GEOLOGY/GEOPHYSICS 101 Program 18

INTRUSIVE IGNEOUS ROCKS

Hi. Welcome back to Geology 101. Today we're studying Lesson 14, "Intrusive Igneous Rocks".

Hope you're doing well in the course. Well, let's take a look at igneous rocks.

You know, it wasn't always recognized that igneous rocks cooled underground; in fact, James Hutton was the first to recognize the igneous nature of granite. He did this from cross cutting relationships with the sedimentary rocks that the rocks had cut across, and, in fact, Hutton had a long standing disagreement with another geologist named Murchison, who thought that the granites formed by precipitating as salts out of sea water as the ocean evaporated, but today we recognize that most of Earth's crust originated from magma that cooled below ground; in fact, 95 percent of the Earth's crust is igneous in origin, and it's related, of course, to mountain building processes, and we recognize that the continental crust is, in fact, covered by a thin veneer of sediments and soil.

We'll take a look at those sediments and soil in future programs.

Well, since Hutton's time we have learned much about the formation of magmas from studying the minerals in them, but we can't observe the cooling of the magma directly, but we know that large bodies of rock like the Sierra Nevadas represent huge bodies of molten rock which cooled underground. Although we can't study the rocks as they form, we can study the minerals in these rocks.

You see, the minerals represent the conditions in the rock at the time the rock cooled. The minerals remember the conditions, so the composition and the texture of the mineral grains gives us clues to the origin of the rock. On the other side of the coin, laboratory studies of cooling melts also help us to interpret the clues that we find left behind in those minerals, so in this lesson we'll

Well, I'm not going to go through the learning objectives with you this time. Those are there in the study guide, and by this time you should know where to find them, and you should know how to look at the learning objectives. Don't forget that they're there. You still have to study them.

Well, there are many possibilities for combinations of atoms to form minerals, but luckily for us, the main rock forming minerals are silicates and are made from the eight most abundant elements in the Earth's crust, and there are only eight common rock forming silicates. You should be familiar with these. Most of these eight common silicate minerals are really mineral series or solid solutions of one form or another, so I'll review these very quickly with you.

Basically there are two categories of silicate minerals that we need to worry about. One of these are called the ferromagnesian minerals. "Ferro" means iron; "magnesian" means magnesium, so the ferromagnesian are minerals which contain iron and magnesium, but they also may contain the other metallic ions as well.

I'll run through the list of these with you.

Okay, the non-ferromagnesian minerals are those which do not contain iron and magnesium, a nice, simple contradiction there.

It's worthy of note here that there are also some non-silicate rock forming minerals, such as calcite, halite, and gypsum, but these are common in sedimentary rocks, but not in igneous rocks, so we'll defer talking about them until we get into the topic of sedimentary rocks.

Okay, let's turn our attention now to the classification of igneous rocks. Igneous rocks, like all kinds of rocks, are classified based upon both composition and texture. Since the igneous rocks are composed mainly of these silicate minerals, we want to kind of get a sense of how the rocks are classified based upon the presence of these various kinds of minerals. You can refer to Table 10.1 on page 222 and also Figure 10.10 on page 221 in the textbook to get a sense of this classification.

Well, it turns out that there are actually hundreds of different types of igneous rocks recognized, but only a few main categories for our purposes, so we classify rocks on mineral composition, which reflects the chemical composition, and basically we're looking at the presence or absence of quartz and the type of dominant feldspar. So with this category in mind, we can look at three basic categories of light, intermediate, and dark, but the rocks are also classified by the sizes of the mineral grains, which we refer to as "texture".

The size of the mineral grains reflects the rate at which the rocks cool, so there are basically two kinds of textures. There are coarse grain textures, also called "phaneritic", and there are fine grain textures, which are also called "aphanitic".

Okay, of course, if crystals don't have time to form at all, the rocks may be glassy, so by "coarse" we mean visible with the naked eye, and this, of course, reflects a slow cooling rate.

By "fine" we mean that the grains are too small to see individually without a microscope, and this reflects a rapid cooling rate.

There's another texture called "porphyritic", which refers to two distinct sizes of crystals within the rock. The large crystals are called "phenocrysts"; the smaller ground mass is called a "matrix", and a porphyritic rock represents a cooling in steps as we'll see later on, so within this category between fine and coarse grain, the mineral composition is pretty much the same, so let's look at a simplified classification as I talk about the rocks and show you some examples of them.

Okay, in the light category we have the rock called "granite", granite is phaneritic texture, and you can see here, you can make out the individual coarse grains. Granite is a rock composed almost entirely of microcline or orthoclase feldspar and quartz, but it has a small amount of ferromagnesian minerals in it, which you see in the picture here as the dark spots. People refer to granite sometimes as "salt and pepper" texture. The salty part is orthoclase and quartz; the pepper part is the ferromagnesian minerals; in this case, it's biotite mica.

The fine grained equivalent of granite is rhyolite or the volcanic development. Here you see it's light in color. You don't see so much of the dark spots, but still it's much lighter in color than some of the rocks that we see here in Hawaii.

Okay, in the intermediate side, the coarse grain variety is called "diorite", which I don't have an example of because it looks so much like granite that you really can't tell the difference easily, but the fine grained version is called andesite. Andesite, as you can see, is darker in color. It's erupted from andesite volcanoes. In andesite, the main minerals are plagioclase feldspar with virtually no quartz with up to 50 percent of ferromagnesian minerals, 30 to 50 percent, I think, the textbook will tell you if you look at it.

Okay, on the darker side of things, we have the fine grained version is called "basalt". This is a particularly dark piece of basalt. It also has some glassy pieces in it, so the flashes of light you see are of the glass, but you can see that the basalt itself is much darker in color than the andesite. Basalt, of course, is very common in the Hawaiian-type volcanoes.

The coarse grained version of the dark rocks is called "gabbro". Here you see gabbro. This particular gabbro has a high percentage of plagioclase felzbar, but there's also quite a bit of pyroxene in here as well, so these dark rocks, which are also called "mafic" rocks have usually about half of the mineral composition is ferromagnesian or more, and the rest of it is plagioclase feldspar and, once again, almost no quartz.

Okay, there's one other type of rock called "altramafic". "Altramafic" means more than mafic or darker than mafic. The altramafic rocks consist of almost entirely ferromagnesian minerals. In this case, this particular piece consists almost entirely of olivine, which is the green color you see, and I don't believe you can see the little tiny flecks of pyroxene in this sample, but it contains quite a bit of little tiny peppery flakes of pyroxene as well. There's no fine grained or phaneritic equivalent to the altramafic rocks because they're melting temperature is too high; it's about 2,000 degrees Celsius, so they probably form at great depth, probably in the mantle, and when we find pieces of altramafic rock, they either have risen as a solid mass or, in this case, were present as xenoliths in lava. We talked about xenoliths, I think, in the last program.

Okay, let's turn our attention now to the Bowen Reaction Series. The Bowen Reaction Series is a chemical model that helps us to understand the origin and variety of igneous rocks that we find spread around the Earth.

There are several questions. For example,

Questions like these prompted Bowen to undertake experiments in the laboratory in the early part of the Twentieth Century. Bowen studied feldspars. What he did was to take the right proportions of silica, aluminum, calcium, and sodium, and grind them up into a powder, and put them into a crucible and melt them in an oven. After the mixtures were melted, he let them cool slowly to a certain temperature, then cracked the whole thing open and saw what was inside. He studied the resulting solids. He did this for several different temperatures. What he found was fairly interesting. He found that the first feldspars that formed were richer in calcium than the liquid from which they were derived; in other words, the crystals were extracting calcium preferentially over the sodium. He found that as the temperature lowered that the feldspars formed a continuous series of plagioclase feldspars, which represented various degrees of solid solution.

What exactly does that mean?

That means that the feldspars became increasingly rich in sodium as the temperature fell, and this happened over the entire range of cooling from about 1100 degrees down to about 800 degrees Celsius. He also found that once the calcium feldspar began to form, that as the temperature lowered, the calcium crystals that had already formed began to react with the magma, so that sodium replaced the calcium, and he found that if he let the whole thing cool slowly and uniformly that the resulting crystals wound up having exactly the same composition as the original melt.

What does all this mean?

Well, Bowen theorized that if you can remove somehow the early formed crystals from the magma that the resulting magma can change in composition. It's the change of the composition of the magma that we're concerned with here. Further studies by Bowen and others since then found that the ferromagnesian minerals do exactly the same thing; that is, they form a series, an order of crystallization.

The difference is that the ferromagnesian minerals form a discontinuous series.

"Discontinuous" means that there are several separate minerals involved because there are a wider variety of ferromagnesian minerals that there are nonferromagnesian, so on the ferromagnesian side of things, the first mineral to form is olivine, and olivine, you may remember, is the simplest of all the ferromagnesian minerals with a single tetrahedra. The olivine forms a solid solution similar to feldspar only it does it over a narrower temperature range; in fact, the first olivines to crystallize are richer in magnesium than the later olivines to crystallize, but the point of this is that when the olivine forms, the liquid magma that results is depleted in iron and magnesium but enriched in silica.

You're taking out some of the iron, and magnesium, and some of the silica but in different proportions than in the original magma because basically to form olivine you have to use only one part of silica for each two parts of iron and magnesium, so the gist of this is that at a particular temperature, as the temperature falls, olivine begins to dissolve as the pyroxenes begin to crystallize; in other words,

Orthoclase, or microcline, muscovite mica, and quartz form from the remaining liquid, and it's no coincidence here that this is the composition of a typical granite: the orthoclase, feldspar, and the quartz.

You'll note from looking at the Bowen Reaction series that basaltic rocks consist of minerals near the top of the series. Andesitic minerals are near the center, and granitic materials are near the bottom, so can we use the Bowen Reaction Series now to understand some of these questions that I asked at the beginning of this section.

For example, the oscillatory zoning of the feldspar crystals. Try to imagine, if you will, inside a magma chamber where a feldspar crystal begins to form. Now, within the magma chamber, the temperature may range from relatively warm to relatively cool. Convection currents in the magma chamber may circulate crystals as they begin to form, so if a feldspar crystal forms in the center of the magma chamber where it's hot, it forms calcium rich feldspar.

When that crystal, then, gets carried into a cooler region, the calcium begins to react with the sodium in the magma, and the feldspar that crystallizes on the outside of the crystal, as the crystal grows from the center, is now richer in sodium, but then if that crystal gets carried back into the hotter part of the magma, the calcium gets deposited on the newly born crystal, so that we find this oscillatory zoning representing layers of calcium rich versus sodium rich versus calcium rich and so on as the crystal grows.

We can also understand how phenocrysts form if we think about this. If the magma is cooled to a certain temperature where a particular type of crystal, such as olivine or plagioclase begins to form and held at that temperature, then no further cooling can take place, so no further crystal growth takes place, so if the magma is erupted, then, at that temperature, it consists of the already grown crystals as phenocrysts, and the rest of the magma, now erupted as lava cools at the surface to form fine grain, so we have the two distinct grain sizes. I might also note here that resorbed olivine crystals are very common in Hawaii in basalts; "resorbed" meaning when the olivine crystals have actually reacted with the magma, and we sometimes find little pieces of pyroxene connected to the olivine crystals as if the pyroxene was replacing it, so what we've got here is this process called "differentiation", and in the beginning of the course we noted this process of differentiation of the Earth into different layers. The process of changing the composition of molten magma by removing crystals is called "differentiation", sort of like freezIng salt water or koolaid.

You can try this at home. You freeze some koolaid into your ice cube tray. You'll find that as the ice freezes, it becomes increasingly sweeter and more concentrated in kool aid near the center, so these processes do occur in the nature, but nowhere near to the extent envisioned by Bowen; in fact, there's far too much granite on the surface of the Earth for all of it to have been created from the amount of basalt in Earth's crust by this process alone, but there are other processes.

If crystals settle out of the magma, the early formed crystals are more dense, so they can settle to the bottom leaving the remaining liquid changed in composition permanently, and the accumulation of making minerals is observed near the bottom of many large bodies.

We know that Hawaiian volcanoes change composition as they age as well. That can be due to the fact that once the magma is cut off from its source of fresh magma, the magma chamber, differentiation occurs within the magma chamber causing the magma to change in composition.

There's also the possibility of partial melting. "Partial melting" is just the reverse of fractional crystallization or this process of differentiation; in fact, if you take mantle material and melt it, the first liquid contains a greater fraction of ferromagnesium minerals and less silica than the original solid. Since the liquid is less dense than the solid, it can move upwards leaving the mantle depleted in silica and enriched in ferromagnesian minerals, exactly the type of differentiation process we envisioned occurring as the Earth separates.

We can also form a granitic liquid from an altramafic melt like the mantle, but it takes about 100 parts of altramafic rock to form one part of granite, but the first liquid to form from the melting mantle rocks has a composition that's nearly identical to the Hawaiian basalts.

Okay, so it seems that either fractional crystallization or partial melting can cause differentiation of the Earth's atoms, so the "Earth Revealed" video elaborates on this model when discussing the origin of igneous rocks of various types so pay attention and look for these, and we'll return to summarize the suspected origins of the three major types of igneous rocks after the video, so let's watch the video.

Major Funding for "Earth Revealed" was provided by the Annenberg C.P.B. Project.

A volcanic eruption is one of the most awe inspiring sights in all of nature, but whether an eruption is sudden and explosive, such as Mt. St. Helens or more subdued like the eruptions of Kilauea in Hawaii, volcanic activity is not an unusual event.

Volcanic eruptions are the pulse of geologic activity in the Earth's interior. They graphically demonstrate the geologic processes inside the Earth can have a direct impact here on the surface. The lava that's produced during a volcanic eruption is, of course, rock that melted somewhere beneath the volcano.

Rock melts in a variety of geologic settings in the crust and upper mantle of the Earth depending on the temperature and pressure conditions and on the composition and water content of the rock, but lava represents only a tiny proportion of the magma that forms within the Earth. Most of this magma either seeps into voids and fractures within rocks of the Earth's crust, or it rises and intrudes into the cooler rocks above as enormous globs that require thousands or even millions of years to cool; in fact, most of the rock of this planet was formed from the slow cooling and crystallization of magma deep underground.

This is known as intrusive igneous rock. The study of igneous rocks began in the 1780s when Scottsman James Hutton became embroiled in a controversy with another influential geologist, Abraham Vernor.

Vernor believed that the Earth was once covered entirely by a great ocean from which all rocks formed starting with granite. Hutton disagreed. In exploring the highlands of Scotland, Hutton observed veins of granitic rock slicing across sedimentary strata. He reasoned that the granite must have been injected into the strata as a molten liquid and not been precipitated from a primordial sea. He made a link between granites formed at depth and the quickly cooling deposits of erupting volcanoes. Both are products of molten liquid or magma from Earth's interior.

Thanks to Hutton, geologists recognized a new class of rocks called "igneous", literally fire formed rocks.

Some of the original heat that results in the formation of magma comes from the original formation of the Earth. As the materials came together to form the Earth, they produced an enormous amount of compression. Now, that was some four and a half billion years ago. Some of that heat still remains in the interior of the Earth. Another source is from radioactive isotope. Potassium, thorium, and uranium have all contributed as a result of radioactive decay to heating of the Earth, particularly in the earlier history of the Earth when these isotopes were more abundant.

Now, there's another contribution that comes from the tidal effects of the sun and the moon. As the result of these tides, the Earth is being constantly being squeezed and flexed, and that tends to build up some heat in the Earth's interior as well.

The ascent of magma is a process called "intrusion". If the magma cools underground, it forms an intrusive, sometimes called, plutonic igneous rock.

If it erupts at the surface, the igneous rock is called "extrusive" or volcanic.

James Hutton studied granite because it is among the common plutonic rocks of Scotland, but worldwide there are, in fact, many types of plutonic rocks. Those that contain abundant iron and magnesium are called "mafic" rocks by geologists. Gabbro is an example. Felsic rocks, such as granite, contain abundant silica and aluminum. Finally, many igneous rocks are essentially mixtures of mafic and felsic compositions.

These are known as the "intermediate igneous rocks". Diorite is one of the most abundant examples. Each of these plutonic rocks has a compositionally identical counterpart in volcanic rocks, but although they may be made of exactly the same material, their textures are very different. This can only be due to the fact that each cools and hardens under different conditions. Laboratory studies indicate that if the molten rock cools too quickly, crystals are unable to form since the individual atoms in the melt don't have enough time to build crystal lattices.

If the molten rock cools slowly, however, crystals will ultimately form throughout the magma, and the more slowly the magma cools, the larger the crystal will grow. This dike formed when a hot andesitic magma welled up into a fracture in this gneiss. As the magma cooled, it transferred heat into the surrounding gneiss. Heat transfer was, of course, more rapid here at the contact resulting in the gneiss being cooked by the magma heat. Because crystal size in igneous rocks is a good indicator of relative cooling rate, we can also see that the magma itself cooled more quickly at the contact. The smaller crystals here at the contact tell us that the magma cooled more quickly there than here at the center where the crystals are somewhat larger. Based upon texture, geologists infer that granites crystallize from magma cooling slowly deep underground.

In contrast, rhyolites form from magma that cools slowly at first allowing a small amount of the larger crystals called phenocrysts to develop. Then it cools quickly as the result of an eruption stopping crystal growth altogether.

Textures that result in the crystallization of those minerals tells us about the conditions under which that rock formed. Some rocks have very large crystals that are easily visible to the naked eye, and we call that a phaneritic texture. Your everyday granite has a phaneritic texture, and that shows that rock had to crystallize at some depth several kilometers down in the Earth's crust. That rock represented a different time, and it's exposed now at the surface only due to later tectonic activity.

Other igneous rocks crystallize near the surface. Volcanic rocks, for example, record a very rapid crystallization of the same kind of magma, but as the crystallization becomes more rapid, the crystals are forced to be smaller and smaller becoming eventually aphanitic or invisible to the eye, or even faster the minerals don't have time to grow, and the result is volcanic glass, which we call obsidian, so we're learning to read textures from the rock, and we learn the conditions under which it forms.

By studying the crystal textures of igneous rocks, geologists have discovered that not all minerals form from a magma at the same time and temperature. Rather magma crystallizes one or two mineral types at a time. The early forming crystals, those that could withstand higher temperatures, such as plagioclase or olivine preserve symmetrical shapes that could only have resulted from the crystal formation taking place in a liquid. Later forming crystals, such as potassium feldspar and quartz exhibit irregular shapes indicating that they crystallized last from small compartment of liquid confined in the spaces remaining between the early forming crystals.

Lab work by Norman Bowen and other experimentalists has given us precise information about how magma crystallizes. Bowen called the sequence of mineral crystallization in magma a "reaction series". This is because as magma cools early forming minerals may react with the magma to form new minerals at lower temperatures. Some minerals react continuously with the magma, and so are constantly undergoing change. Others react discontinuously; that is, only at certain specific temperatures.

The first minerals to crystallize are the calcium plagioclases, which would crystallize up about 1,100, a little more than 1,100 degrees Celsius. Along with the calcic plagioclases we expect to find crystallization of minerals like olivine, but as the temperature falls, different minerals begin to crystallize; in fact, some of the earlier minerals begin to convert into some of the minerals that crystallize at lower temperatures, and in discontinuous series, we see olivine being dominated then and overtaken by pyroxene, the pyroxene being changed into hornblende, hornblende into biotite, and there's a conversion, usually just a pair of minerals as we go to lower and lower temperatures.

The low temperature minerals in Bowen's Reaction Series consists also of plagioclases. The continuous series changes from the calcium rich plagioclase to the sodium rich plagioclases. As we get down to 800, 700 degrees Celsius, we have more sodic rich plagioclases. We have minerals like K feldspars, biotite, muscovite, and quartz crystallizes. These are the more felsic constituents of Bowen's Reaction Series.

The actual temperatures at which magma forms or as crystallizes are influenced greatly by its water content. In general, the greater the water content, the lower the temperatures. By showing that magma crystallizes in a piecemeal fashion, Bowen provided an explanation for why plutonic rocks contain so many kinds of crystals, and volcanoes, so few, but Bowen's work has other important implications. It suggests that different kinds of igneous rocks can form from a single parent magma.

Bowen developed this series in response to the idea of what was the origin, the question what is the origin of granite. He felt that all magmas originally started out as basaltic material, but through a process of differentiation, the magma evolved and changed. The early crystallizing materials, the materials that were crystallized at higher temperature would tend to settle out or become separated in some way from the magma. This process is called "differentiation", and it demonstrates how a magma might evolve, so that you can change from what was originally a "mafic" magma into a "felsic" magma consisting of the lower melting minerals.

Geologists find evidence of differentiation preserved in many plutonic rocks; for example, some intrusions are layered due to settling out of early forming mafic minerals. As these minerals left the upper part of the magma body, only felsic minerals were left to crystallize there. At volcanoes, the composition of lava and ash may change in time reflecting differentiation in the magma underneath.

The effect of cooling rate, water content, and magma compositions on the resulting textures of intrusive igneous rocks can be investigated in a more complete and quantitative way using laboratory experiments, but a lab alone cannot recreate the geologic processes that cause a magma to form. These processes operate deep within the Earth's interior, and they're usually tied to tectonic processes and the movement of plates.

Igneous activity is directly related to plate tectonics. Ocean ridges, zones of spreading in the Earth's oceans is due to the up welling of molten magma derived from partial melting in the mantle. In melting mantle, the result is a magma that will crystallize to form basalt, and that's why basalt is the dominant rock type of ocean ridges, of ocean islands, and ocean crust. It's subduction zones at ocean crust made of basalt again that's subducted, taken back down into the Earth's mantle, and it's heated. It then is partially melted, and magma rising from that partial melt rises to form the great volcanoes above the rim of fire. Particular types of magma are associated with specific types of plate boundaries.

Plate Tectonic Theory explains why we have magmas on the sea floor, which are very different from magmas that form on continents. The magmas on the sea floor are enriched in iron and magnesium tending to be on the basaltic side of the scale, higher temperature minerals in accordance with the Bowen's reaction series. In subduction zones, relatively cool, wet rocks on the sea floor and on a sea floor lithosphere are subducted under continents, and because of the presence of so much water, melting begins at a low temperature, thus a partial melting takes place, and we tend to get rocks that are higher in silica associated with subduction zones and andesite volcanoes in South America rather than the basaltic type rocks in the mid-oceanic ridges.

At convergent plate boundaries, andesite and its plutonic equivalent diorite are among the most common igneous rocks, but geologists are uncertain about how their parent magma forms. We think that much of the magma developed as a result of the melting of basalt from the upper part of the descending lithospheric plate. There may also be components that are derived from the marine sediments that may also be carried down. These marine sediments may have interstitial water that can also contribute to and become dissolved in the magma.

There may be substantial amounts of serpentine, they're being brought down with the basalt. These high grade minerals and could also contribute water. Now, the presence of water is significant because it helps to bring about a fluxing of the remaining amount of materials. It lowers the melting point, and it helps in the formation of magma.

Now, as this magma moves up toward the surface, is it basaltic in composition? Is it more felsic? We aren't completely certain of the nature of the magma. As this magma moves up toward the surface, it encounters the base of the Earth's crust mainly made up of felsic constituents. These are lower melting materials. As we look at Bowen's Reaction Series we see that these minerials from the continental crust are really the low melting material, so they are very likely to become incorporated into the magma. In fact, partial melting of continental crust, mantle, and subducting slabs probably all contribute to the formation of andesite and diorite, but these aren't the only igneous rocks that form at convergent plate boundaries. Many other types also occur.

Partial melting in the mantle triggered by upwelling associated with subduction will form basaltic magma. This magma may collect, cool, and harden at the base of the crust. In some cases, though, the magma may move rapidly up towards the surface, still of basaltic composition, and erupt as a basaltic lava at the Earth's surface. If it's held for some time within the Earth's crust assimilation may take place, and large amounts of the crust may be melted and incorporated into this magma, so that it becomes higher in felsic constituent, and these felsic constituents, then, increase the viscosity, so that the magma doesn't so easily move up to the surface to erupt as a volcano, but instead it lodges itself within the crust of the Earth as a pluton.

Plutons, huge masses of igneous rock, also form when continental crust and sediment are partially melted by the heat of rising mafic magmas. The molten continental material cools to form one of the most characteristic rocks of convergent plate boundaries, granite. Magma rises from the Earth's interior because it is bouyant; that is, less dense than the rock materials surrounding it.

It ascends in several ways. At divergent plate boundaries, magma simply fills up the spaces opened as the crust is pulled part. By contrast, at convergent plate boundaries, rock enclosing a magma body may be pushed aside as the magma passes through it, or the magma may fracture the overlying country rock through a process called "stoping". Bits of fractured country rock sink into the magma chamber where they may be preserved upon cooling in the form of fragments called "xenoliths".

In part, magma may even melt its way through the crust, increasing in volume and changing in composition. Once formed, igneous intrusions are classified according to their geometric shapes and overall size. Small common intrusions possessing sheetlike shapes that cut across the bedding or fabric of preexisting rocks are called "dikes".

Sills are related to dikes, but instead of cutting across strata as dikes do, sills intrude between layers forming sheets parallel to the strata.

A laccolith is a shallow sill-like structure that bulges upward in its central portion, also causing a hump or gentle rise in the overlying crust. the largest intrusions typically having the shape of spheroidal globs are the plutons. These range from masses called "stocks", which crop out over areas tens of square kilometers to gigantic intrusions exposed over hundreds of square kilometers. Geologists call these huge plutons "batholiths".

Plutons often rise in groups, so a batholith can, in fact, be composed of many plutons merged together. Watching a lava lamp can give us a good deal of insight into the way a batholith forms. When rocks melt in the lithosphere of the Earth, the magma begins to rise because it's less dense than the surrounding rock, just as these bouyant blobs rise through the surrounding oil when the base of the lamp is heated. As the bubbles of magma make their way toward the surface of the Earth, they begin to expand and pack together coalescing and cooling into a huge mass of plutonic igneous rock. A composite igneous intrusion like this can be immense encompassing tens, or in the case of a batholith, several hundred individual plutons. When erosion eventually strips away the several kilometers of rock covering the batholith, the massive plutonic rock remains as an elongate mountain range. Plutonic rocks are composed of a tightly intergrown mass of crystal, which is usually more resistant to weathering than the sedimentary and metamorphic rocks which surround the magmatic intrusion. Geologists use batholiths to try to understand the tectonic history of the Earth's crust.

The Sierra Nevada batholith here in Eastern California is an excellent example. The hundreds of individual plutons which make up this batholithic mountain range represent a series of magmatic intrusions that began about 200 million years ago and continued over the next 120 million years. Most of this plutonic rock is chemically very similar to andesite, a volcanic rock that's found forming in continental volcanic arcs, such as the Andes. These arcs exist because oceanic crust is being subducted beneath the edge of a continent. Partial melting of the down going rocks generates the andesitic magma, which rises in bouyant plumes to feed the volcanoes or to become part of the batholith below, so the overall structure and composition of the Sierran plutonic rocks tells us that this is a deeply eroded magmatic arc, which formed along an ancient subduction margin.

The study of ancient igneous rocks has not only provided a wealth of information concerning past tectonic processes, it has revealed much about the evolution of Earth's crust. One ancient type of igneous rock is comateite, a rare class of basalt formed from much higher temperatures than the hottest basalt lavas known today. Comateites suggest the Earth had a much warmer interior. Anorthsite is another kind of rock that is relatively rare on Earth and almost entirely embedded in ancient plutonic belts cutting across the older portions of the continents. Anorthosite may be rare on Earth, but it is the primary type of rock found in the highlands of our nearest neighbor in space, the moon. When the astronauts came back from the moon, they brought back samples of a kind of rock called anorthosite. Apparently it's very common on the surface of the moon, but it tends to be relatively rare here on Earth.

I have an example of anorthosite here. This one comes from the San Gabriel Mountains, and it seems to represent a body of rock that was formed at great depth in the interior of the Earth. It may have started out as a rock of basaltic composition, a magma of basaltic composition, but it was held at great depth for a long period of time, long enough that the crystals had an opportunity to grow very large. I don't know if you can see, but this is made up of extremely large crystals. It was evidently under tremendous pressure at that depth as well, and so this magma has undergone an extreme amount of differentiation; that is to say there was a separation of the crystals from the melt.

Apparently these crystals settled to the bottom of the chamber where they became almost purely made up of this one kind of crystal. In this case, it's a calcium rich plagioclase,and a kind of rock made almost exclusively of this plagioclase is called, as I say, "anorthosite". I have another example here also from the San Gabriel Mountains that shows, gives us an idea of how this differentiation may have proceeded. The crystals grew large. We can almost imagine the magma surrounding them in this area here and the crystals settling down. These are single large crystals. You can tell by their uniform shape and the straight sides, large crystals growing in what was molten rock surrounding it, and these crystals then settled to the bottom of the chamber. This sample is well over a billion years old, and all of the other known anorthosites are very old as well.

The presence of very ancient anorthosite both on Earth and the moon suggests that these worlds may once have been more similar in composition, but Earth's interior, still geographically alive, has allowed our planet to continue evolving. While the moon, now geologically dead, remains the same. Indeed, the formation, ascent and cooling of magma is one of the most significant agents of change in Earth's vast interior.

Throughout geologic time, the great heat generated within the interior of the Earth has caused rocks to melt. Most of the Earth is composed of intrusive igneous rock. Continents are largely masses of granitic rock beneath a thick blanket of soil and sediment.

Oceanic rock is mostly gabbro coated with basalt and mud. Even the rocks of the Earth's mantle were formed from the slow cooling of altramafic magma, and so far evidence from the interior planets of our solar system indicate that they, too, formed in a similar way.

Geologists can't study igneous intrusions directly, but by analyzing the minerals within these rocks and mapping their physical relationships, Earth scientists are gaining an intimate understanding of the evolution of the lithosphere. Research into intrusive igneous processes is now more important and interesting than ever. For example, it's used in determining the location of metallic ores, previous metals and gems, for interpreting the complexities of plate tectonics, and in studying the histories of other planets similar to our own.

Intrusive igneous rocks remind us that a great deal of geologic activity takes place in a vast, unseen arena. There in the Earth's interior, magma continued to form, move about and crystallize, setting the stage for the growth of an ocean basins, the continents, and the mountain ranges in the future.

Well, I really liked this video. It seems to tie all this stuff about minerals, and atoms, and igneous rocks together really well, and we see that the Earth's processes operate according to the same chemical laws that we find in the laboratory.

Now, the chemical solutions that we deal with, the chemicals that we deal with in the Earth are much more complex, and pressure environments, but still in the laboratory we can model the behavior of chemicals and go out and apply this to the real world.

I also want to note the importance of both heat and gravity in the formation of igneous rocks. Heat, of course, is obvious. It's what causes the rocks to melt in the first place, but gravity allows crystals to settle, leaving behind a liquid of a different composition. It also allows the first liquid formed from partial melting to rise, leaving behind a solid of a different composition.

Bowen's model, you see, helps us to understand something we couldn't understand otherwise, and, in fact, it's one of the first instances where laboratory work help us to understand Earth processes.

Okay, let's take a look now at plutons. "Plutons" or intrusive bodies are large masses of igneous rock, which have solidified below ground. There are lots of different ways that you might characterize or classify these igneous intrusive bodies, but the easiest way is simply to look at them as discordant versus concordant.

Discordant intrusions cut across country rock structure. By country rock then we mean the existing rock, not the book's kind of country rock. Okay, concordant structures, on the other hand, are parallel to country rock structures, so let's look first at concordant plutons.

The first of these is a sill. Think of a window sill, okay. It's flat; it's horizontal; it's tabular; and in the case of igneous intrusions, the sill is typically squeezed between layers of sedimentary rock, so that it follows the bedding planes of the sedimentary rock.

On the other hand, is a laccolith A laccolith is like a sill except that it's lands are dome-shaped, has a central thicker portion, as if the magma swells up between the layers. As far as the discordant plutons go, the largest of these is the batholith. "Batholith is a large irregular-shaped intrusive body that's at least a hundred square kilometers on the surface expression; that's at least ten by ten. Most batholiths are much larger than this.

A good example of this is the Sierra Nevadas in Eastern California, which as the video shows, is really a collection of smaller plutons all sort of stuck together. The batholith may be as much as 20 kilometers deep although they formerly were thought to extend all the way to the bottom of the crust before we got a better understanding of how they form.

If there's an outcrop of an irregularly shaped pluton that's too small to be a batholith, it's simply called a "stock", so a stock is like a baby batholith; in fact, many stocks are actually offshoots of a larger batholith.

Okay, the last of these discordant structures is a dike. A "dike" is very similar to a sill. It's tabular, but nearly vertical, in shape. They serve as feeders for volcanic eruption, and here in Hawaii we find many such dikes. For example, here on Oahu, near Castle Junction as you head toward Kailua from Honolulu, the whole region is dissected by hundreds of dikes in many swarms.

Okay, let's turn our attention, finally, to the sources of magma. Using Bowen's Reaction Series as a model, we have the opportunity to understand the origins of these various types of magma. It seems likely that partial melting of mantle rocks is the likely source of basalt, and we can consider basalt to be the most primitive kind of magma; in fact, the first liquid that forms from melting mantle rocks matches the composition of both sea floor and Hawaiian type basalts.

The subtle differences between the two can be explained chemically very nicely by considering the depth at which the magma originates, and there's plenty of mantle material available to account for all the basalt present now and all the basalt through geologic time, that which has been destroyed by subduction.

You recall that the mantle is about 80 percent of Earth's volume, so there's no scarcity of mantle rock down there from which to form the basalt. It's the intermediate, and the felsic, or the light colored rocks that are the problem. It seems that they're probably recycled crust, either continental or oceanic basalt, or some combination of the two. One way that you can get these, either intermediate or felsic lavas from magmas is from partial melting of oceanic basalt as it's subducted at a convergent plate boundary, and the sediments may be partly incorporated, the friction from the sliding of the plate adds additional heat, and water trapped along with the sediments lowers the melting temperature significantly by interfering with the silica bonding.

The problem is that it's not completely clear why intermediate magmas form sometimes and felsic magmas at other times. There's probably a combination of factors because none of the factors alone seem to be able to account for the great volumes of granite that we find on the continental crusts. One way might be melting of sedimentary rock. Sedimentary rock, as we'll see later, is much richer in felsic constituents. In later programs, we'll take this up as well.

If the lower continental crust melts at the bottom, especially under the deep roots of mountain ranges where it's subjected to greater heat and pressure, this could also account for the difference between felsic and intermediate magmas. Another possibility is underplating of continental crust by mafic magma. As the mafic magma rises from the mantle due to partial melting, it could stick to the bottom of the continental crust and act like a hot plate underneath the crust to cause partial melting. There's also the possibility of the mixing of magmas.

Since the mafic magmas are less viscous, they should rise faster through the overriding rocks, and, therefore, the less viscous bubbles of mafic magma may overtake the blobs of slower rising felsic magma mixing with it. The composition of the resulting magma depends upon the relative amounts of two different ones.

Well, this picture that we've built here is not really a direct observation. Obviously, we can't go down there and see what's happening, but it's certainly a plausible interpretation of the available data, and it's fair to mention that there are a number of variations on this basic picture that are based upon slightly different interpretations of the data, but one thing is clear. The magma is formed by some process. We know that because the stuff is there. The rocks are there. The plate tectonics model combined with Bowen's chemical model provide a framework to help us understand this. Even though it hasn't provided the final answers, it's given us some good answers, and modifications simply await new data and new understanding of how the Earth works.

Okay, I want to remind you of the lesson assignment for next time. We'll switch our focus here and look at weathering and soils. This will be Lesson 15 in the Study Guide, so be sure you

Study hard but enjoy learning about our planet, and I'll see you next time.