Making Magma

Since volcanoes are places where magma is erupted onto the surface of the Earth, to understand them it is necessary to first understand magma.  In this supplement we will try to explain the fundamentals of magma generation in the Earth without relying too heavily on mathematics, physics, and chemistry.

Magma is molten rock, but is usually not 100% liquid.  In most cases it also contains some mineral crystals, and especially near the surface may also contain gas bubbles:

A blob of magma containing some crystals (green) and some gas bubbles.

Magma is always formed deep inside the Earth.  When it exists onto the surface of the Earth it is called lava.

Melting Rocks

Before discussing how magma is made it is important to understand how rocks melt.  Unlike familiar substances like butter, wax, or ice, rock does not have a single melting point.  Rock has two melting points: a lower temperature, which marks the onset of melting, and a higher temperature, which marks the conclusion of melting.  The lower temperature is called the solidus and the higher temperature is called the liquidus.  At temperatures between the liquidus and solidus a mixture of liquid and solid occurs, and the liquid is called a partial melt.  This kind of melting is indicated in this sequence of diagrams, which proceeds from lower to higher temperatures:

T = 990EC.  Rock is just below its  solidus temperature, and therefore is all solid.  There are 4 minerals in this rock indicated by the colors white, green, blue, and brown.	T = 1,020EC. Rock is slightly above its solidus temperature (1,000EC) and has consequently begun to melt.  The partial melt is colored red.  The brown mineral melts first, and is completely melted at 1,020EC.	T = 1,100EC. Rock is in middle of melting range.  The blue mineral has completely melted as well as part of the green and white.	T = 1,180EC. Rock is  just below liquidus temperature (1,200EC) and only a few crystals of the white mineral remain.	T = 1,210EC. Rock is  just above liquidus temperature, and therefore is all liquid.

The difference between the solidus and liquidus temperatures is called the melting interval, and is on the order of 150-200EC for most rocks.

Liquid rock is less dense than solid rock, and therefore tends to rise due to buoyancy as soon as it is created.  Experiments have shown that as soon as a few percent of a rock has melted, the liquid escapes.  Therefore in order to produce magma, all that is necessary is for a rock to rock to be slightly above its solidus temperature, where it will undergo partial melting.  The partial melt will rise, blend with partial melts from other regions, and produce a large body of magma.

Practice Quiz 5.1.  What is the solidus?

• The temperature at which melting begins
• The temperature below which a composition is 100% solid
• The lower of the two melting point for rocks
• The onset of partial melting
• All of the above

Composition of  Partial Melts

Because some minerals melt sooner than other minerals, the composition of the partial melt is different from the composition of the original rock.  This important result was determined by melting rocks in the laboratory, and observing the composition of the partial melts produced by the experiments.  We can summarize the relationship between the compositions of the original rock and of its partial melt with reference to the compositional spectrum of igneous rocks (Supplement 2) as follows:

        The partial melt is always one step more felsic than the original rock

  The relationship is shown schematically below.  Make sure you recall the meaning of the terms ultramafic, mafic, intermediate and felsic.

In this example when a rock with mafic composition melts it will produce a partial melt with intermediate composition.

The generation of all  magma starts out with melting of mantle rock.  Since the mantle is ultramafic, this means that the first magmas produced are always mafic.

Practice Quiz 5.2.  What is the composition of the partial melt produced by melting an intermediate igneous rock?

• Ultramafic
• Mafic
• Intermediate
• Felsic
• Cannot be determined from the information given

Making Rock Melt

There are three processes that can make rock melt.  Two of these have analogies on the surface of the Earth, and are fairly readily understood.  The third is difficult for many students because it is unfamiliar.

• Raising the temperature above the solidus.  This is the obvious method, but isn't as important as you might think.  The problem is the lack of a source of extra heat inside the Earth.  In fact, the only plausible candidate for a heat source is magma itself, which is mobile and can in some cases carry heat upward and cause overlying rocks rocks to melt.  But while this is an important process in some places, it doesn't explain the origin of the original magma.
  
  In addition, magma temperature varies with its composition:

  The temperature of magma depends on its composition.  Felsic magmas are the coolest (700EC at the surface) and mafic the hottest observed today (1,100EC at  the surface).  Ultramafic magmas only occurred early in Earth's history (before 2.5 billion years) and produced lavas with temperatures in excess of 1,300EC.

  As a result of this variation, the ability of a magma to melt other rocks depends on the composition of both. Mafic magmas are the hottest, and can cause melting in rocks of intermediate to felsic composition.  Felsic magmas, on the other hand, are too cool to induce cooling in other rocks.
   

• Lowering the solidus by adding a contaminant.  If you've ever lived where it snows, you know that salt is an invaluable tool in the fight against ice.  People, and some municipalities, salt their icy sidewalks and roads in order to melt the ice.  The idea is that the combination of ice + salt melts at a lower temperature than ice alone.  The reason this is so is because salt readily dissolves into water, but not into ice.  The water produced by melting of salted ice is always salty.
  
  The same principle applies to rocks, but in this case it is usually H₂O (steam) that is added to promote melting.  The combination of rock + steam melts at a significantly lower temperature than rock alone, as much as 400EC (depending on the pressure).  This is because the steam dissolves much more easily into magma than rock.  The partial melts produced by this mechanism contain a lot of dissolved H₂O, which is very important in how they are erupted if they feed volcanoes.  This kind of melting is called hydration melting.
   

  T = 900EC. Rock is below its solidus, and is consequently all solid.	T = 900EC; same temperature. Rock is now immersed in a steam bath, and instantly partially melts.  The partial melt contains a high concentration of dissolved H2O.

  Dehydration  melting typically occurs in subduction zones, when water released off of the descending plate rises and triggers melting in the overlying mantle.  These magmas have high concentrations of H₂O, which often leads to explosive volcanic eruptions.

  Dehydration melting at subduction zones.  The downgoing plate contains water bound in minerals.  This water is released when the minerals are heated as the plate descends into the hot mantle.  The water is released as steam, which rises and causes the overlying mantle to partially melt via hydration melting.  These partial melts are H2O-rich because they are formed by hydration melting.  The partial melts rise, pool, and may eventually feed volcanoes at subduction zones like Mt. Pinatubo (Philippines), Mt. St. Helens (USA) or Mt. Fuji (Japan).

• Lowering the solidus by decompression.  This is the hardest mechanism to understand, but the most important, as it is the cause of most of the magma generated inside the Earth.  It occurs when rock rises toward the surface, and therefore experiences lower pressures as it ascends.  The lower pressure causes the solidus temperature of the rock to fall.  Even though the temperature of the rock remains the same, or may actually decrease slightly, the rock melts when it finds itself above its solidus temperature. This kind of melting is called decompression melting.
  
  To understand decompression melting we need to introduce the idea of a phase diagram.  A phase diagram, or map, is simply a diagram showing the conditions under which certain phases are stable.  In this case we are interested in the phases solid rock and liquid rock, and by 'conditions' we mean the temperature and pressure.  We expect that solid rock will be stable under conditions of low temperature, but what about the effect of pressure?
  
  The diagrams below show the phase diagram for ultramafic rock.  Notice that two lines separate the phase diagram into three regions: all solid (blue), all liquid (pink), and a mixture of solid + liquid (orange).  Being in the solid + liquid mixture region is akin to being at T = 1,020EC, 1,100EC, or 1180EC in the melting diagram above.  The lines are the solidus and the liquidus.  Also notice that depth, and as a consequence pressure, increases downwards.  This is to imitate a cross-section of the Earth, where the pressure is low at the surface and increases downwards.  The phase diagram shows the condition of mantle rock from 600EC to 1,800EC, and from the surface (essentially 0 kilobars) to a depth of 90 kilometers (60 miles, 30 kilobars).  Given a combination of temperature and pressure you can determine whether the rock would be all solid, all liquid, or a mixture of solid + liquid.  For example, under the conditions P = 10 kilobars (corresponding to a depth of 30 km), T = 1,000EC the rock would be all solid, as shown in the figure on the right.
   

  Phase diagram for ultramafic rock.  Melting will occur whenever the rock finds itself in the orange or pink regions.	The coordinates given by T = 1,000EC and P = 10 kilobars lie in the blue field, and therefore the rock would be all solid.

  The key feature of this phase diagram is the slope of the solidus line.  The temperature of the solidus increases with increasing pressure.  Thus the solidus is at 1,100EC at the surface, but over 1,500EC at 90 kilometers depth.  Remember, all that is required for a rock to partially melt is that it lie at a temperature above its solidus.  A rock that lies anywhere in the fields colored orange or pink will produce magma.
  
  Under normal conditions the Earth is entirely within the field of all solid--which is just a fancy way of saying it's not melted at all.  The collection of states that rock in the Earth feels, from the surface downward, is called the geotherm, and is shown on the phase diagram below as a green line.  Remember, the key point about the geotherm is that it lies everywhere below the solidus in the blue field, and therefore does not produce any melting.

  Phase diagram for ultramafic rock showing the geotherm.

  Under certain conditions, large masses of rocks deep inside the Earth rise.  They rise either because they are slightly less dense than surrounding rocks (because they are slightly hotter), or because they are "sucked" up by low pressure above (for example, below a mid-ocean ridge where plates are diverging), or because they are caught up in convection cells within the Earth's sublithospheric mantle.  When a large mass of mantle rock rises its temperature changes very little.  To be sure, as it rises it finds itself in a cooler environment, and heat begins to be lost at the borders of the mass of rock, but if it is sufficiently large the loss of heat from its interior will be minimal and its temperature will remain nearly constant.  The ascent of a large blob of mantle can be shown on the phase diagram, and will have the following features: (a) it will start along the geotherm; and (b) it will follow a path that is nearly vertical, indicating its temperature remains constant while its pressure decreases.  This path is shown on the phase diagram below as a heavy black arrow:

  Phase diagram for ultramafic rock showing the geotherm and the path of a rising blob of rock (black arrow).  As blob rises it pressure decreases but its temperature remains essentially constant.  This produces a vertical path on this diagram.  As blob rises it eventually intersects and crosses the solidus, and melting begins.

  At this point it is important to summarize:
  

  • As pressure is decreasing the temperature of the mass of rock remains constant.  Thus it carries its temperature, inherited from below, upward. 
  • As pressure is decreasing the temperature of the solidus is decreasing. 

  As a consequence of these trends, the temperature of the mass of rising rock eventually will equal the solidus, and as it continues to rise will eventually cross it and enter the field of solid + liquid.  We call this event melting!  Thus the rock melts not because its temperature increases but because its solidus temperature decreases, a result of decreasing pressure.

  Examples where rising rock occurs in the Earth's sublithospheric mantle.  The lithosphere is shown in purple; everything below that is fluid rock of the sublithospheric mantle.  Ultramafic rock rises under mid-ocean ridges (MOR) to fill in the space vacated when the plates move apart, and also under hot spots.  Decompression melting explains the origin of magma at both of these settings.

Practice Quiz 5.3.  What kind of rocks can mafic magma melt?

• All rocks except ultramafic rocks
• Mafic rocks only
• Intermediate rocks only
• Intermediate and felsic rocks only
• Felsic rocks only

Practice Quiz 5.4.  What is the contaminant that lowers the solidus to produce melting in the Earth's mantle?

• Metallic iron
• Si
• Ultramafic magma
• H₂O as steam
• Quartz minerals from the mantle

Practice Quiz 5.5.  Where does hydration melting typically occur?

• At mid-ocean ridges
• Near the core/mantle boundary
• Under hot spots
• In the Earth's crust
• In subduction zones

Practice Quiz 5.6.  Which of the following statements is not true about decompression melting?

• It occurs below mid-ocean ridges
• It occurs below hot spots
• It occurs in the mantle
• It can occur while the temperature of the rock is decreasing
• It requires the addition of water

Practice Quiz 5.7.  Which of the following statements is not true about the geotherm?

• It describes the temperatures and pressures within the Earth
• It lies above the solidus over much of its length
• It is not a straight line
• It lies entirely within the field of "All solid"

Practice Quiz 5.8.  Why does decompression melting occur?

• Because the temperature of the rock increases faster than its solidus temperature
• Because the liquidus temperature decreases to equal the soildus temperature
• Because the solidus temperature decreases to equal the rock temperature
• Because the solidus intersects regions with abundant H₂O
• Because the solidus temperature increases faster than the rock temperature

Practice Quiz 5.9.  What is necessary for rock to melt?

• Its temperature exceeds the solidus temperature
• Its temperature exceeds the liquidus temperature
• Its temperature exceeds both the solidus and liquidus temperature
• Its temperature lies on the geotherm
• Its temperature lies below the geotherm