Further Reading

Collins, Alan S., Ian C. W. Fitzsimons, Bregje Hulscher, and Theodore Razakamanana. "Structure of the Eastern East African Orogen in Central Madagascar." In Evolution of the East African and Related Orogens, and the Assembly of Gondwana, Special Issue of Pre-cambrian Research 123, edited by Timothy M. Kusky, Mohamed Abdelsalam, Robert Tucker, and Robert Stern, 111-134. Amsterdam; Elsevier, 2003. de Wit, Maarten J. "Madagascar: Heads It's a Continent, Tails It's an Island." Annual Reviews of Earth and Planetary Sciences 31 (2003): 213-248. Handke, Michael J., Robert D. Tucker, and Lewis D. Ash-wal. "Neoproterozoic Continental Arc Magmatism in West-Central Madagascar." Geology 27 (1999): 351-354.

Kusky, Timothy M., and Julian Vearncombe. "Structure of Archean Greenstone Belts." In Tectonic Evolution of Greenstone Belts, edited by Maarten J. de Wit and Lewis D. Ashwal, 95-128. Oxford: Oxford Monograph on Geology and Geophysics, 1997. Tucker, Robert T., Timothy M. Kusky, Robert Buchwaldt, and Michael Handke. "Neoproterozoic Nappes and Superimposed Folding of the Itremo Group, West-Central Madagascar." Gondwana Research 12 (2007): 356-379.

Windley, Brian F., Adriantefison Razafiniparany, Theodore Razakamanana, and D. Ackemand. "Tectonic Framework of the Precambrian of Madagascar and Its Gondwanan Connections: A Review and Reappraisal." Geologtsche Rundschau 83 (1994): 642-659.

magma Molten rock beneath the surface of the Earth is known as magma. When magma reaches the surface, it is known as lava, which may flow or explosively erupt from volcanoes. The wide variety of eruption styles and hazards associated with volcanoes around the world can be linked to variations in several factors, including different types of magma, different types of gases in the magma, different volumes of magma, different forces of eruption, and different areas that are affected by each eruption. Different types of magma form in distinct tectonic settings, explaining many of the differences above. Other distinctions between eruption styles are explained by the variations in processes that occur in the magma as it makes its way from deep within the Earth to the surface at a volcanic vent.

Most magma solidifies below the surface, forming igneous rocks (igneous is from the Latin word for fire). Igneous rocks that form below the surface are called intrusive or plutonic rocks, whereas those that crystallize on the surface are called extrusive or volcanic rocks. Rocks that crystallize at a very shallow depth are called hypabyssal rocks. Some common plutonic rock types include granites, and some of the most abundant volcanic rocks include basalts and rhyolites. Intrusive igneous rocks crystallize slowly, giving crystals an extended time to grow, thus forming rocks with large mineral grains that are clearly distinguishable with the naked eye. These rocks are called phanerites. In contrast, magma that cools rapidly forms fine-grained rocks. Aphanites are igneous rocks in which the component grains can not be distinguished readily without a microscope and are formed when magma from a volcano falls or flows across the surface and cools quickly. Some igneous rocks, known as porphyries, have two distinct populations of grain size. One group of very large crystals (called phenocrysts) is mixed with a uniform groundmass or matrix filling the space between the large crystals. This indicates two stages of cooling, as when magma has resided for a long time beneath a volcano, growing big crystals. When the volcano erupts it spews out a mixture of the large crystals and liquid magma that then cools quickly, forming the phenocrysts and the fine-grained groundmass.

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