400 Billions of years ago, ier elements sank to- floated on heavier ele- now dif

Question

400 Billions of years ago, ier elements sank to- floated on heavier ele- now differentiated into sity downward (Fig. ce relative to that of Residual temperature eases downward Igneow pluton world) becaus rocks a igneou rise up 10 A volcanic region 15 A nonvolcanic region 25 Figure 4.3 Geothermal gradients (i.e., change in temperature per depth) differ from place to place (c.g. gradients are steeper in volcanic regions). Also. geothermal gradients decrease downward of ash t Lavas c base of Q4.3 Imagine a rock that melts in a laboratory oven at 800 °C. Applying the graph in Figure 4.3 for a volcanic region, at what depth (in kilometers) would you expect our rock to melt? Your answer is probably around 22 kilometers. But what if your were told that at a depth of 22 kilometers a temperature of 1,000 C is required to melt our rock? Q4.4 Explain why the temperature required to melt our rock down deep is 200 °C higher than that in the labora- tory. Hints: (a) Which is more dense, solid rock or liquid rock? (b) Qualitatively speaking, how does the weight of overlying rock affect the melting point of rock at depth centric layers r part of ght suspect arth's liquid (Fig. 4.4)? re prodaced 4.5 By the way, can you name the oaly substance for 4.5 By the way, can you name the only substance for which the solid phase is less dense than the liquid phase? ly less than Hint: Were this not true, Earth would be barren of life. arth's what is ndicates ore? Confining pressure (kilobars) 50 100 200 300 400 500 600 magmas ), which First ks melt the . 4.3). is approximately 1000 times atmospheric pressure at sea level.) Figure 4.4 Confining pressure increases with depth. (One kilobar

Explanation / Answer

4.3. From the graph we can see that the temperature of 800°C is reached at a depth of about 22/23 km beneath the surface of the earth. This result is got by constructing a vertical line from the 800 mark and extending a horizontal line from the point of intersection of the previous line with the volcanic region curve.

4.4 Now let's come to the more interesting part of the question. Why at these depths we are needing more temperature than the temperature required at the lab!

Let's consider the difference between the two locations and we will get to the answer. With increasing depth we are also increasing the pressure of the system. At these great depths the pressure of the vertical column of rock overlying the location is huge.

Now let's come to the process of melting. In the process of melting we generate a liquid from a corresponding solid. By this process we increase the volume of the system. This increased volume in turn exert an excess pressure. Now,if we decrease the pressure of the system,then it helps in expanding the volume,but if we increase the pressure then it restricts the solid to melt and increase the total volume of the system. So,at greater pressure we need more energy and hence more temperature to begin the process of melting.

This is the cause of excess temperature required to melt the rock in the question.

4.5 Water us the only substance for which the solid phase(ice) is less dense than liquid(water).

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