The color of the lithosphere is complex because this rigid, outermost layer is not a single, uniform object but a vast structural shell encompassing different materials. Its appearance varies dramatically from the familiar surface rocks to the deep, unexposed mantle materials below. To understand its color, the lithosphere must be broken down into its distinct compositional parts, as the overall color is a spectrum dictated by its geological and chemical makeup.
Defining the Lithosphere
The lithosphere is defined by its mechanical properties as the cool, strong, and rigid outer layer of the Earth. It extends from the surface down to a depth ranging from about 5 kilometers beneath the oceans to as much as 200 kilometers in stable continental interiors. This layer is structurally composed of the entire crust and the uppermost, non-flowing part of the mantle.
The lithosphere is fractured into large segments known as tectonic plates, which move slowly over the underlying asthenosphere. The asthenosphere is a warmer, more ductile layer of the upper mantle that allows the rigid lithosphere to glide across it. The boundary between the crust and the mantle portion of the lithosphere is the Mohorovičić discontinuity, marking a significant change in chemical composition.
The continental lithosphere is generally thicker and less dense, primarily composed of silica-rich rocks like granite. In contrast, the oceanic lithosphere is thinner and denser, with its crustal component mainly consisting of iron- and magnesium-rich basalt.
The Variability of Crustal Color
The color of the visible lithosphere—the crust—is highly diverse, exhibiting a wide range of browns, reds, grays, and whites. This variability is a direct result of the specific minerals present, as well as the effects of weathering and exposure to air and water.
The continental crust, rich in minerals like quartz and feldspar, tends to be lighter in color. Quartz is often colorless or white, and feldspars range from light cream to salmon pink, contributing to the lighter appearance of continental rocks such as granite. Vibrant colors, especially reds, yellows, and deep browns, are caused by the oxidation of iron-bearing minerals. When iron combines with oxygen and water, it forms iron oxides, which are responsible for the reddish hues seen in soils and sedimentary rocks.
Oceanic crust, in contrast, is dominated by mafic rocks like basalt, which are rich in iron and magnesium. These minerals give the rock a naturally darker, often black or dark gray appearance. The color of the crust is thus a mosaic determined by local mineralogy and geochemical processes.
Color Below the Surface
The deeper, non-crustal part of the lithosphere consists of the uppermost rigid mantle, and its color is much more consistent than the crust above it. This mantle rock is primarily composed of an ultramafic rock called peridotite. Peridotite is dense and rich in iron and magnesium, with a silica content below 45%.
The characteristic color of unweathered peridotite is dark green, which is mainly due to the high concentration of the mineral olivine. Olivine is the dominant mineral in peridotite, making up more than 40% of the rock. The presence of other minerals like pyroxene can also contribute to a dark greenish-grey or brownish-black appearance.
This dark green mantle portion of the lithosphere is rarely seen because it remains deep below the crust. It is only brought to the surface in specific geological settings, such as where oceanic lithosphere has been thrust onto a continent, or as fragments carried upward by volcanic eruptions. This deep, dense, dark green layer completes the color profile of the Earth’s rigid outer shell.