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<p>mantle material (<a href="page.php?w=peridotite">peridotite</a> in the upper mantle).</p>

<p><big> Negative geothermal gradient </big></p>
<p>Negative geothermal gradients occur where temperature decreases with depth. This occurs in the upper few hundreds of meters near the surface. Because of the low <a href="page.php?w=thermal_diffusivity">thermal diffusivity</a> of rocks, deep underground temperatures are hardly affected by diurnal or even annual surface temperature variations. At depths of a few meters, underground temperatures are therefore similar</p><p>
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