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5: Igneous rocks and Volcanism

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    Learning Objectives

    • Explain the origin of magma it relates to plate tectonics
    • Describe how the Bowen’s Reaction Series relates mineral crystallization and melting temperatures
    • Explain how cooling of magma leads to rock compositions and textures, and how these are used to classify igneous rocks
    • Analyze the features of common igneous landforms and how they relate to their origin
    • Explain partial melting and fractionation, and how they change magma compositions
    • Describe how silica content affects magma viscosity and eruptive style of volcanoes
    • Describe volcano types, eruptive styles, composition, and their plate tectonic settings
    • Describe volcanic hazards

    Igneous rock is formed when liquid rock freezes into solid rock. This molten material is called magma when it is in the ground and lava when it is on the surface. Only the Earth’s outer core is liquid; the Earth’s mantle and crust are naturally solid. However, there are a few minor pockets of magma that form near the surface where geologic processes cause melting. It is this magma that becomes the source for volcanoes and igneous rocks. This chapter will describe the classification of igneous rocks, the unique processes that form magmas, types of volcanoes and volcanic processes, volcanic hazards, and igneous landforms.

    Pahoehoe lava flow in HawaiiGFDL or CC BY-SA 3.0], via Wikimedia Commons" width="377px" height="251px" src="/@api/deki/files/7523/04.1_P%25C4%2581hoehoe_and_Aa_flows_at_Hawaii-300x200.jpg">
    Figure \(\PageIndex{1}\): Lava flow in Hawaii

    Lava cools quickly on the surface of the earth and forms tiny microscopic crystals. These are known as fine-grained extrusive, or volcanic, igneous rocks. Extrusive rocks are often vesicular, filled with holes from escaping gas bubbles. Volcanism is the process in which lava has erupted. Depending on the properties of the lava that is erupted, the volcanism can be drastically different, from smooth and gentle to dangerous and explosive. This leads to different types of volcanoes and different volcanic hazards.

    An intrusive igneous mass now exposed at the surface by erosion
    Figure \(\PageIndex{1}\): Half Dome, a mass of intrusive igneous rock in Yosemite National Park, now exposed by erosion.

    In contrast, magma that cools slowly below the earth’s surface forms larger crystals which can be seen with the naked eye. These are known as coarse-grained intrusive, or plutonic, igneous rocks. This relationship between cooling rates and grain sizes of the solidified minerals in igneous rocks is important for interpreting the rock’s geologic history.

    • 5.1: Classification of Igneous Rocks
      Igneous rocks are classified based on texture and composition. Texture describes the physical characteristics of the minerals, such as grain size. This relates to the cooling history of the molten magma from which it came. Composition refers to the rock’s specific mineralogy and chemical composition. Cooling history is also related to changes that can occur to the composition of igneous rocks.
    • 5.2: Bowen’s Reaction Series
      Bowen’s Reaction Series describes the temperature at which minerals crystallize when cooled, or melt when heated. The low end of the temperature scale where all minerals crystallize into solid rock is approximately 700°C (158°F). The upper end of the range where all minerals exist in a molten state is approximately 1,250°C (2,282°F). These numbers reference minerals that crystallize at standard sea-level pressure, 1 bar.
    • 5.3: Magma Generation
      Magma and lava contain three components: melt, solids, and volatiles. The melt is made of ions from minerals that have liquefied. The solids are made of crystallized minerals floating in the liquid melt. These may be minerals that have already cooled Volatiles are gaseous components—such as water vapor, carbon dioxide, sulfur, and chlorine—dissolved in the magma. The presence and amount of these three components affect the physical behavior of the magma and will be discussed more in this page.
    • 5.4: Partial Melting and Crystallization
      Even though all magmas originate from similar mantle rocks, other things, like partial melting and crystallization processes, can change the chemistry of the magma. This explains the wide variety of resulting igneous rocks that are found all over Earth. Because the mantle is composed of many different minerals, it does not melt uniformly. As minerals with lower melting points turn into liquid magma, those with higher melting points remain as solid crystals. This is known as partial melting.
    • 5.5: Volcanism
      When magma emerges onto the Earth’s surface, the molten rock is called lava. A volcano is a type of land formation created when lava solidifies into rock. Volcanoes have been an important part of human society for centuries, though their understanding has greatly increased as our understanding of plate tectonics has made them less mysterious. This section describes volcano location, type, hazards, and monitoring.


    Igneous rock is divided into two major groups: intrusive rock that solidifies from underground magma, and extrusive rock formed from lava that erupts and cools on the surface. Magma is generated from mantle material at several plate tectonics situations by three types of melting: decompression melting, flux melting, or heat-induced melting. Magma composition is determined by differences in the melting temperatures of the mineral components (Bowen’s Reaction Series). The processes affecting magma composition include partial melting, magmatic differentiation, assimilation, and collision. Volcanoes come in a wide variety of shapes and sizes, and are classified by a multiple factors, including magma composition, and plate tectonic activity. Because volcanism presents serious hazards to human civilization, geologists carefully monitor volcanic activity to mitigate or avoid the dangers it presents.

    Thumbnail: 10-meter (33 ft) high fountain of pāhoehoe lava, Hawaii, United States. (Public domain; Mbz1).

    5: Igneous rocks and Volcanism is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher.

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