Which Plate Tectonic Boundary Causes Crustal Formation?

Types of Crust

• The Earth is comprised of two types of crust. Oceanic crust, which forms the oceanic plates, is basaltic in nature. This crust is thin, averaging three to 4.5 miles, and dense, averaging three grams per cubic centimeter. Continental crust, which forms the continental plates, is granitic in nature. This crust is thick, averaging 21 to 44 miles, and less dense, averaging 2.7 grams per cubic centimeter. Crust is created during the interaction of tectonic plates, referred to as plate boundaries. Each type of crust is created by a different type of plate tectonic boundary.

Creation of Continental Crust

• Continental crust is primarily formed by convergent boundaries between oceanic plates and continental plates. As these plates collide, the denser oceanic plate is subducted beneath the more buoyant continental plate. As the oceanic plate descends into the asthenosphere, the crust melts, creating magma. This magma mixes with melted continental crust, creating magma chambers. Eventually this magma will either cool under the ground, producing intrusive igneous rock such as granite, or erupt from volcanoes, producing extrusive igneous rock. Either way, this tectonic process results in the creation of new continental crust.

Creation of Oceanic Crust

• Oceanic crust is primarily formed by divergent boundaries, where plates are being forced apart. This can occur with oceanic or continental plates. The Mid-Atlantic Ridge is an example of a divergent zone involving oceanic plates. Here, the North American and Eurasian plates are spreading apart at a rate of around one inch per year. The Red Sea Rift is an example of a divergent zone involving continental plates. Here, the African and Arabian plates are spreading apart, creating the Red Sea. Ultimately, this divergence will produce a new ocean seafloor. At all these locations, convective currents cause magma to rise to the surface. These forces push the plates apart, with magma seeping into the resulting cracks. When the magma cools, it forms new oceanic crust.

Other Methods for Creation of Crust

• In addition to the two primary methods of crustal formation, hotspots also create new crust. Hotspots represent thermal plumes of magma that occur underneath plates, rather than at inter-plate boundaries. The Yellowstone super volcano is an example of a continental hotspot. The thick, gas-filled nature of its continental lava is what makes this volcano so explosive and dangerous. The Hawaiian Islands are an example of an oceanic hotspot. This chain of islands was produced by oceanic basalt lava, which is fluid and less explosive.