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Is the Continental Crust 'Granitic'?

Granite. Photo from

Q: I have been wondering a few of things over the last few days after being asked a question, and can't find an answer that I find satisfying. They are:

  1. If granite is an intrusive igneous rock, how is it that most of the continental crust is made up of it? I mean, if it was intrusive, what did it all intrude into in the first place?

  2. Following on from that: When and how did the first continental crust form? If it was granite, then what did it intrude into in the first place?

  3. Did all the continental crust first form in one place as one land mass, and if so why?

From Mr Anthony Hopkins (November 2009)

Reply by Dr Ted Nield

Continental crust is indeed “granitic”, and has the general composition typical of granitic rocks, made up of mostly aluminium silicates (the SiAl). The lower density of these rocks is what is responsible for the continents lying high on the Earth’s surface, relative to the more dense ocean floors - which though thinner crust, are made of magnesium silicates (SiMa) and are much denser.

SiMa is the primitive crustal rock, from which all other geomaterials derive, because it itself comes from the upper mantle of the Earth at ocean floor spreading centres. Continental rocks are fractionated from this material during the processes of plate destruction at subduction zones. Processes grouped together under the heading of “magmatic differentiation” remove denser elements and produce refined rocks progressively richer in silica and aluminium. These intermediate and acid igneous rocks (like granite, andesite and rhyolite) then form the basic materials from which continent-derived sediments are formed. They of course reflect the “granitic” or “acid” (rich in silica) chemistry of their precursors.

As the Earth has recycled oceanic crust many times during its history, as plate tectonic processes have formed, broken up and reformed the massive land areas known as “supercontinents” (see my book!) the total amount of refined continental crustal material has grown larger, because being buoyant it can never be lost and keeps accumulating on the surface of the Earth like scum.

So first of all, continents have at their cores a number of ancient “shield” areas whose composition is basically granitic. Most of these are a mixture of granite intrusives and deep-crustal rocks, extremely old, born in ancient mountain-building episodes and exposed today by erosion. Massive intrusions of granite are created, now as in the deep past, in mountain roots during orogenic phases occurring around their edges – as in the Rockies and the Andes and Himalayas today. These granite bodies are formed by the remobilization of older continental crustal material that has been accumulating for as long as plate tectonic processes have operated on Earth.