IGNEOUS ROCKS: SOLIDS FROM MELTS

Igneous rocks all begin their history as molten material called magma. Since evidence suggests that at one time Earth's entire surface was molten, it is probably fair to say that all inorganic rock material started out as a magma. The study of igneous rocks is particularly intriguing to geologists because under the microscope, the physical laws of nature are strictly obeyed. Through these data, an understanding evolved of how Earth's crust formed. The chemistry of igneous rocks also gives insight to plate tectonic processes occurring deep beneath the surface.

How Do Igneous Rocks Differ from One Another?

Hand specimen identification of igneous rocks is somewhat of an art. It takes practice and an agile tongue to recognize and pronounce many of the words that geologists use in their description. Igneous rocks are classified using two different criteria:

• Texture
• Chemical and Mineral Composition

Most of the time the rock name, such as basalt or granite, reflects the chemical composition. The minerals and textures are used as adjectives that modify the rock name. You might see a name like an olivine basalt or a quartz porphyry rhyolite.

Texture

The first thing that a geologist does with an igneous rock is to look at the size of the mineral grains. The answer is almost always either fine-grained or coarse-grained. Coarse grains are visible to the naked eye. If the rock is made entirely of coarse grains, it is given a phaneritic texture. Fine grains are not visible to the naked eye. A rock composed completely of fine grains has an aphanitic texture. A phaneritic rock with one mineral that is much larger than the others or an aphanitic rock with one mineral type that is visible to the naked eye is said to be porphyritic. A friend of mine once had a Dalmatian named Porphyry.

First Clue: Volcanic Rocks

Because volcanic rocks are extruded into the atmosphere or ocean, they cool much faster than magmas that cool deep in the crust. Crystals do not have enough time to grow large so any crystals that form have an aphanitic texture. Microscopic examination of volcanic rocks reveals that they are mostly composed of fine-grained glass indicating that they cooled too fast for much crystallization to take place. The lava from the center of a lava flow is better insulated from cooling than other parts of a flow so crystals will tend to be a little larger there but most of the rock is still glass.

Second Clue: Laboratory Studies of Crystallization

Crystallization from a magma has some similarities to ice forming from water. The major difference is that water is pure H2O whereas magmas are a mixture of many elemental components (including water). When water freezes, the individual molecules take fixed positions in the crystal structure through the formation of hydrogen bonds and van der Waals bonds. Individual minerals do the same thing but different minerals start to crystallize at different times depending on the temperature of the magma and the magma composition.

Time consuming laboratory experiments with melts of basalt in the early 20th century by M. L. Bowen help to identify and understand the crystallization process. Basalts are the most common constituent of oceanic crust. In fact, if the oceanic crust were completely melted and allow to cool slowly, it would form a basalt.

Third Clue: Granite--Evidence of Slow Cooling

In the late 1700's one of Geology's founding fathers, James Hutton, examined numerous granites around his native Scotland. He was particularly intrigued that granites seemed to fracture the surrounding rocks and then flow into the cracks like a liquid. Hutton noted that the bordering rocks were usually discolored and otherwise altered. He also recognized that granites are composed of an interlocking set of crystals. He correctly deduced that the alteration was the result of exposure to great heat from the granite and that granite had flowed into the cracks in a molten state. Because the granite was well insulated deep in the crust, it cooled very slowly, allowing every part of the melt to crystallize into one mineral or another. Unlike basalt, granites contain no glass.

Granites are the most common constituent of continental crust. In fact, if the continental crust were completely melted and allow to cool slowly, it would form a granite.

Intrusive Igneous Rocks

Intrusive igneous rocks constitute one of two major categories of igneous rocks. Because they cooled slowly, they always exhibit a phaneritic texture. These rocks are often called plutonic rocks with reference to Pluto, the Roman god of the underworld. Solid rock bodies formed of plutonic rocks are called plutons.

Extrusive Igneous Rocks

Extrusive igneous rocks constitute the other major category and are always either aphanitic or porphyritic. Because these rocks are erupted by volcanic processes, they are usually called volcanic rocks.

In truth, there is a third category called hypabyssal igneous rocks. They have a medium-grained texture and, as one might expect, they form between the plutons and volcanoes. These rocks solidify in the conduits and cracks the magma creates on its way to the surface. The most common rock is diabase which is simply a basalt that had more time to cool so a lot of medium-sized crystals formed. Glass is still a component of diabase.

Chemical and Mineral Composition

Because a molten magma can cool slowly deep beneath the surface to form a plutonic rock or can cool rapidly on the surface as a volcanic rock, it stands to reason that every extrusive rock has an intrusive equivalent and vice versa. We will create a table that exhibits volcanic rocks (aphanitic textures) the upper row and their intrusive equivalents (phaneritic textures) in the lower row. I prefer this configuration over the one in the book (Fig. 5.4) which shows intrusive (deep) rocks on top of volcanic (surface) rocks. We will divide these rows based on the relative proportion of silica in the rocks. This has a tendency to correspond with specific mineral types found in the rocks.

COMPOSITION	ULTRAMAFIC	MAFIC	INTERMEDIATE		FELSIC or SIALIC
TEXTURE
Aphanitic (extrusive, volcanic)	Komatiite	Basalt	No Quartz	+ Quartz	No Quartz	+ Quartz
			Andesite	Dacite	Trachyte	Rhyolite
Phaneritic (intrusive, plutonic)	Peridotite	Gabbro	Diorite	Tonalite	Syenite	Granite

General Classification Chart for Igneous Rocks

Note that I also include a few more rock names than your book does. We will discuss these rocks in the sections below. I arrange the rocks in the opposite direction from the book for several reasons. Going from left to right we see:

1. a decrease in the temperature at which melting starts.
2. a decrease in the amount of magnesium and iron.
3. a decrease in the amount of water dissolved in the magma.
4. an increase in the amount of sodium and potassium.
5. an increase in the amount of silicon and aluminum.
6. an increase in the viscosity of the magma.
7. an increase in the explosivity of volcanic eruptions.

Ultramafic Rocks--Ultramafic rocks are rich in ferromagnesian minerals (from which the ma-f in mafic is derived) such as olivine and pyroxenes and depleted in feldspars (<10%). Because of this composition, ultramafic rocks have the highest melting temperatures. They tend to be very dark to greenish and are denser than most common rocks. Today, all ultramafic rocks form in the mantle where it is hot enough to sustain them in a liquid form. These intrusive rocks are referred to as peridotites. There are several varieties of peridotites:

1. dunite--composed of >95% olivine.
2. pyroxenite--composed of >95% pyroxene.
3. lherzolite--composed of a mixture of olivine and pyroxenes.
4. harzburgite--a lherzolite in which the pyroxene is the mineral enstatite

Although we don't see volcanic ultramafic rocks today, early in Earth's history, during the Archaean (4.0-3.5 billion years ago) ultramafic lavas were abundant. This indicates that Earth was a much hotter place early in its history. These lavas, called komatiites, were first described in South Africa. They are also abundant on the Canadian Shield and shield areas on other continents.

Mafic Rocks--Mafic rocks are also rich in ferromagnesian minerals but, unlike ultramafic rocks, they contain abundant calcium-rich plagioclase feldspar. Basalts never contain quartz. The rocks tend to be very dark. Basalt, the volcanic rock, is black. Because the feldspars are light colored and have more time to grow in intrusive rocks, gabbros tend to be dark with gray or white speckles. Basalts comprise most of the sea floor and are also common around continental volcanoes. Gabbros are less abundant on the continents because they tend to form in the lower part of the oceanic crust.

Intermediate Igneous Rocks--Depending on the silica content of the magma, free quartz may or may not be present in intermediate igneous rocks. These rocks tend to have a grayish appearance. Intermediate volcanic rocks with no quartz were named by Charles Darwin as he crossed the Andes from Chile into Argentina. He called them andesites. Andesites are common in all continental volcanic mountain ranges. If quartz is present, the rock is called a dacite. The dome growing in the crater of Mt. St. Helens is a dacite. Diorites and tonalites, not surprisingly, tend to form beneath areas of continental volcanism. Because of their coarse grain size and subequal amount of light and dark minerals, these intrusive rocks are often said to have a "salt and pepper" appearance. These rocks can be exposed, along with other plutonic rocks, when mountains are uplifted and the overlying volcanic rocks are eroded away. Many of the rocks in the Sierra Nevada Mountains of California are diorites and tonalites. Their ages span more than 100 million years, suggesting that the area was once a long-lived volcanic region.

Felsic or sialic rocks--Felsic rocks are rich in feldspar. Since feldspar is rich in silicon (Si) and aluminum (Al), many geologists refer to them as sialic rocks. These rocks have two kinds of feldspar that are often difficult to distinguish: 1) orthoclase, a potassium feldspar, and 2) a sodium-rich plagioclase feldspar. These rocks commonly have abundant quartz as well. Minor amounts of biotite, the black mica, and amphibole minerals may also be present, as well as muscovite, the clear mica. In general, sialic rocks have a light color. Granites, and their volcanic equivalents, the rhyolites, are the most common felsic rocks. These rocks are also usually associated with areas of continental volcanism. In fact, the association of basalts, andesites, and rhyolites (B-A-R) is diagnostic of continental volcanism in the ancient rock record. Syenites and trachytes are restricted to environments where quartz is less abundant, such as areas of continental rifting.

How Do Magmas Form?

Studies of seismic waves traveling through the planet reveal that the top of the mantle and the entire lower mantle, all the way down to the outer core, is composed of solid material. Only a narrow region of the mantle exhibits evidence of partial melting. In this section we'll see how igneous melts are produced.

How Do Rocks Melt?

Most of what we know about the melting of rocks comes from careful, long-term, laboratory experiments. From this work, we know at what temperatures different minerals melt under different pressure-temperature conditions and rock compositions. The amount of water in the rock, for instance, can significantly affect the temperature at which a mineral begins to melt.

Temperature and Melting

Most rocks do not melt completely at a given temperature because they are composed of more than one mineral. Instead, a partial melt takes place in which one or more mineral constituents of the rock begin to melt. You can imagine that the composition of a magma will vary with regard to how much of each mineral has melted to contribute to the composition of the magma, in other words the ratio of solid:liquid.

Pressure and Melting

The melting temperature of a rock increases as pressure increases. If the pressure increases faster than the temperature, then the rock will remain solid. This is the reason that the entire lower mantle remains solid even though the temperature is much hotter than in the upper reaches of the mantle.

Conversely, if pressure is relieved, the very hot rock will rapidly melt. This decompression melting explains why rock rising on a convection current beneath a mid-ocean ridge melts to produce magma. This is very important to the generation of basaltic magmas. A 5-10% partial melt of a peridotite generates a basalt.

Water and Melting

Dissolving a little of one substance into another lowers the melting point and boiling point of the solution. Ocean water does not freeze while fresh water onshore at the same temperature ices over. Cooks add salt to hot water to lower the boiling point. The melting temperature of feldspars, quartz, and other rock-forming minerals decreases as the amount of water around them increases. We refer to this as fluid-induced melting.

The Formation of Magma Chambers

Because the liquid form of most substances, including rocks and minerals, is less dense than the solid form, the liquid will tend to flow in the direction of less pressure. In the crust, that direction is almost always upward. The liquid seeps upward through pores and fractures. As they rise, they bump into other drops and form bigger and bigger drops. The magma may rise for thousands of years, constantly growing in volume. It may mix with melts of other compositions. It may be hot enough to melt some of the minerals in the "country rock" through which it travels. Other pieces of country rock may fall into the magma, producing xenoliths or foreign rocks that appear to be floating with the igneous rock. Eventually, the liquid coalesces into a subterranean magma chamber. Two major options occur in the magma chamber. In one scenario, the hot magma causes the overly rock to bulge and crack, allowing the magma to squirt up to the surface, erupting as an aphanitic volcanic rock. If this does not happen then the magma just sits there and cools over many thousands of years to form phaneritic plutonic rocks.

Where Do Magmas Form?

Magmas usually form along the boundaries of tectonic plates. We have already seen how volcanic eruptions along mid-ocean ridges produce new sea floor, at divergent plate boundaries. Subduction zones, at convergent plate boundaries, produce volcanic chains.

Geologists often refer to the geothermal gradient. This is the rate at which temperature increases with depth. In most areas, this rate is around 30º C/km. In spreading and subduction areas, the geothermal gradient is much higher. Other areas experience anomalously high heat flow such as the Great Basin of the North American West. This region is experiencing tension, or stretching, so the crust is thinner than normal allowing more heat to escape from below. Within the past 15 million years tremendous volumes of volcanic rock was erupted throughout the Great Basin.

Magmatic Differentiation

We saw above in the section on pressure and melting that a basaltic magma can be created by the partial melt of a peridotite. That same basaltic magma can be shown to be the source of a number of other igneous rock types. The idea that rocks of various composition could arise from a single magma was born in the laboratory through the careful study of the sequence in which minerals appear as a melt cools. N. L. Bowen was a pioneer in this type of experiment in the 1920's. Starting with a high-temperature basaltic melt, for instance, he saw that the first mineral to crystallize, as temperature was gradually lowered, was olivine. As the olivine crystallizes, the overall composition of the melt changes since the chemical constituents of the olivine are no longer in the liquid state.

Bowen's Reaction Series: The sequence of mineral crystallization from a basaltic melt.

If the olivines that crystallized from the basaltic melt all settled to the floor of a magma chamber, then a rock of pure olivine would result. We saw earlier that such a rock is a variety of peridotite that we call a dunite. Continued crystallization of mafic minerals would eventually crystallize a pyroxene, then an amphibole, and then a biotite mica. Meanwhile, felsic minerals also begin to crystallize out. The first is a Calcium (Ca)-rich plagioclase feldspar. As the temperature drops, these plagioclases take more and more sodium (Na) into their structures. Eventually, all of the Ca and Na are used up. If any Potassium (K) is left in the melt, orthoclase will start to crystallize, followed by muscovite. Finally, the last mineral to crystallize is quartz.

Other rock types could form from the same basaltic melt as crystallization progressed:

lherzolite (olivine + pyroxene) (This is usually an enstatite pyroxene, making the lherzolite a harzburgite!) gabbro (olivine + pyroxene + Ca-plagioclase) pyroxenite (pyroxene) diorite (amphibole + biotite + Na-plagioclase) granite (amphibole + biotite + Na-plagioclase + orthoclase + quartz + muscovite)

This process is referred to as fractional crystallization.

Fractional Crystallization: Laboratory and Field Observations

Your book describes field evidence for fractional crystallization seen in the Palisades along the Hudson River in New York. Another place where fractional crystallization played an important role is in the Bushveld Complex of South Africa. Like the Palisades, the Bushveld was a large intrusion of basaltic magma. Unlike the Palisades, the Bushveld covers more than 65,000 km2 and is 8 km thick, in places. It is also extremely rich in chromium and platinum and is the world's largest source for these two metals.

The Bushveld is characterized by numerous repeating layers of dunite on the bottom followed by harzburgite and then by pyroxenite. Sometimes the pyroxenite is capped by a gabbro. This suggests that the Bushveld was created over a long period of time by numerous injections of basaltic magma that may or may not have mixed with earlier magmas.

Granite and Basalt: Magmatic Differentiation

Magmatic differentiation goes hand in hand with fractional crystallization since magma composition changes after minerals crystallize out. If that magma then migrates to a new magma chamber due to tectonic pressures, it could form a new rock type but not be physically associated with the early rocks that crystallized from the melt. Some evidence for this is seen in Iceland. The source magma for Icelandic volcanic rocks is unquestionably basaltic and yet there are numerous, small rhyolite domes and lava flows.

Forms of Magmatic Intrusions

Geologic mapping around intrusive bodies allow geologists to categorize plutons into several categories which are discussed below.

Plutons

The generic pluton is called a stock. It is the result of a single intrusive event and covers an area of < 100 km2. If igneous activity continues for a long period of time, numerous stocks will intrude a region. These bodies will begin to coalesce, almost indistinguishably if their compositions are similar, and form a larger igneous body called a batholith. Batholiths cover > 100 km2 and sometimes cover thousands of square kilometers. The Sierra Nevada Batholith in California is a good example. Another example, in western North Carolina is the "Whiteside" Batholith that is found in Jackson, Transylvania, and Henderson counties.

 

Sills and Dikes

Two very common types of intrusive bodies are dikes and sills. Both are tabular, usually intruding along preexisting fractures or faults, or along geological/stratigraphic contacts between rock types. Although these can occur at any depth, they are often associated with shallow intrusions. Dikes are discordant intrusions and usually serve as magma conduits to higher intrusive bodies or to volcanoes on the surface. They tend to grow along fractures and faults. Sills are concordant plutons and tend to grow along stratigraphic contacts.

Veins

Veins are essentially smaller dikes and sills that usually contain a single major mineral, such as quartz or calcite. Pockets of other minerals may or may not be present within the vein. Hydrothermal veins, as the name implies, are veins formed by minerals that were dissolved in hot water that once flowed through the rocks. Often these veins contain pyrite and may have other sulfide minerals and/or gold and silver.

Igneous Activity and Plate Tectonics

There is little doubt that most igneous activity occurs at plate margins associated with either sea floor spreading or subduction. Mid-ocean ridges are the product of sea floor spreading. Island arcs, such as the Aleutians, and continental arcs, such as the Cascade volcanoes, result from subduction. Of the two, the mid-ocean ridges are the most significant.

Mantle plumes, or hot spots, such as Hawaii are a third source of igneous rocks. Most hot spot volcanoes are not associated with plate margins. Iceland, however, is a place where a hot spot happens to be centered on a mid-ocean ridge.

Spreading Centers as Magmatic Geosystems

Since the Bronze Age, the chalcopyrite deposits of the Troodos Complex on the island of Cyprus, in the eastern Mediterranean, have been exploited for their copper. The deposits are situated within a suite of rocks that geologists call an ophiolite suite. The word ophios means snake in Greek. The most abundant mineral in the ophiolite suite is serpentine, a green Mg-rich sheet silicate mineral that feels a lot like snake skin. Serpentine forms when other Mg-rich silicates such as olivine and enstatite (pyroxene) come into contact with hot water circulating through them.

An ophiolite is a slice of oceanic lithosphere that has been incorporated into the continental lithosphere during the collision of two plates. The Thetford Mines, Québec ophiolite is one of a string of these pods that extend from Newfoundland to northwestern Georgia. They were emplaced about 490 million years ago during the Taconic Orogeny. In North Carolina, there are several well-known localities. The Webster-Addie district, near Sylva may be the best exposed. One variety of serpentine is the mineral asbestos. The Thetford mines area is the largest asbestos district in North America. Several copper mines and an abandoned chromite mine are also found within the ophiolite.

Ophiolites have a characteristic igneous stratigraphy that is apparently the result of fractional crystallization of a basaltic melt. They can typically be divided into an ultramafic layer, a mafic layer, and a sedimentary layer. The basic stratigraphy is shown below with the deepest rocks on the bottom:

Sedimentary Layer	Red radiolarian cherts are the predominant sedimentary rock. The strata indicate deposition in a deep marine environment.
Mafic Layer (Oceanic crust)	Pillow Lavas Pillow lavas are only formed by underwater volcanic eruptions*. Sheeted basaltic dike complex These vertical dikes fed the eruptions that created the overlying pillow lavas. Gabbro Gabbro is the plutonic equivalent of the volcanic rock basalt. Instead of being erupted to the surface where it could quickly cool, this magma remained deep, cooling slowly, and crystallizing coarse-grained crystals. *Unlike the ultramafic layer below, these layers do not repeat. Base-metal sulfide deposits are sometimes found in the pillow lava  layer.
Mohorovicic Discontinuity
Ultramafic Layer Mantle Lithosphere	Pyroxenite** Pyroxene minerals make up >95% of all pyroxenites. Harzburgite** Pyroxenes and olivine are found together in harzburgites. Dunite** Olivine makes up >95% of all dunites. **These layers tend to repeat numerous times in a complete or broken bottom to top order. Chromite deposits may be associated with the ultramafic layer.

An ophiolite is a slice of oceanic lithosphere. The boundary between the crust and the underlying mantle, known as the Mohorovicic Discontinuity, or Moho, occurs between the ultramafic (mantle) and mafic (crust) layers. During metamorphism, hot water was flushed through the Thetford Mines ophiolite and altered the ultramafic layer. Olivine and pyroxene, the two major mineral groups in the ultramafic layer, are predominantly magnesium silicates. These chemicals react with the hot water to form the clay mineral known serpentine, a hydrated magnesium silicate. A major form of serpentine is chrysotile, more commonly known as asbestos.

To learn more about ophiolites, go to http://jersey.uoregon.edu/~mstrick/GeoTours/Josephine%20Ophiolite/JoOphiolite.html.

Input Material: Peridotite in the Mantle

Temperatures in the asthenosphere are hot enough to partially melt the olivine-rich peridotite at this depth. Minor amounts of garnet and pyroxene are also thought to be present in the peridotite. Most estimates suggest that < 1% of the rock actually melts but this tiny amount gives the asthenosphere enough plasticity to flow slowly and form convection cells.

Process: Decompression Melting

As asthenosphere peridotites rise on the convection currents their melting temperature fall due to the decreased pressure. Decompression melting of these rocks is responsible for the generation of the vast amounts of basaltic magma at mid-ocean ridges. These are often referred to as Mid-Ocean Ridge Basalts (MORBs).

Output Material: Oceanic Crust Plus Mantle Lithosphere

Because the melting temperatures of the garnet and pyroxene are lower than that of olivine, the magma that is generated is richer in Si and Fe than the parent peridotite. The magma collects in the magma chamber beneath the mid-ocean ridge. Mid-ocean ridge basalts form most of the oceanic crust as intrusive gabbros in the bottom part of the chamber. Cracks above the chamber filled with magma making its way to the surface. This area becomes the sheeted basaltic/gabbroic dike complex. The portion of the magma that actually flowed to the surface is erupted as underwater pillow lava flows.

The residual portion of the peridotite from which the magma was derived remains beneath oceanic crust. Because it is depleted in garnet and pyroxene, due to decompression melting, it remains in the solid state as the lower part of the lithosphere. Geologists refer to these rocks as the mantle lithosphere.

Rocks Formed at Subduction Zones

The other area of major magma generation is at subduction zones. Large amounts of water-saturated oceanic crust are carried deep beneath the surface on the top of the down going slab. Hot water circulating through the oceanic lithosphere reacts with the basaltic glass to form amphibole, mica, and clay minerals while peridotites in the mantle lithosphere react to form serpentine and other sheet silicate minerals. All of these minerals have OH- ions incorporated into their chemical formulas. Release of this water at depth appears to stimulate fluid-induced melting to form magmas of various compositions.

Although some sediment is scraped off the top as the slab is subducted in a deep ocean trench, most of the water-laden rocks are carried downward. Some of it is squeezed out due to increasing temperatures and rises into the overlying mantle wedge. Some is involved in the metamorphic reaction that converts the basalt to amphibolite, a metamorphic rock. The amphiboles in this rock allow some water to be carried to even greater depths. At depths > 100 km, the amphibolite undergoes another metamorphic reaction to form eclogite, a rock composed of garnet and pyroxene, neither of which can accommodate OH- ions in its structure. This causes the OH- ions to be released directly into the already partially molten asthenosphere. Because the addition of water lowers the melting temperature of the asthenosphere, new magmas are generated. These rise into the mantle lithosphere and crust of the overriding plate.

Most of these magmas are basaltic in composition although they have a chemistry that is slightly different from MORBs. These melts change composition as they rise through the crust due to fractional crystallization and magma differentiation. Because these magmas are hotter than the melting temperature of quartz, mica minerals, and most feldspars, particularly in continental crust, they can melt a portion of the country rock, increasing the amount of Si and Al in the magma. It is through these processes that andesites and some rhyolites are generated.

Mantle Plumes

Mantle plumes are a source of significant intraplate volcanism. Two important mantle plumes are located in the United States. One is centered a little to the southeast of the island of Hawaii beneath oceanic lithosphere. The other is centered under Yellowstone National Park's continental lithosphere. Basalts erupted in these two areas are very similar to MORBs, suggesting they they result from decompression melting rather than fluid-induced melting.

The cause of these pencil-shaped mantle plumes that rise from deep in the mantle to form hot spots on the surface is still under hot debate. Some people think that they don't exist at all! As time goes on, I find myself more and more skeptical. I think there is an easier explanation but I'll warn you that this is pure scientific speculation:

The fairly recent understanding that the extinction of the dinosaurs is related to the impact of a large meteor or asteroid stimulated many new avenues of research. We now understand that impacts of large rocks traveling at tens of thousands of km/hr, although rare, do occur at a much higher frequency than once was thought. Think about this: The oceanic lithosphere is only about 8 km thick; the crust is only a few km thick. A speeding impactor a km or two in diameter could easily excavate a crater that exposed the upper mantle. The pressure relief caused by the excavation could induce massive amounts of melting. This would be even more enhanced, initially, by the the influx of ocean water that would contribute a fluid-induced melting component.

As we continue to discover how important the impacts of extraterrestrial objects are on our planet, I suspect that new sections will need to be added to this and other textbooks. There are still many new and exiting things waiting to be discovered in geology.