Erosion of volcanoes will immediately expose shallow intrusive bodies such as volcanic necks and diatremes (see Figure 6). A volcanic neck is the “throat” of a volcano and consists of a pipelike conduit filled with hypabyssal rocks. Ship Rock in New Mexico and Devil’s Tower in Wyoming are remnants of volcanic necks, which were exposed after the surrounding sedimentary rocks were eroded away. Many craterlike depressions may be filled with angular fragments of country rock (breccia) and juvenile pyroclastic debris. When eroded, such a depression exposes a vertical funnel-shaped pipe that resembles a volcanic neck with the exception of the brecciated filling. These pipes are dubbed diatremes. Many diatremes are formed by explosion resulting from the rapid expansion of gas—carbon dioxide and water vapour. These gases are released by the rising magma owing to the decrease in pressure as it nears the surface. Some diatremes contain kimberlite, a peridotite that contains a hydrous mineral called phlogopite. Kimberlite may contain diamonds.
Figure 7: Idealized cross section of a divergent plate boundary showing the structure of the oceanic lithosphere.
Below the collection of lavas and dikes in layer 2 are found gabbro and diorite. They represent the plutonic rocks formed as a result of differentiation of the MORB magma that fed the volcanic activity along the rift. (Differentiation is the process in which more than one rock type is derived from a single parent magma.) These coarse-grained intrusives account for about 4 to 5 kilometres of layer 3, which rests on a sequence of layered ultramafic rocks. The rocks were formed by the gravitative accumulation of mafic minerals from the original MORB magma that filled a large chamber below the ridge axis. Below this layered sequence is mantle rock that is highly deformed and depleted (of elements such as lanthanum, cerium, sodium, and potassium that have been removed by repeated partial melting). Because seismic waves cannot distinguish between layered ultramafic rocks, which are not true mantle rocks, and ultramafic mantle rocks, the Moho actually is positioned between layer 3 and the layered ultramafics. The sequences consisting of layer 1 (limestone and chert sedimentary rocks), layer 2 of MORB lavas and dikes, and layer 3 of gabbro and diorite and the ultramafic rocks are known as ophiolites. Many geologists believe that ophiolites formed at oceanic ridges were emplaced by tectonic forces at convergent plate boundaries and then became exposed in highly deformed orogenic (mountain) belts. In fact, the same sequences of rocks were first reported in the Alps and were considered deep-seated intrusions. Some geologists still argue that all ophiolites were not formed at divergent plate boundaries.
Away from the axis of divergence, the composition of the volcanic rocks becomes more diverse. Most of the magmatism is related to hot spots, which are hot rising plumes of mantle rock that are anchored beneath the moving lithospheric plates (see Figure 7). The Hawaiian Islands owe their existence to the magmatism associated with a hot spot that currently is located just southeast of the large island of Hawaii. This mantle plume not only provides magma for the eruptions at Kilauea Volcano but also is responsible for the submarine volcano named Loihi that will eventually become a new island. Most of the islands are built on a tholeiite basalt base, but the caps of the volcanoes are alkali basalts. The final episodes of volcanic activity on an island are extremely undersaturated; nephelinites and olivine melilite nephelinites are common products. The alkali basalts have differentiated to more silica-rich compositions, with hawaiites, mugearites, and trachytes being erupted in minor amounts. The two active volcanoes on Hawaii, Mauna Loa and Kilauea, are still erupting tholeiite basalts. Tholeiites on all the islands far from the ocean ridge crests are different from MORB in that they are enriched in lanthanum, cerium, sodium, and potassium. Early in Earth’s history, a high-magnesium, high-temperature mafic magma called komatiite erupted from hot spots. Since most komatiites are only found in Archean regions, they are thought to be evidence for Earth being hotter than when it was initially formed. The youngest komatiite was recently discovered on the island of Gorgona, Colom.