Ultramafic describes rocks that are extremely rich in magnesium and iron, with very low silica content, typically less than 45% by weight. These are among the densest, darkest rocks on Earth, and they make up the vast majority of our planet’s upper mantle. If you’ve encountered this term in a geology class or while reading about Earth’s interior, here’s what it actually means and why it matters.
The Chemistry Behind the Name
“Mafic” comes from combining magnesium (ma) and ferric iron (f-ic). The prefix “ultra” simply means these rocks take that iron-and-magnesium richness to the extreme. While ordinary mafic rocks like basalt contain roughly 45 to 52% silica, ultramafic rocks fall below 45%. Some definitions place the cutoff around 50%, but the key point is the same: these rocks are silica-poor and metal-rich.
The International Union of Geological Sciences (IUGS) formally classifies a rock as ultramafic when more than 90% of its mineral content consists of dark, iron-and-magnesium-bearing minerals. That’s what geologists call a high “color index,” and it’s why ultramafic rocks tend to be dark green to black in their fresh, unaltered state.
What Ultramafic Rocks Are Made Of
The dominant mineral in most ultramafic rocks is olivine, the glassy green mineral you might recognize from peridot gemstones. Depending on the specific rock type, olivine can make up anywhere from 50% to nearly 100% of the rock by weight. The rest is typically filled in by two types of pyroxene (minerals in a closely related family) plus smaller amounts of other minerals.
The IUGS classifies ultramafic rocks using a triangle diagram based on the proportions of olivine and two pyroxene types. This gives rise to the common varieties:
- Dunite: Almost entirely olivine, 90 to 100% by weight. The simplest ultramafic rock.
- Harzburgite: Olivine plus one type of pyroxene (orthopyroxene). This is the most common ultramafic rock in Earth’s upper mantle.
- Lherzolite: Olivine plus both types of pyroxene. Often considered the closest match to the composition of undepleted mantle rock.
- Wehrlite: Olivine plus the other pyroxene type (clinopyroxene), less common than the others.
All of these fall under the broader family name “peridotite,” which is essentially a catch-all term for coarse-grained ultramafic rocks dominated by olivine.
Where Ultramafic Rocks Come From
Almost all ultramafic rock originates deep in Earth’s mantle, the thick layer between the crust and core that starts roughly 5 to 70 kilometers below the surface depending on whether you’re under ocean or continent. The mantle is overwhelmingly peridotite, making ultramafic rock the most voluminous rock type on the planet, even though you rarely see it at the surface.
So how does mantle rock end up where humans can study it? The main route is through tectonic collisions. When oceanic plates collide with continental plates, slabs of oceanic crust and the mantle rock beneath them can get shoved up onto land in a process called obduction. The resulting geological formation is called an ophiolite: a stacked sequence of ocean floor sediments, volcanic basalt, and underlying ultramafic mantle rock, all now exposed at the surface. The island of New Caledonia in the southwest Pacific hosts one of the world’s most studied ophiolites, where a massive sheet of peridotite was thrust onto the island roughly 32 million years ago.
Ultramafic rocks also reach the surface through volcanic pipes. Kimberlites, the rocks that carry diamonds from deep in the mantle, are ultramafic. Small fragments of mantle peridotite sometimes hitch a ride upward in other volcanic eruptions as well, arriving at the surface as chunks called xenoliths.
Physical Properties
Fresh, unaltered peridotite is dense stuff. Theoretical density values for mantle peridotite run around 3,300 kg/m³, noticeably heavier than typical crustal rocks like granite (around 2,700 kg/m³). In practice, though, ultramafic rocks found at the surface are almost always chemically altered, which can drop their density considerably, sometimes down to 2,370 kg/m³ in heavily altered samples.
That alteration also changes color. Fresh ultramafic rock is dark green to nearly black. Once water gets to it, the olivine and pyroxene transform into new minerals (more on that below), and the rock often turns lighter green, gray, or even reddish-brown on weathered surfaces.
Serpentinization: What Water Does to These Rocks
One of the most important things about ultramafic rocks is what happens when they meet water. Olivine and pyroxene are unstable at Earth’s surface conditions, and when water infiltrates the rock, it triggers a set of chemical reactions called serpentinization. The original minerals transform into serpentine (a soft, waxy, green mineral), along with smaller amounts of brucite and magnetite.
The reaction also produces hydrogen gas. Iron locked inside olivine gets oxidized by water, splitting water molecules and releasing molecular hydrogen. This hydrogen can accumulate in vent fluids, and the process makes the surrounding water highly alkaline, reaching pH values of 9 to 11. In some systems, enough hydrogen builds up to reduce nickel and iron into native metal alloys.
Serpentinization is not just a geological curiosity. The hydrogen it generates may have fueled some of the earliest metabolic reactions on Earth, making ultramafic-hosted hydrothermal vents a leading candidate environment for the origin of life. Today, unique ecosystems thrive at seafloor serpentinization sites, sustained by chemical energy rather than sunlight.
Valuable Metals in Ultramafic Rocks
Because ultramafic rocks are so enriched in metals, they host several economically important ore deposits. The key resources include:
- Nickel and cobalt: When ultramafic rocks weather in tropical climates, intense chemical breakdown concentrates nickel and cobalt into thick laterite soils. These laterite nickel deposits are major global sources of both metals.
- Chromium: Chromite, the only commercially important chromium ore, forms as concentrated pods within mantle peridotite. These podiform chromitite deposits are a primary target in ultramafic terranes worldwide.
- Platinum group elements: Platinum, palladium, and related metals occur at elevated concentrations in ultramafic rocks, sometimes as distinct platinum group minerals within the original mantle rock.
- Iron: Weathered ultramafic surfaces develop iron-rich crusts called ferricrete, which have been mined in some regions.
- Scandium: An increasingly valuable rare metal found in some ultramafic laterite profiles.
Ultramafic Rocks and Carbon Storage
The same chemical reactivity that drives serpentinization also makes ultramafic rocks promising for permanently storing carbon dioxide. When CO₂-charged water contacts the magnesium, calcium, and iron in these rocks, it dissolves the silicate minerals and the released metal ions combine with the CO₂ to form stable carbonate minerals. This is carbon mineralization: converting a gas into solid rock.
The potential scale is staggering. One estimate suggests that injecting CO₂-rich fluids into peridotite formations at depth could store between 100,000 and 100 million gigatons of CO₂. For context, humanity currently emits roughly 37 gigatons per year. The challenge is that ultramafic rocks tend to be relatively impermeable, making it difficult to push fluids through them at useful rates. Field tests are underway, but the technology is still in early stages.
How Ultramafic Differs From Mafic
The distinction between mafic and ultramafic comes down to degree. Mafic rocks like basalt and gabbro contain 45 to 52% silica and are rich in iron and magnesium, but they also contain significant amounts of calcium-rich feldspar, giving them a more varied mineral makeup. Ultramafic rocks drop below 45% silica and are almost entirely composed of olivine and pyroxene, with little to no feldspar. Mafic rocks are common at Earth’s surface as ocean floor basalt. Ultramafic rocks dominate beneath that, in the mantle, but are relatively rare at the surface unless tectonic forces have brought them up.