{"id":229,"date":"2018-04-19T02:57:55","date_gmt":"2018-04-19T06:57:55","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/7-1-magma-and-how-it-forms\/"},"modified":"2023-07-04T12:50:38","modified_gmt":"2023-07-04T16:50:38","slug":"magma-and-how-it-forms","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/magma-and-how-it-forms\/","title":{"raw":"7.1 Magma and How It Forms","rendered":"7.1 Magma and How It Forms"},"content":{"raw":"[caption id=\"attachment_226\" align=\"alignright\" width=\"400\"]\"Composition Figure 7.2<\/strong> Element abundance in Earth's crust. Source: Karla Panchuk (2021), CC BY 2.0. Modified after Steven Earle (2016), CC BY 4.0. Image source<\/a>.[\/caption]\r\n\r\nIgneous rocks form when melted rock cools, and melted rock originates within Earth as magma<\/strong>.\u00a0 Magma compositions vary, but will have eight main elements in different proportions. The most abundant elements are oxygen and silicon, followed by aluminum, iron, calcium, sodium, magnesium, and potassium. These eight elements are also the most abundant in Earth's crust (Figure 7.2).\r\n\r\nAll magmas have varying proportions of lighter elements such as hydrogen, carbon, and sulphur. Lighter elements are converted into gases like water vapour, carbon dioxide, hydrogen sulphide, and sulphur dioxide as the magma cools.\r\n

What Controls Magma Composition?<\/h1>\r\nMagma composition depends on two things:\r\n
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  1. The composition of the rocks that melted to form the magma.<\/li>\r\n \t
  2. The conditions under which the melting happened<\/li>\r\n<\/ol>\r\nMost igneous rock in Earth's crust comes from magmas that formed through partial melting<\/strong> of existing rock, either in the upper mantle or the crust. During partial melting, only some of the minerals within a rock melt. This happens because different minerals have different melting points<\/strong> (temperatures at which they melt). The melt is less dense than the surrounding rock, and will percolate upward without the source rock having melted completely. The result is magma with a different composition than the original rock. Partial melting produces melt that has more silica than the original rock, because minerals higher in silica have lower melting points.\r\n\r\nTo see how partial melting works, consider the mix of materials in Figure 7.3A. It contains white blocks of candle wax, black plastic pipe, green beach glass, and pieces of aluminum wire. When the mixture is heated to 50 \u00b0C in a warm oven, the wax melts into a clear liquid (B), but the other materials remain solid. This is partial melting.\r\n\r\n[caption id=\"attachment_227\" align=\"aligncenter\" width=\"650\"]\"A: Figure 7.3<\/strong> Experiment to illustrate partial melting. (A) The original components are white candle wax, black plastic pipe, green beach glass, and aluminum wire. (B) After heating to 50\u02daC for 30 minutes, the wax has melted. (C) After heating to 120\u02daC for 60 minutes, the plastic has also melted and mixed with the melted wax. (D) The liquid has been poured off and allowed to cool, making a solid with a different overall composition from the original mixture. Source: Karla Panchuk (2021), CC BY-NC-SA 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.<\/a>[\/caption]\r\n\r\nWhen the mixture is heated to 120 \u00b0C, the plastic melts and mixes with the wax, but the aluminum and glass still remain solid (C). This is still considered partial melting because solid materials remain. When the plastic and wax mixture is poured into a separate container and allowed to cool (D), the resulting solid has a very different composition from the original mixture. The plastic and wax are analogous to more silica-rich minerals with relatively lower melting points than other minerals in the same rock.\r\n\r\nOf course, partial melting in the real world isn\u2019t as simple as the example in Figure 7.3. Many rocks are much more complex than the four-component system used here. Some mineral components of rocks may have similar melting temperatures, and begin to melt at the same time. The melting temperature of a mineral may change in the presence of other minerals. Also, when rocks melt, the process can take millions of years, unlike the 90 minutes required to melt the pipe and wax in the experiment.\r\n
    \r\n\r\nConcept Check: Melting Point of a Rock<\/strong>\r\n\r\n
    \r\n\r\nA rock contains three minerals, A, B, and C, with melting points of 700 \u00b0C, 900 \u00b0C, and 1300 \u00b0C, respectively. What is the melting point of the rock?\r\n
      \r\n \t
    1. 700 \u00b0C, the lowest<\/strong> melting point of the three minerals<\/li>\r\n \t
    2. 1300 \u00b0C, the highest<\/strong> melting point of the three minerals<\/li>\r\n \t
    3. 700 \u00b0C to 1300 \u00b0C, the range<\/strong> covering the melting points of all the minerals<\/li>\r\n \t
    4. 967 \u00b0C, the average<\/strong> melting point of all three minerals<\/li>\r\n<\/ol>\r\nTo check your answers, navigate to the below link to view the interactive version of this activity.\r\n\r\n<\/div>\r\n[h5p id=\"70\"]\r\n<\/div>\r\n

      Why Rocks Melt<\/h1>\r\nThe obvious answer to the question \u201cWhy do rocks melt?\u201d is that rocks have been heated above the melting point of one or more of the minerals that make them up. Magma from partial melting of mantle rocks rises upward through the mantle, and may pool at the base of the crust, or rise through the crust. Moving magma carries heat with it, and some of that heat is transferred to surrounding rocks when the magma touches them. If the melting temperature of a rock is less than the temperature of the magma, the rock will begin to melt. This is melting by conduction.<\/strong>\r\n\r\nHeat transfer is not the only way of melting rocks, however, and it doesn't explain another conundrum: where melted mantle rocks come from. The rock that makes up the mantle begins to melt at approximately 1330 \u00b0C, and the mantle is almost entirely at temperatures above 1330 \u00b0C. Nevertheless, the mantle is in fact solid<\/em> with the exception of small amounts of melt in its uppermost parts.\r\n\r\nThe answer to this paradox is that higher pressures within the mantle increase the melting point of rock compared to what it would be at Earth\u2019s surface. For example, at the depth where the mantle temperature is 2300 \u00b0C, the melting point of mantle rock is actually 3800 \u00b0C. Clearly, a lot of additional heat would have to be added to melt those rocks, but there are other ways that melting happens. One way is to remove some of the pressure. This is called decompression melting<\/strong> (Figure 7.4, left).\r\n\r\n[caption id=\"attachment_228\" align=\"aligncenter\" width=\"1024\"]\"\" Figure 7.4<\/strong> Melting triggers. Left- Decompression melting occurs when a parcel of rock rises or the overlying crust thins. Right- Flux-induced melting occurs when volatile compounds such as water are added. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2016), CC BY 4.0. Image source.<\/a>[\/caption]\r\n\r\nAnother way to trigger melting is to disrupt the chemical environment by adding water. This is called flux-induced melting<\/strong>, or fluid-induced melting<\/strong>. When a substance such as water is added to hot rocks, the melting points of the minerals within those rocks decreases. If a rock is already close to its melting point, the effect of adding water can be enough to trigger partial melting. The added water is a flux<\/strong>, hence the name flux-induced melting. In Figure 7.4 (right), the parcel of rock represented by the dashed box is not hot enough to be right of the line where dry mantle rocks melt, but it is to the right of the line where wet mantle rocks melt.\r\n
      \r\n\r\n<\/a>Practice with Melting Types & Phase Diagrams<\/strong>\r\n
      \r\n\r\nThe image shown here is a phase diagram representing the pressure and temperature conditions under which granite and basalt will melt, with and without water. The questions that follow will help you get oriented with the phase diagram, and then test your understanding of decompression melting and fluid-induced (or flux) melting.\"\"\r\n\r\nImage description: The image shows a phase diagram with a vertical axis labelled \"pressure increases\" and a downward pointing arrow. The horizontal axis is labelled \"temperature increases\" with an arrow pointing to the right. The diagram has four lines labelled A through D in order from left to right. Lines A and B are curved upward to the right, and lines C and D tilt downard to the right. Lines are labelled as follows: A- wet granite melts; B- wet basalt melts; C- dry granite melts; D- dry basalt melts. There are three points labelled x, y, and z. Point x is between lines A (\"wet granite melts\") and B (\"wet basalt melts\"). Point y is directly across from x, and just to the left of line C, \"dry granite melts.\" Point z is straight above point y, and between lines C and D (\"dry basalt melts\").\r\n
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      1. Which part of the diagram represents conditions at Earth's surface?\r\n