Origin of Magma

Most magma originates in the uppermost mantle. The greatest quantities are produced at divergent plate boundaries in association with seafloor spreading. Lesser amounts form at subduction zones, where oceanic lithosphere descends into the mantle. In addition, magma can originate far from plate boundaries.

Earth’s crust and mantle are composed primarily of solid, not molten, rock. Although the outer core is fluid, this ironrich material is very dense and remains deep within Earth. So, where does magma come from?

INCREASE IN TEMPERATURE. Most magma originates when essentially solid rock, located in the crust and upper mantle, melts. The most obvious way to generate magma from solid rock is to raise the temperature above the rock’s melting point. Although the rate of temperature change varies considerably from place to place, it averages about 25 °C per kilometre in the upper crust. This increase in temperature with depth, known as the geothermal gradient, is somewhat higher beneath the oceans than beneath the continents. As shown in Figure 4.1, when a typical geothermal gradient is compared to the melting point curve for the mantle rock peridotite, the temperature at which peridotite melts is everywhere higher than the geothermal gradient. Thus, under normal conditions, the mantle is solid. Tectonic processes exist that can increase the geothermal gradient sufficiently to trigger melting. In addition, other mechanisms exist that trigger melting by reducing the temperature at which peridotite begins to melt.

DECREASE IN PRESSURE: DECOMPRESSION MELTING. Pressure also increases with depth. Melting, which is accompanied by an increase in volume, occurs at higher temperatures at depth because of greater confining pressure. Consequently, an increase in confining pressure causes an increase in the rock’s melting temperature. Conversely, reducing confining pressure lowers a rock’s melting temperature. When confining pressure drops sufficiently, decompression melting is triggered. Decompression melting occurs where hot, solid mantle rock ascends in zones of convective upwelling, thereby moving into regions of lower pressure. This process is responsible for generating magma along divergent plate boundaries (oceanic ridges) where plates are rifting apart.

ADDITION OF VOLATILES. Another important factor affecting the melting temperature of rock is its water content. Water and other volatiles act as salt does to melt ice. That is, volatiles cause rock to melt at lower temperatures. Further, the effect of volatiles is magnified by increased pressure. Deeply buried “wet” rock has a much lower melting temperature than “dry” rock of the same composition (see Figure 4.1). Therefore, in addition to a rock’s composition, its temperature, depth (confining pressure), and water content determine whether it exists as a solid or liquid.

In summary, magma can be generated in three ways: (1) when an increase in temperature causes a rock to exceed its melting point; (2) in zones of upwelling a decrease in pressure (without the addition of heat) can result in decompression melting; and (3) the introduction of volatiles (principally water) can lower the melting temperature of hot mantle rock sufficiently to generate magma.

Task 4. Answer the following questions, using the vocabulary from Task 1.

1. Where does most magma originate?

2. What is the geothermal gradient?

3. How does increase in temperature influence the origin of magma?

4. Describe decompression melting.

5. How does the introduction of volatiles trigger melting?

6. What are the three ways magma can usually be originated in?

Task 5. Find English equivalents for the following (see the text). Try to build up your own sentences with them.

Породжуватись (створюватись) у самому верхньому шарі мантії; переходити у мантію; межі тектонічної плити; тверда, а не рідка гірська порода; залишатись глибоко в Землі; зовнішнє ядро Землі; верхня мантія Землі; точка (температура) плавлення; значно відрізнятись; за звичайних умов; зменшувати температуру; вміст води; тиск, який зростає з глибиною; визначати; зони апвелінгу; достатньо.

Task 6. Give a short summary of the text from Task 3.