9.2 The Temperature of earth’s Interior

As we’ve disputed in the context of metamorphism, Earth’s interior temperature increases with depth. However, as displayed in number 9.10, that rate of boost is no linear. The temperature gradient is around 15° to 30°C/km within the upper 100 km; it climate drops off considerably through the mantle, increases an ext quickly at the base of the mantle, and then increases gradually through the core. The temperature is about 1000°C at the base of the crust, about 3500°C at the basic of the mantle, and around 5,000°C at earth’s centre. The temperature gradient in ~ the lithosphere (upper 100 km) is fairly variable depending upon the tectonic setting. Gradients space lowest in the main parts that continents, greater in the vicinity the subduction zones, and higher still in ~ divergent boundaries.

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Figure 9.10 generalized rate the temperature increase with depth in ~ Earth. Temperature boosts to the right, therefore the flatter the line, the steeper the temperature gradient. Our expertise of the temperature gradient originates from seismic tide information and also knowledge the the melting points of earth materials.

Figure 9.11 price of temperature increase with depth in Earth’s top 500 km, compared with the dried mantle rock melt curve (red dashed line). LVZ= low-velocity region

Figure 9.11 shows a usual temperature curve for the top 500 kilometres of the mantle, in comparison v the melting curve because that dry mantle rock. In ~ the depth interval in between 100 and 250 km, the temperature curve comes an extremely close come the melt boundary for dry mantle rock. At this depths, therefore, mantle rock is either an extremely nearly melted or partially melted. In part situations, whereby extra heat is present and the temperature line crosses over the melting line, or wherein water is present, it might be completely molten. This an ar of the mantle is well-known as the low-velocity zone since seismic waves space slowed in ~ rock the is near its melting point, and also of course it is also known together the asthenosphere. Below 250 km, the temperature remains on the left side of the melt line; in various other words, the mantle is heavy from right here all the way down to the core-mantle boundary. 

The reality that the temperature gradient is much much less in the main part of the mantle 보다 in the lithosphere has actually been taken to show that the mantle is convecting, and also therefore that warm from depth is being carried toward the surface faster than it would certainly be v only warm conduction. As we’ll check out in chapter 10, a convecting mantle is vital feature of key tectonics.

The convection the the mantle is a product of the deliver of warm from the core to the reduced mantle. Together in a pot the soup top top a warm stove (Figure 9.12), the product near the heat resource becomes hot and also expands, making it lighter than the product above. The force of buoyancy reasons it come rise, and cooler product flows in indigenous the sides. The mantle convects in this way because the warm transfer from below is not perfectly even, and also because, also though mantle product is solid rock, that is sufficiently plastic come slowly flow (at prices of centimetres per year) as long as a steady pressure is used to it.

As in the soup pot example, earth’s mantle will certainly no much longer convect when the core has cooled to the suggest where over there is not enough heat transfer to overcome the toughness of the rock. This has already happened on smaller sized planets choose Mercury and Mars, and on earth’s Moon.

Figure 9.12 Convection in a pot that soup on a hot stove (left). As long as warm is being moved from below, the liquid will certainly convect. If the warmth is turned turn off (right), the fluid remains hot for a while, yet convection will certainly cease.

Why is the within of planet hot?


The warmth of earth’s interior originates from two key sources, each contributing about 50% that the heat. Among those is the frictional warmth left over from the collisions of large and small particles that produced Earth in the first place, plus the subsequent frictional warmth of redistribution of product within earth by gravitational forces (e.g., sinking of steel to form the core).

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The other source is radioactivity, specifically the spontaneous radioactive degeneration of 235U, 238U, 40K, and 232Th, which room primarily current in the mantle. As shown on this figure, the complete heat produced that method has to be decreasing with time (because these isotopes are obtaining used up), and is now roughly 25% of what it was when earth formed. This method that Earth’s inner is slowly coming to be cooler.