The magma is typically less stiff and solidifies at a somewhat higher temperature than high-silica magma. In most cases, this type of magma has large amounts of MgO and FeO in its analyses and, when it solidifies, produces abundant ferromagnesian minerals and some feldspar, but no quartz. Most low-silica magma is produced by melting of the Earth's mantle. Rocks formed from low-silica magma are typically dark in colour because of the presence of ferromagnesian minerals. These rocks are described as ultramafic .
Ultramafic rocks have extremely high melting points.
We do not think any ultramafic magma exists on Earth at the present day, although in the early part of the history of our planet we find ultramafic lavas, indicating that the Earth was hotter than at present. Solid ultramafic rocks occur in the mantle, however, from which solid chunks are sometimes brought up to the surface in volcanoes and as slices in mountain ranges. In addition to the silica and other oxides listed in chemical analyses of rocks, most magmas contain dissolved water and other gases such as carbon dioxide. These volatile components typically escape as a magma is erupted, but how they behave has an enormous influence on eruption style. In a low-silica magma, volatiles typically form bubbles in the magma as the magma approaches the Earth's surface.