EART10 Geologic Principles
Lecture #3
Igneous Rocks and Volcanism


Earth scientists recognize three rock types: igneous, sedimentary and metamorphic. Igneous rocks are rocks that have crystallized from a magma or lava. They are the most abundant rock type in the Earth. Sedimentary rocks form by lithification of minerals and rock pieces. They occur only at or near the Earth's surface. They are very important since they are the source and reservoir of all fossil fuels. Metamorphic rocks form by modification (change of form) of existing rocks of any type. This occurs at high temperatures and pressures, often with the aid of fluids.

Today's lecture is about igneous rocks.

Igneous rocks crystallize from magma or lava. Magma is hot, usually silica rich, melt with suspended minerals and dissolved gasses. Lava (see figure) is melt on the surface of the Earth. Since there are no original melts left in the mantle or crust, magma must involve melting of rock. Where melting occurs is important to the type of igneous rock produced and to its mode of occurrence. Different magmas make different igneous rocks. The composition, and hence, the minerals present in the rocks vary in predictable ways from place to place. So too do the structures associated with crystallization of magma. The massive volcanoes that make up the islands of Hawaii and the massive granitic bodies that outcrop in Yosemite are examples of the varied structures associated with igneous rocks. In this lecture we will discuss the different types of igneous rocks and how we characterize them, how and where melts form, and the different structures that result from magma movement.

Texture

Let's start with the rocks themselves. Igneous rocks are distinguished by two primary characteristics: texture and the mineral assemblage (or composition). Texture refers to the grain of the rock, in particular to the size of crystals within it. Minerals crystallize from melt. Construction of a large crystal requires proper proportions of the components (silicate complex ions and cations), space to grow, and time, since the reactions proceed at a finite pace. Melts that cool rapidly don't have time to grow large crystals. We describe them as aphanitic, meaning "not apparent" since the crystals are too small to be apparent to the naked eye. In the extreme, melts that cool very rapidly form glasses, which are not mineralic since they do not contain crystals with regular, ordered structures. Melts that crystallize (cool) slowly have time to grow large crystals. If we can see crystals with the naked eye, then the texture is described as phaneritic (yes, that's how it's spelled, not phanitic). Texture tells us about the environment of crystallization. Phaneritic rocks are intrusive, meaning they are emplaced in the subsurface and not erupted. This allows them to cool slowly since the surrounding rock does not efficiently conduct heat away. Aphanitic rocks are extrusive, erupted at or very near the surface and not permitted time to grow large crystals. Some rocks show both textures, with a few large crystals within an aphanitic matrix. This texture if referred to as porphyritic and the large crystals are called phenocrysts. This texture is indicative of rocks that have a two-stage history of both slow and rapid cooling. Phenocrysts form during the slow cooling phase, the aphanitic matrix forms during rapid cooling. One last texture, indicative of environments that produce museum-quality crystals, is pegmatitic. Rocks with this texture are pegmatites and contain crystals larger than 2 cm on average. These are obviously intrusive rocks, but their rarity implies something special in their formation (more later).

Igneous Rock Textures

Aphanitic
Extrusive
Rhyolite

Phaneritic
Intrusive
Granite

Porphyritic
Mixed
Rhyolite Porphyry

images copyright EOS University of British Columbia

Mineral Assemblage/Composition

Most magmas are rich in silica, being comprised of >45% SiO2. The other important elements are Na, Ca, Fe, Mg, Al and K. The minerals that form are mostly silicates. Olivine, Pyroxenes, Amphiboles, Micas, Feldspars and Quartz are chief constituents. The relative proportions of these minerals determines igneous rock type. We recognize five types: Granite (Rhyolite), Granodiorite (Dacite), Diorite (Andesite), Gabbro (Basalt) and Peridotite (Not seen). For each type, the first name applies to intrusive rocks (phaneritic texture), the second to extrusive rocks (aphanitic texture). Granites and Rhyolites share the same composition and same minerals, only the crystal size varies between them. Ditto Granodiorites and Dacites, Gabbros and Basalts, etc.

These types are easily enumerated on a single graph. Looking at this we find that Granites are the richest in silica (>66%), composed chiefly of Potassium Feldspar, with Quartz, Muscovite and some minor Plagioclase (rich in Na). Granites are light in color. You find them in Yosemite and the Monterey Headlands.

Granodiorite is the next richest in silica. They are distinguished from Granite by having more Plagioclase than Potassium Feldspar. They have more Biotite and some Amphiboles also. Distinguishing between a Granite and a Granodiorite in hand sample is difficult. It is impossible for the extrusive forms (Rhyolite and Dacite). For our purposes they are the same.

Diorite is the next richest in silica (~50%). They contain lots of Plagioclase and little Potassium Feldspar or Quartz. Amphiboles are common. The composition of Diorite is very close to the average composition of continental crust. If continental crust was melted down and stirred, the magma would be equivalent to a Diorite.

Gabbro comes next. Gabbros have more dark minerals (pyroxene, olivine and amphibole) than light minerals (Feldspar). Thus they are darker. Oceanic crust is largely Gabbro with Basalt at the surface where cooling was rapid.

Peridotite is mostly Olivine with some Pyroxene and a little Plagioclase. Extrusive rocks of Peridotite composition do not occur, hence there is no name for them. Call them Justinites if you wish. Peridotite is the main rock in the mantle. In other words, the mantle has the same composition as Peridotite.

Intrusive Igneous Rocks

Granite

Granodiorite

Diorite

Gabbro

Peridotite

images copyright EOS University of British Columbia

Why and how do rocks melt?

To produce an igneous rock we need to melt rock to produce magma. To completely melt a rock often requires temperatures well in excess of 1000 degrees Celsius. We know that temperature within the Earth increases with depth and that temperatures of 1000 degrees can be found at 100 km depth. Yet the mantle is not molten. Hmmm. Obviously there is something more involved in the production of melt than just temperature.

There are four important concepts in melting. They are: partial melting, the relation of pressure to melting, the role of water in melting, and the density of melt relative to the solid. Let's examine each of these.

  • Partial melt: Unlike ice, rocks do not melt completely at one temperature. This is because they are a multi-component system whereas ice is single component (water). A reasonable analog is a chocolate cookie. The chips melt before the cookie dough, thus heating a cookie you could melt the chocolate, but not melt the entire cookie. The composition of the melt (chocolate) differs from the remainder (dough). In a rock, certain minerals and certain crystals melt before others. Like a cookie, the composition of the melt is different than the solid residue and different from the sum until melting is completed. This means that the partial melt of a particular rock will not produce that rock upon crystallization. For instance, partial melt of a Diorite does not crystallize to form a Diorite. In fact, it will form a Granite. In general, the lower the melt percentage (percentage of the rock that melts), the more different the melt is from the solid.

  • Pressure and melting: As pressure increases (as depth increases), melting temperature of rocks or components of rocks increases. For instance, a rock that melts entirely at 1000 degrees at the surface may require a temperature of 1300 degrees to melt at 100 km depth. This explains why the mantle is solid even though temperatures are very high: the melting temperature of the rock increases faster than the geotherm (temperature in the Earth).

  • Water and melting: The presence of free water lowers the melting point (solidus-the temperature at which liquid first appears). The effect is similar to salt on ice. Ice alone is a single component system with a melting temperature (solidus and liquidus, the temperature at which only liquid is present) of 0 degrees C. The addition of salt, NaCl (Halite), results in a multi-component system with a lower melting point. In the Earth, the presence of water during melting can greatly affect the composition of the melt. This is not true of salt on ice.

  • Melt is less dense than solid: Melts are buoyant and will ascend if possible. Melts produced at depth migrate upwards, often a long way (50 km or more!).
  • Putting these four concepts together, we can explain the occurrence of melt. We find from global studies that melts occur in three settings: mid-ocean ridges, subduction zones and hotspots. The cause of melting and composition of the melt produced varies between the three.

    1. Mid-ocean ridges: By far and away the most productive source of magma, generation of melt at mid-ocean ridges is entirely responsible for production of oceanic crust (which is Gabbro and Basalt). At ridges, the crust and upper mantle is pulled apart (rifted), allowing hot mantle rock to ascend. As the rock ascends, the pressure acting on it decreases and so does its melting point (solidus). If it ascends faster than heat can conduct out of it, it will melt. This means of melting, known as pressure-release melting, usually results in a 1 to 10% partial melt, implying that a large amount of rock must be affected to produce the oceanic crust. The upper mantle is Peridotite. When this is partially melted, it produces a melt of gabbroic composition.

    2. Subduction zones: Responsible for the ring of fire, melt production above subduction zones seems, at first glance, counterintuitive. Remember that at subduction zones one plate is diving beneath another. The descending plate is cooler than the mantle it sinks through, so why is there melt? The reason is water. Water is released from hydrated minerals, chiefly the Amphiboles, Clays, and Micas. This is because water in these minerals is only stable at low pressures, not the higher pressures of the mantle. The release of water lowers the solidus temperature of the mantle above the subducting plate (or slab). As a result, partial melting takes place. As mentioned earlier, the presence of water during melting can affect the composition of the melt. Whereas gabbroic melts are produced by the partial melting of peridotite at mid-ocean ridges, gabbroic to granitic magmas can be produced in subduction zones. The magma ascends through the mantle and crust. During the ascent, the magma interacts with the surrounding rocks. When magma ascends through oceanic crust, the magmas that emerge are gabbroic to dacitic (basaltic to andesitic). When they ascend through continental crust, they emerge as dacites, granodiorites and granites (andesites to rhyolites). By far the most common rock produced is dacite/andesite. In fact, the name andesite comes from the Andes, an example of subduction zone volcanism on continental crust.

    3. Hotspots/Mantle plumes: The last place we find melts is in hot spots. Hot spots got their name because they are either large igneous rocks provinces or linear chains of volcanoes that seem to arise from a single point. Examples of hot spots are: the Hawaiian island chain, the Snake River plain and Iceland. The presence of melt here is exceptional since there need not be anything anomalous happening in the crust and upper mantle. In this setting, magma ascend from very deep in the Earth's mantle, probably from the boundary between the core and the base of the mantle. Melting can occur here because of the very high temperatures within the core and the resultant flow of heat out into the mantle. The magmas produced are basaltic and are more or less identical to mid-ocean ridge basalts.

    Magmatic Differentiation/Crystallization of Magma

    Rocks do not melt at one temperature, nor do they crystallize at one temperature. There is a well defined sequence by which minerals crystallize from a cooling melt. This was studied by Bowen early in this century in an attempt to understand the origin of different igneous rocks. The sequence is now known as Bowen's Reaction Series. It actually is two series, one called the "continuous" series, the other called "discontinuous." The continuous series applies to the Plagioclase feldspars. With decreasing temperature the series moves from Ca-rich (Anorthite) to Na-rich (Albite). The discontinuous series applies to the other major igneous rock-forming minerals. With decreasing temperature, the sequence goes: Olivine (isolated tetrahedra) to Pyroxene (chains) to Amphibole (double chains) to Biotite (sheets). The two sequence merge, with Potassium Feldspar, Muscovite and Quartz crystallization with continued cooling.

    Okay, so what? So some minerals crystallize at higher temperatures than others. Why should we care? Let's think about this. Olivine and Ca-rich Plagioclase Feldspars are the first minerals to crystallize. Olivine contains Mg, Fe and silica. Anorthite contains Ca, Al and silica. These elements are found in abundance in almost every magma, allowing these minerals to crystallize. But say the magma is granitic; granites don't have Olivine or Plagioclase Feldspar in them in any abundance. So what happens to these crystals when they form? Good question.

    The answer lies in equilibrium between crystals and melt. Although the temperature of the magma may be low enough to allow Olivine to crystallize, it is not in equilibrium with a granitic magma which is much more silica rich and lower in Mg and Fe. Because of this, Olivine reacts with the melt to produce Pyroxene. In the continuous series, the Ca-rich crystals react with the melt, giving up Ca and taking in Na, which would be more abundant in a granitic melt. Through this process, the crystals maintain equilibrium with the melt, producing the right mineral assemblage for the rock type.

    But what if the crystals don't stay in equilibrium, then what?

    Fractional crystallization: If crystals settle out from the melt (remember they are solid and denser) or the melt rises quickly leaving the crystals behind, then the two (crystals and melt) can't stay in equilibrium. This causes the melt composition to evolve. Since the first minerals to crystallize are mafic (Olivine and Pyroxene), the melt becomes more felsic (richer in silica) and the rocks it will produce move from Gabbro toward Granite. That this happens can be seen in some exposed magma chambers where bands of distinct minerals occur in layers at the base of a magma chamber. Pegmatites, igneous rocks with very large crystals, form from melts that have evolved significantly toward the granitic end. The melt is very rich in silica and elements that don't fit easily into minerals. As a result, pegmatites are important sources of some rare elements such as Be, Li, and U.

    Forms of Magmatic Intrusion

    Magma must first be emplaced in the crust before lava can erupt. Often, the magma never becomes lava, that is it never reaches the surface. Even when it does, it is usual for most of the magma to remain below the surface. As it cools, it forms a number of different types of intrusive structures. These structures are called plutons (after Pluto). The most common are dikes and sills. Dikes are near vertical fissures filled with magma (solidified), frequently opened up as cracks to accommodate buoyant magma. Sills are near horizontal layers filled with magma, often following the original layering of the rock that magma has intruded. Laccoliths are similar to sills, but the roof is upraised, producing mounds. The most spectacular plutons are batholiths: huge bodies of igneous rock of irregular shape, usually constructed of more than one igneous intrusion (or diapir, literally an ascending magma blob). Batholiths are associated with subduction zones and are usually granitic. They are thought to involve much recycled continental crust, meaning that the melt produced above the subducting plate melts a large volume of the continental crust that it ascends through. This produces a more felsic, silica rich magma since continental crust is much more felsic and silica rich than the mantle. Batholiths can be huge. The Sierra Nevada batholith and Southern California batholith run pretty much the length of California. Larger still is the Coast Range batholith in British Columbia which extends from the international border into Alaska.

    Eruption of Magma

    The eruption of magma (lava) can be explosive or non-explosive in nature. Although non- explosive eruptions are more common, explosive eruptions are more interesting. Explosive eruptions are caused by high gas pressures. Remember that magma is a mix of melt, suspended crystals and dissolved gasses. As magma ascends and pressure decreases, gasses come out of solution, similar to a soda bottle when you open it. Explosions happen when the gas can't escape quickly enough as it is released from solution. This happens when the magma is rich in water and carbon dioxide (the dominant gases) and viscous, such that it does not flow readily. These properties (lots of water and thick) are characteristic of Granitic/Rhyolitic melts. Rhyolitic melt is very viscous, meaning it flows only with difficulty, because of the high silica content. Silica in the melt polymerizes as it does it crystals, albeit not in a regular, ordered array. This makes the melt less runny. Since the production of Rhyolitic melts usually involves the presence of water or recycling of water-rich continental crust, the magma has high gas content (steam).

    Explosions produce pyroclasts or tephra. Pyroclasts are hot, broken fragments of rock. Tephra is essentially synonymous, ranging from volcanic ash (very fine, < 2mm), to lapilli (< 64mm), up to volcanic bombs (>64mm). These are ejected during explosions and can be pre- existing rock shattered and thrown by the explosion, or rock produced by cooling of exploded lava. Eruption columns carry hot gas and tephra (ash) upwards, sometimes to altitudes of 45 km (stratosphere) or greater. Ash in the upper atmosphere blocks short- wave radiation from the Sun, resulting in global climatic cooling. Ash clouds can drift great distances, covering the globe in a matter of days.

    Eruption columns can collapse if the source of hot gas turns off or if the air around the column gets too hot, such that hot material within the column is no longer buoyant. Tephra in the column then rains down rapidly, often in the form of a fast moving wall of ash and poisonous gases. Lapilli and bombs accumulate around the base of the column, building up a volcanic edifice.

    Non-explosive eruptions are more common. This is because most volcanism is basaltic (mid-ocean ridge and hot spot) and basaltic lava is not very viscous, being silica poor. Because it flows readily, it can flow long distances down subtle slopes, producing lava rivers. When the source of magma is more voluminous, it can poor out in giant flood basalts, massive eruptions that can cover hundreds of square kilometers. These outpourings dramatically affect landscape. They are also believed capable of affecting global climate. Flood basalts may represent the first stage, that is the initiation, of hot spot volcanism. The massive outpouring of lava also delivers huge amounts of gas, gas which can alter the atmospheric composition enough to affect climate. The Deccan Traps in India erupted about the time that Dinosaurs went extinct. Although we now attribute that event to a large meteorite collision, the outpouring of gas associated with the Traps may have contributed.

    Volcanoes

    Volcanoes come in two primary varieties: shield volcanoes and stratovolcanoes (or composite volcanoes). Shield volcanoes are built of successive lava flows and limited tephra. They are usually basaltic. Since basalt flows easily, the slope are gentle, steepening on the edges where lava is cooler and more viscous. A prominent example is Mauna Kea on the big island of Hawaii. In fact, each of the Hawaiian isles is a shield volcano (or combination of several). We see only a small portion of the entire volcanic edifice. Most is beneath water.

    Stratovolcanoes form from more viscous (rhyolitic and andesitic) lavas. They are built up of tephra, thrown out onto steep sloped cones. Tephra dominates because eruptions are usually explosive, emitting little flowing lava. Long-lived stratovolcanoes will eventually emit flowing lava, which inter beds with tephra, producing a durable, steeply sloping, volcanic edifice. The combination of tephra and lava flows is why these volcanoes are also referred to as composite. They can grow very tall. Mt Rainier and Mt St. Helens are well known examples.

    Features of volcanoes: Volcanoes are topped by craters, funnel shaped depressions from which tephra, gasses and lava are emitted. Collapse of the crater following large eruptions results in calderas. Crater lake in the Cascades is a ill-name example of a caldera. Calderas are frequently water filled. Within them, one may find small conical lava domes produced by ascent of very viscous lava remaining after the main eruption. This lava is cooler, low in gas content, and moves very slowly, forming very steep cones.

    Underwater Eruptions

    Because the majority of volcanism occurs at mid-ocean ridges, most volcanism takes underwater. Mid-ocean ridges produce basaltic lavas, so the eruptions are seldom explosive. Unlike subaerial eruption, underwater eruptions are cooled so quickly that the basaltic lavas cannot flow far. The result are pillow-shaped blobs of basalt known as pillow basalts. Most of the ocean floor, beneath the layer of accumulated sediment, is pillow basalt. If you were to drill down into the floor, you would find the pillow basalts give way to more massive flows, riddled with dikes, and with increasingly gabbroic texture as cooling rates decrease.






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    Document Last Modified Monday, November 17 1997 09:01