostly from a rock called lherzolite. It’s a combination of three minerals: olivine (the dominant one), clinopyroxene, and orthopyroxene. Olivine is a dense, glassy, green mineral made of magnesium, iron, silicon, and oxygen. The pyroxenes are similar in chemistry but have a different crystal structure.
Garnet and spinel also appear in the upper mantle, and their presence shifts depending on depth. At shallower depths, spinel is more common alongside the olivine and pyroxene mix. Deeper down, where pressure increases, garnet takes over spinel’s role. Rarer minerals like diamond and graphite (both pure carbon, just arranged differently) also form in the upper mantle under the right pressure and temperature conditions.
The Transition Zone: 410 to 660 Kilometers
Between 410 and 660 kilometers deep, the mantle enters a transition zone where familiar minerals transform under crushing pressure. Olivine doesn’t disappear chemically; it rearranges its atoms into higher-pressure forms. First it becomes a mineral called wadsleyite, then deeper still it transforms into ringwoodite. Both have the same basic ingredients as olivine but pack their atoms into tighter crystal structures.
This zone is scientifically fascinating because wadsleyite and ringwoodite can hold water within their crystal structures, potentially up to about 2.5% of their weight. A 2014 study published in Nature reported the first direct evidence of this: a tiny ringwoodite crystal trapped inside a diamond from Brazil, carried up from transition zone depths. If the entire transition zone contains even a fraction of this water capacity, it could hold more water than all the oceans on Earth’s surface combined.
The Lower Mantle
Below 660 kilometers, pressure forces yet another transformation. The dominant mineral becomes bridgmanite, a dense crystal with a structure scientists call perovskite. Bridgmanite is the single most abundant mineral on Earth, making up the bulk of the lower mantle, which extends all the way down to the core boundary at about 2,900 kilometers. It’s composed of magnesium, iron, silicon, and oxygen, similar to the minerals above it but squeezed into a much more compact arrangement.
At the very base of the mantle, in a thin layer called D” (pronounced “D double-prime”), pressures exceed 120 billion pascals and temperatures climb above 2,000°C. Here, bridgmanite undergoes one final transformation into a form called post-perovskite. This phase change is thought to explain unusual patterns in how seismic waves bounce off the boundary between the mantle and the core.
Solid Rock That Flows Like Putty
One of the most common misconceptions about the mantle is that it’s molten. It isn’t. The mantle is solid rock at virtually every depth. But “solid” doesn’t mean “rigid” when you’re talking about geological timescales and extreme heat.
The key is how rock behaves under different conditions of temperature and pressure. In the lithosphere (the crust plus the uppermost mantle, down to roughly 100 kilometers), rock is cool enough to be stiff and brittle. Below that lies the asthenosphere, where temperatures are high enough that rock loses much of its strength. The asthenosphere has the same chemical composition as the rest of the mantle, but its physical behavior is completely different. The rock there is weak and easily deformed, sometimes described as a slush-like material with tiny pockets of melt between solid grains.
Over thousands to millions of years, mantle rock flows through a process called viscoplastic deformation. At the atomic level, defects in the crystal structure of minerals slowly migrate, allowing the solid to creep and shift. Recent research shows this flow isn’t always perfectly smooth. At small scales, deformation can happen in discrete bursts as groups of atomic-level defects move together in avalanches. But when viewed at the scale of continents and ocean basins, these bursts average out into what looks like steady, continuous flow. This is the engine behind plate tectonics, volcanic activity, and the slow recycling of Earth’s surface.
How Scientists Know What’s Down There
No one has ever drilled to the mantle. The deepest borehole ever made, Russia’s Kola Superdeep Borehole, reached only about 12 kilometers, barely scratching the crust. So how do scientists know what the mantle contains?
The most direct evidence comes from xenoliths, chunks of mantle rock carried to the surface by volcanic eruptions. Southern Africa is especially rich in these samples because thousands of ancient volcanic pipes called kimberlites punched through the crust there, dragging pieces of the mantle upward. Scientists have cataloged over 120 geochemically calibrated mantle xenoliths from this region alone, using them to determine the minerals, temperatures, and pressures at specific depths.
The second major tool is seismic waves from earthquakes. These waves change speed and direction depending on the density and stiffness of the rock they pass through. By comparing wave speeds predicted from xenolith mineral data with actual seismic observations, scientists can check whether their models of mantle composition are correct. Seismic data revealed the boundaries of the transition zone (the speed changes at 410 and 660 kilometers), confirmed the phase change to post-perovskite near the core, and mapped variations in temperature and composition across different regions of the mantle.
Laboratory experiments fill in the rest. Using diamond anvil cells, researchers squeeze tiny mineral samples to pressures exceeding 130 billion pascals while heating them with lasers above 2,000°C, recreating conditions found deep in the lower mantle. These experiments confirmed that bridgmanite transforms into post-perovskite and helped determine exactly where in the mantle each mineral transition occurs.