4.1 Molten Materials

Charlene Estrada

We may receive all the heat we need on the Earth’s surface from sunlight, but if you dig a hole just a few feet deep, you might notice that the surrounding soil is much cooler. This happens because much of the sediment is insulated from the sun’s energy. But, if you could dig miles and miles into the crust, you would find that the temperature of the rock would increase!

Figure 4.1.0. This temperature vs depth plot shows how temperature increases with depth. The rate of change is called a Geothermal Gradient. By: Bkilli1, CC BY-SA 3.0,  Wikimedia Commons.

This increase in temperature with depth in the Earth’s crust is called the geothermal gradient. We can expect temperatures to increase about 25°C for each kilometer of depth. This sounds like a significant increase in heat, but once we pass the 100 km mark, temperatures really take off!

As we learned in the second chapter, the Earth’s iron core is wickedly hot! Such heat powers plate tectonics, and when combined with other factors, it can lead to partial melting in the upper mantle. This melted, molten rock occasionally rises to the Earth’s surface and when it does, watch out!

Is it Magma or is it Lava?

You may have heard both magma and lava used to refer to molten rock. This can confuse, especially since both terms apply to hot, melted rock. So what’s the difference, anyway? It comes down to the location. Magma is hot, molten rock that exists beneath the Earth’s surface. It can be found in the crust right below a volcano or within the mantle. For a volcano to erupt, it must have a source of magma.

Parts of a volcano. An active volcano always has a magma chamber beneath the cone.
Figure 4.1.1. Parts of a volcano. An active volcano always has a magma chamber beneath the cone.

Lava, on the other hand, is only observed at the surface following a volcanic eruption. It will have the same composition as the magma from which it originated. Besides erupting only at the surface, lava cools quickly because it is exposed to the comparatively cold Earth’s atmosphere (or sometimes water!), and we see different igneous rock textures as a result.  Igneous rocks from magma that solidified at depth are coarse-grained or intrusive because they cool slowly, but those from lava are fine-grained, or extrusive.

Lava fountain
Figure 4.1.2. Lava is the eruption or flow of molten material above ground.

Melting Solid Rock. How does magma form?

Figure 4.1.3. Notice the spheres on the left of this figure. Molten rock that appears on the mantle or crust is called magma. Molten rock that appears on the atmosphere is called lava, and it reaches the surface through volcanic processes. “Magmatism and Volcanism” By Woudloper, CC BY-SA 3.0.

Magma and lava contain three components—melt, solids, and volatiles (dissolved gases). The liquid part, called melt, is made of ions from molten minerals.

All you need to melt a solid rock is heat, right? Wrong! It may be counterintuitive, but most geological processes that melt rock do not involve increasing the temperature. Most lava at volcanoes is around 700 to 1300°C, which is the typical temperature of our upper mantle. However, our mantle as a whole is solid, so something else is required to cause rock to melt. That “something else” can be a sudden decrease in pressure or introducing liquid water, which will lower the melting point of rocks in the mantle. The two main mechanisms through which rocks melt are decompression melting and flux melting.

Decompression Melting

Our mantle is solid, but under high temperatures and pressures, it flows over very long timescales in a process known as convection. Convection forms circular cells of movement for the rock within the mantle, and sometimes leads to the upwelling of hot mantle material at divergent plate boundaries and hot spots.

Rock is a poor heat conductor, so as rock in the mantle rises with upwelling or convection, its temperature does not significantly change. Nevertheless, when that rock rises, the pressure of the rock decreases. This happens because the depth decreases, meaning that the weight of the column of rock above it decreases. It is the decrease in lithostatic pressure that causes the rock to melt. This process of the rock melting due to a sudden change in pressure is called decompression melting, and it typically occurs at hot spots and divergent boundaries, such as the Mid-Atlantic Ridge [1].

At a divergent boundary, where two plates move apart, the crust above molten material in the magma thins, causing less overriding pressure. Some of the mantle material begins to melt as magma and rise to the thinning rift to create new crust.
Figure 4.1.4. A divergent boundary between two rifting tectonic plates. As these plates move away,  the lithostatic pressure decreases and the mantle melts. The magma rises to the rift as new crust, in the seafloor spreading center.

Flux Melting

Figure 4.1.5. In a subduction zone, melting occurs in the sinking plate. Water, trapped in minerals, is released at depth. The addition of water melts the rock (rising diapirs in the image). Notice the location of magma and volcanoes on the over-riding plate. “Subduction” By K. D. Schroeder CC-BY-SA 4.0

At subduction zones along the Earth’s lithosphere, the descending slab is always made of oceanic lithosphere. This slab contains some hydrated minerals that, when exposed to elevated temperatures and pressures during subduction, will become released as volatile gases such as water vapor.

These volatile gases rise and interact with the overlying plate in the subduction zone. The addition of the volatiles does not change the pressure or temperature of the rock, but it does lower a property called the melting point. The decrease in melting point with those added volatiles suddenly makes it possible for the rocks in the subduction zone to melt at the same pressures and temperatures they have been experiencing, which is why we observe volcanoes at this type of plate boundary. Magmas producing the volcanoes of the Ring of Fire, associated with the subduction zones bordering the Pacific Ocean, are a result of flux melting [1].

Magma Composition

In 1980, Mount St. Helens blew up in the costliest and deadliest volcanic eruption in United States history. The eruption killed 57 people, destroyed 250 homes, and swept away 47 bridges. Mount St. Helens today still has minor earthquakes and eruptions and now has a horseshoe-shaped crater with a lava dome inside. The dome is made of pb_glossary id=”1812″]viscous[/pb_glossary] lava that oozes into place [1].

Video 4.1.1 Geologists from the US Geological Survey explained the main eruption event as well as the signs that announced it and the aftermath (7:00).

Volcanoes do not always erupt in the same way. Each volcanic eruption is unique, differing in magnitude, style, and composition of the erupted material. One key to what makes the eruption unique is the chemical composition of the magma that feeds a volcano, which determines (1) the eruption style, (2) the type of volcanic cone that forms, and (3) the composition of rocks that are found at the volcano.

The words that we use to describe the composition of igneous rocks (Ch. 3), also the ones we use to describe the composition of the magma. Mafic magmas are low in silica and have darker magnesium (Mg), and iron (Fe)-rich minerals, such as olivine and pyroxene. Felsic magmas are higher in silica and have lighter-colored minerals such as quartz and orthoclase feldspar.

The higher the amount of silica in the magma, the higher its viscosity. Viscosity is a liquid’s resistance to flow or movement within the Earth or on its surface. Viscosity determines what the magma will do. Mafic magma is not very viscous and will flow smoothly to the surface.

Video 4.1.2 Review the impacts of viscosity on magmas, lavas and the igneous rocks derived (4:57).

Volcanoes with a mafic composition will typically not have very explosive eruptions, but the lava will be fast moving. This mafic lava often moves down mountainsides and cools rapidly into unique textures that are either ropey called “Pahoehoe” or rough called “A’a“.

Pahoehoe lava flows are thin and ropey whereas aa is rough and blocky.
Figure 4.1.6. Mafic lava flows. Left: Close-up view of A’a forming during an eruption of Pacaya Volcano in Guatemala. Field of view, approximately 1 m across. Right: Rubbly reddish-brown A’a lava flow viewed from Chain of Craters Road, Hawai’i Volcanoes National Park. Pahoehoe is visible in lighter grey in the foreground.

Felsic magma is very viscous, and it does not flow smoothly. Most felsic magma will stay deeper in the crust and will cool to form intrusive igneous rocks such as granite and diorite. If felsic magma rises into a magma chamber, it may be too viscous to move, so it tends to get stuck.

However, intermediate magma is also highly viscous, and it contains dissolved volatile gases. These gases become trapped by the magma, and the magma chamber begins to build in pressure. When the magma finally can erupt as lava, it does so very violently and explosively, as we have seen at Mount St. Helens.



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