How is dense magma able to rise and punch through continental crust?

Question

I am trying to understand how a dense basaltic lava is able to rise and punch through the continental crust, such as in mantle plumes or the Great East African Rift.

My understanding of plate tectonics is that continental crust is made from less dense silicates, where as oceanic crust is made from denser silicates.

The origin of continental crust is from the differentiation of Earth, where silicates rose to form the mantle and even lighter silicates rose, then cooled, to form the crust of early cratons.

If the mantle that forms oceanic crust is composed of dense silicates, how is it able to rise up and punch through the continental crust, pushing it apart to form ocean basins, as in areas of rifting or mantle plumes?

My intuition tells me a dense material should not be able to rise from depth and displace a lighter one.

Answer 1

Your intuition is quite correct: Other factors being equall, a dense magma will not rise through lighter rocks. The exception is if it is under pressure, in which case a fluid magma will escape through 'pressure release' pathways, either laterally or vertically (or in complex geologic structures, in a variety of irregular pathways). Magmas may begin to crystallize at depth. As they do so, the volatile components, such as water, carbon dioxide, halogen acids, and sulphur dioxide, become relatively enriched in the residual magma, where they can build up enormous gas pressures. The most spectacular results are in Plinian-type volcanoes, and in many less dramatic eruptions. Consider, for example, pumaceous lava, which is essentially gas bubbles frozen-in during the process of escape. At depth there are no such bubbles. but the gasses are still there, and still exerting a huge pressure.

Otherwise, a magma may simply rise because of thermal expansion reducing the density to less than that of the host rock. Most likely the magma will rise as a combination of both mechanisms.

Also, you mention the Great East Africa Rift, which is notable for some extraordinary carbonatite lavas and related carbonatite-silicate hybrids, which are less dense than almost all of the other African crustal rocks.

Answer 2

The origin of continental crust is from the differentiation of Earth, where silicates rose to form the mantle and even lighter silicates rose, then cooled, to form the crust of early cratons.

Not quite. The lighter silica rich rocks that compose the continental crust form by partial melting of pre-existing denser basaltic rocks. When you partially melt a basalt, the liquid is richer in silica, thus it is less dense.

If the mantle that forms oceanic crust is composed of dense silicates, how is it able to rise up and punch through the continental crust, pushing it apart to form ocean basins, as in areas of rifting or mantle plumes?

That is because basaltic magma is less dense than basaltic solid rocks. Basaltic magma may very well be less dense than any silica rich rock it encounters. Furthermore, as Gordon mentioned, there is the issue of overpressure.

Mantle plumes are mostly solid - these are areas that are hotter than their surroundings but are still solid. However, hotter material expands and becomes less dense. This allows the lighter rocks, which are chemically similar to their surroundings, to flow upwards. Melting occurs only at a later stage, either because you depressurise the mantle rocks at shallower depths or because the mantle plume heats up other crustal rocks which melt more easily.