{"id":75,"date":"2020-07-09T12:25:17","date_gmt":"2020-07-09T16:25:17","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/geolmanual\/chapter\/overview-of-igneous-rocks\/"},"modified":"2021-09-19T19:44:14","modified_gmt":"2021-09-19T23:44:14","slug":"overview-of-igneous-rocks","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/geolmanual\/chapter\/overview-of-igneous-rocks\/","title":{"raw":"Overview of Igneous Rocks","rendered":"Overview of Igneous Rocks"},"content":{"raw":"
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How Igneous Rocks Form<\/h1>\r\n

Magma Composition<\/h2>\r\nBefore any igneous rock can form, molten material\u2014known as magma<\/strong>\u2014must be produced. That means you must have a rock to melt in the first place to make the magma that will eventually cool to become an igneous rock. The composition of the original rock (or rocks) that melted is one of the factors that controls the composition of the igneous rock that forms once the melt cools.\u00a0 Other factors are how much of the original rock actually melts, and the cooling process of the magma.\r\n

Rocks are often made up of a mixture of minerals. For each mineral, there is a unique set of conditions (such as pressure and temperature) under which that mineral can melt. For a rock with a mixture of minerals, this means that under certain conditions, some of the minerals in the rock may melt, while other minerals remain solid. Because some minerals melt at lower temperatures than others, temperature conditions determine which minerals will add their chemical components to the magma that forms. If temperatures are low enough, some of the minerals might not melt at all. Therefore, even if the same<\/em> types of rocks are melting, different magma compositions can be generated simply by melting at different temperatures!<\/p>\r\n\r\n

Cooling & Mineral Formation<\/h2>\r\n

Eventually, magma will start to rise through Earth\u2019s lithosphere, because it's more buoyant than its source rock. When the magma moves away from its source region, it encounters new thermal conditions, and begins to cool. As the magma cools, the temperature begins to drop beneath the melting points of different minerals. The sequence in which minerals crystallize is the opposite of the melting sequence, such that minerals with high melting points form first as the magma cools.\u00a0 The order is summarized in Bowen's reaction series<\/strong> (Figure 3.2) named after Normal L. Bowen, who performed early experiments on cooling melts.<\/p>\r\n \r\n\r\n[caption id=\"attachment_259\" align=\"aligncenter\" width=\"923\"]\"\"<\/a> Figure 3.2 |<\/strong> Bowen\u2019s reaction series, showing the progression of mineral crystallization as magma temperatures drop from ~1400 \u00b0C to ~500 \u00b0C. Note the corresponding names for igneous rock composition and common rock types within each compositional group. Source: Karen Tefend (2015) CC BY-SA 3.0. View source<\/a>[\/caption]\r\n\r\n<\/div>\r\n\"Reaction series\" refers to the sequence of chemical reactions between elements within magma that result in the formation of minerals as the temperature falls. On the diagram, the sequence proceeds from top to bottom. The length of the arrow indicates the range of temperatures at which a particular mineral can form.\r\n<\/span>\r\n\r\nThe first mineral to crystallize in a cooling magma of ultramafic composition is olivine. Once the temperature falls below this range, olivine crystals will no longer form; instead, other minerals such as pyroxene will start to crystallize. Note that more than one mineral might be forming at a given temperature; for example, within a certain range of temperatures, chemical reactions are forming both olivine and pyroxene. <\/span>\r\n

As mineral crystals form in cooling magma, <\/span>they take some chemical elements from the magma into their crystal structure, and exclude others. In the case of olivine, magnesium (Mg) and iron (Fe) are taken in, leaving the remaining magma with less Mg and Fe than before crystallization started. This means that the composition of the magma changes as crystals are forming.<\/span><\/p>\r\n\r\n

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On the left side of Bowen\u2019s reaction series, the minerals olivine, pyroxene, amphibole, and biotite all remove iron (Fe), magnesium (Mg), and manganese (Mn) from magma during crystallization, but do so over different temperature ranges. These iron- and magnesium-rich minerals are referred to as ferromagnesian <\/strong>minerals (ferro = iron) and are usually green, dark gray, or black in colour due to the absorption of visible light by iron and magnesium atoms.<\/p>\r\n

On the right side of Bowen\u2019s reaction series is a long arrow labelled plagioclase feldspar<\/em>. Plagioclase treated separately because it crystallizes continuously over a large temperature range. As the magma temperature drops and plagioclase first begins to crystallize, it will take calcium atoms into its crystal structure, but as the temperature drops, plagioclase takes in sodium atoms in increasing abundance, and less and less calcium. The difference in calcium and sodium content make a difference in the appearance of plagioclase: the higher temperature calcium-rich plagioclase is dark gray in colour, while the lower temperature sodium-rich plagioclase is white.<\/p>\r\n

At the bottom of Figure 3.2 are potassium feldspar, muscovite, and quartz, the low-temperature minerals that are the last to form during cooling (and therefore the first to melt as a rock is heated). These minerals form from magma that has been depleted of iron and magnesium, and so are referred to as non-ferromagnesian<\/strong> minerals. Non-ferromagnesian minerals are much lighter in colour. For example, the potassium-rich feldspar (also known as orthoclase) can be a pale pink or white in colour. The colour of an igneous rock will be affected by its mineral content, so a general knowledge of mineral colour is helpful for identifying and classifying igneous rocks.<\/p>\r\n\r\n

Igneous Rock Composition<\/h1>\r\n
\r\n\r\nOn the right side of the Bowen\u2019s reaction series diagram are the igneous rock composition categories, and examples of common igneous rock names in each category. The compositional categories are defined by the minerals found within them. For example, in ultramafic\u00a0<\/strong>rocks like peridotite or komatiite, you can expect to find abundant olivine, and maybe some pyroxene and Ca-rich plagioclase. In mafic<\/strong> rocks like basalt or gabbro, you can expect to find pyroxene, plagioclase, and possibly some olivine or amphibole. In a felsic <\/strong>(or silicic<\/strong>) rock such as granite or rhyolite, you can expect to see quartz, muscovite, potassium feldspar, and some biotite and Na-rich plagioclase.\r\n
\r\n\r\nHint for remembering the terms mafic<\/em> and felsic<\/em>:<\/strong>\r\n\r\nIn the word mafic<\/em>, the \u201cma-\u201d comes from magnesium, and the \u201cfic\u201d refers to ferric iron.\r\n\r\nThe word felsic<\/em> combines \"fel-\" from feldspar and \"sic\" from silica-rich quartz.\r\n\r\n<\/div>\r\n<\/div>\r\n

Effect of Mineral Content on Rock Colour<\/h2>\r\n
\r\n\r\nThe classification of an igneous rock depends partly on the minerals that may be present in the rock. Minerals have certain colours due to their chemical makeup, meaning that igneous rocks with a particular mineral composition must also have certain characteristic colours. In general:\r\n