{"id":401,"date":"2019-06-11T14:50:25","date_gmt":"2019-06-11T14:50:25","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/9-2-the-temperature-of-earths-interior\/"},"modified":"2021-12-08T16:17:50","modified_gmt":"2021-12-08T16:17:50","slug":"9-2-the-temperature-of-earths-interior","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/9-2-the-temperature-of-earths-interior\/","title":{"raw":"9.2 The Temperature of Earth\u2019s Interior","rendered":"9.2 The Temperature of Earth\u2019s Interior"},"content":{"raw":"As we\u2019ve discussed in the context of metamorphism, Earth\u2019s internal temperature increases with depth. However, as shown in Figure 9.2.1 (right), that rate of increase is not linear. The temperature gradient is around 15\u00b0 to 30\u00b0C per kilometre within the upper 100 kilometres; it then drops off dramatically through the mantle, increases more quickly at the base of the mantle, and then increases slowly through the core. The temperature is around 1000\u00b0C at the base of the crust, around 3500\u00b0C at the base of the mantle, and around 5,000\u00b0C at Earth\u2019s centre. The temperature gradient within the lithosphere (upper 100 kilometres) is quite variable depending on the tectonic setting. Gradients are lowest in the central parts of continents, higher in the vicinity of subduction zones, and higher still at divergent boundaries.<\/a>\r\n\r\n[caption id=\"attachment_398\" align=\"aligncenter\" width=\"857\"]\"\"<\/a> Figure 9.2.1 Right: generalized rate of temperature increase with depth within Earth. Temperature increases to the right, so the flatter the line, the steeper the temperature gradient. Our understanding of the temperature gradient comes from seismic wave information and knowledge of the melting points of Earth\u2019s materials. Left: Rate of temperature increase with depth in Earth\u2019s upper 500 kilometres, compared with the dry mantle rock melting curve (red dashed line). LVZ= low-velocity zone. [Image Description]<\/a>[\/caption]Figure 9.2.1 (left) shows a typical temperature curve for the upper 500 kilometres of the mantle in more detail, along with the melting curve for dry mantle rock. (Mantle rock will melt under conditions to the right of the dashed red line.) In general the mantle is not molten because the temperature lies to the left of the melting curve, but within the depth interval between 100 and 250 kilometres the temperature curve comes very close to the melting boundary for dry mantle rock. At these depths, therefore, mantle rock is either very nearly melted or partially melted. In some situations, where extra heat is present and the temperature line crosses over the melting line, or where water is present, it may be completely molten. This region of the mantle\u2014the asthenosphere\u2014is also known as the low-velocity zone because seismic waves are slowed within rock that is near its melting point. Below 250 kilometres the temperature stays on the left side of the melting line; in other words, the mantle is solid from here all the way down to the D\" layer near the core-mantle-boundary.\r\n\r\nThe fact that the temperature gradient is much less in the main part of the mantle than in the lithosphere has been interpreted to indicate that the mantle is convecting, and therefore that heat from depth is being brought toward the surface faster than it would be with only heat conduction. As we\u2019ll see in Chapter 10, a convecting mantle is an key feature of plate tectonics.\r\n\r\nThe convection of the mantle is a product of the upward transfer of heat from the core to the lower mantle. As in a pot of soup on a hot stove (Figure 9.2.2), the material near the heat source becomes hot and expands, making it lighter than the material above. The force of buoyancy causes it to rise, and cooler material flows in from the sides. The mantle convects in this way because the heat transfer from below is not perfectly even, and also because, even though mantle material is solid rock, it is sufficiently plastic to slowly flow (at rates of centimetres per year) as long as a steady force is applied to it.\r\n\r\nAs in the soup pot example, Earth\u2019s mantle will no longer convect once the core has cooled to the point where there is not enough heat transfer to overcome the strength of the rock. This has already happened on smaller planets like Mercury and Mars, as well as on Earth\u2019s Moon.\r\n\r\n[caption id=\"attachment_399\" align=\"aligncenter\" width=\"700\"]\"\"<\/a> Figure 9.2.2 Convection in a pot of soup on a hot stove (left). As long as heat is being transferred from below, the liquid will convect. If the heat is turned off (right), the liquid remains hot for a while, but convection will cease.[\/caption]\r\n\r\n
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Why is the inside of the Earth hot?<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\n[caption id=\"attachment_400\" align=\"alignright\" width=\"400\"]\"\"<\/a> Figure 9.2.3 Heat flow on Earth from radioactive decay[\/caption]\r\n\r\nThe heat of Earth\u2019s interior comes from two main sources, each contributing about 50% of the heat. One of those is the frictional heat left over from the collisions of large and small particles that created Earth in the first place, plus the subsequent frictional heat of redistribution of material within Earth by gravitational forces (e.g., sinking of iron to form the core).\r\n\r\nThe other source is [pb_glossary id=\"1599\"]radioactivity[\/pb_glossary]<\/strong>, specifically the spontaneous radioactive decay of the isotopes\u00a0235<\/sup>U, 238<\/sup>U, 40<\/sup>K, and 232<\/sup>Th, which are primarily present in the mantle. As shown on Figure 9.2.3, the total heat produced that way has been decreasing over time (because these isotopes are getting used up), and is now roughly 25% of what it was when Earth formed. This means that Earth\u2019s interior is slowly becoming cooler.\r\n\r\n<\/div>\r\n<\/div>\r\n

Image Descriptions<\/h3>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
<\/a>Figure 9.2.1 image description: Temperature increase within the Earth.<\/caption>\r\n
Layer<\/th>\r\nDepth (kilometres)<\/th>\r\nTemperature increase (Celsius)<\/th>\r\nTemperature increase rate (Degrees per kilometre)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
Lithosphere<\/td>\r\n0 to 100<\/td>\r\n0 to 1400<\/td>\r\n14<\/td>\r\n<\/tr>\r\n
Asthenosphere<\/td>\r\n100 to 250<\/td>\r\n1400 to 1700<\/td>\r\n2<\/td>\r\n<\/tr>\r\n
Mantle<\/td>\r\n250 to 1000<\/td>\r\n1700 to 2100<\/td>\r\n0.53<\/td>\r\n<\/tr>\r\n
1000 to 2000<\/td>\r\n2100 to 2600<\/td>\r\n0.5<\/td>\r\n<\/tr>\r\n
2000 to 2890<\/td>\r\n2600 to 3800<\/td>\r\n1.35<\/td>\r\n<\/tr>\r\n
Outer Core<\/td>\r\n2890 to 4000<\/td>\r\n3800 to 4600<\/td>\r\n0.72<\/td>\r\n<\/tr>\r\n
4000 to 5100<\/td>\r\n4600 to 5000<\/td>\r\n0.36<\/td>\r\n<\/tr>\r\n
Inner core<\/td>\r\n5100 to 6370<\/td>\r\n5000 to 5100<\/td>\r\n0.079<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n[Return to Figure 9.2.1]<\/a>\r\n

Media Attributions<\/h3>\r\n