Volcanoes and Volcanic Rocks Module (1) PDF

Videos How Dangerous Are The Northwest's Volcanoes? https://www.youtube.com/watch?v=1Nb2sxWl3aM Transcript: (00:00) at 8:30 2:00 in the morning not very long ago one of the most powerful forces in nature woke up in Portland's backyard but mount st. Helens isn't the only active volcano in the Nort...

Videos How Dangerous Are The Northwest's Volcanoes? https://www.youtube.com/watch?v=1Nb2sxWl3aM Transcript: (00:00) at 8:30 2:00 in the morning not very long ago one of the most powerful forces in nature woke up in Portland's backyard but mount st. Helens isn't the only active volcano in the Northwest we're surrounded by them so what's up are we gonna wake up some morning to a new Mount st. (00:23) Helens right now we recognized about 25 26 places in Washington Oregon where magma has come out of the ground in the last ten twelve thousand years Seth Morin leads the u.s. geological surveys cascade volcano observatory with his team and partners around the region he keeps tabs on the volcanoes in the Cascade Range these are the folks who can help us understand if any of these volcanoes might blow like mount st. (00:47) Helens did many of the volcanoes last erupted thousands of years ago which might make it seem like they're dead but a volcano that's active is a couple hundred thousand years old and has erupt at least in the last ten fifteen thousand years a thousand years seems like a long time to humans but if a volcano lived for just one day those thousand years would be more like just a few minutes in other words these volcanoes aren't done those reasons to believe that it could happen again whether they wake up tomorrow or a (01:19) thousand years from now Seth says around 8:00 pose the greatest risk and each has its own personality [Music] now Baker puts out a lot of gas but doesn't erupt very often neither does Glacier Peak though one past eruption was five times as big as Mount st. Helens in 1980 Mount Rainier is the tallest mountain in the northwest and isn't likely to blow its top but it does have a lot of ice which even small eruptions of hot gas and rock could melt causing devastating mudflows Mount st. (01:52) Helens is still the one to watch it's the most active in the chain erupting once or twice a century on average it's been torn down and remade so many times the current mountain is only about four thousand years old compared to hundreds of thousands of years for other cascade volcanoes like Mount Hood puts not far from st. (02:14) Helens but it's personality is very different not hood is not likely to erupt the way Mount st. Helens did Mount Hood has a history of venting gas and rock off of its sides not huge explosions that's not great news though many people live on and around the mountain and even small eruptions could cause landslides explosions and mud flows that destroy everything in their path the rest of Oregon's volcano is very widely now Jefferson has been relatively quiet for 20,000 years long enough that it may be finished South sister goes off every couple thousand (02:48) years Newberry crater more frequently and like why is Kilauea it can send rivers of lava far and wide then there is crater lake today it's a giant hole in the ground but eight thousand years ago it was Mount Mazama a mountain comparable in size to Mount Ranier the explosion that leveled it 50 times as big as Mount st. (03:11) Helens in 1980 crater lake produced the largest bang that we've had in the Cascades in the last 10,000 years that brings us back to the original question well one of these volcanoes be the next Mount st. Helens it's a tough one to answer set that his team aren't psychics there are more like weather forecasters they use a network of monitoring stations to keep tabs on what each volcano is up to every volcano has its own normal baseline just like people there's a range of course we know it's healthy but there within that everybody's a little bit (03:40) different instead of heart rate and blood oxygen volcanoes have earthquakes ground warping and gas burbs even when they're not erupting comparing these vital signs to the historical record let's Seth and his team know if a volcano is acting normally or if it's time to pay more attention when Mount st. (04:00) Helens woke back up in 2004 the observatory's measurements suggested it probably wouldn't blow like it did in 1980 and they were right that doesn't mean Mount st. Helens is anywhere near done though of all the volcanoes in the Cascade Range there's a good chance the next Mount st. Helens will be Mount st. Helens the forces that make mountains and destroy them will continue to stir deep underground for a very long time we can't stop volcanoes we can't even say for sure when they'll wake back up but by paying attention we won't be caught off guard when they do (04:40) you How to Classify Volcanoes https://www.youtube.com/watch?v=iavbdqsSC1o Transcript: (00:02) Volcanoes These iconic pointy mountains are characteristic of many states such as Hawai'i and Alaska and parts of the Pacific Northwest They look different depending on where we see them Why is that? Lets find out. The type of volcano can help geologists interpret how violent the next eruption might be and to evaluate the potential risk for different types of volcanic hazards. (00:30) Our learning objectives for this lesson are that you will be able to tell the difference between some common types of larger and smaller volcanoes. Let’s start with a definition: A volcano is a cone shaped hill or mountain created by the eruption of magma in the form of lava, tephra and other debris. The presence of a volcano is indicative of an underlying magma source in the crust or mantle, escaping gases that help drive the eruption, a range of potential volcanic hazards, and one or more eruptions needed to build the cone. (01:00) We will begin by examining the distribution of active volcanoes on Earth’s surface as represented by the green triangles on this spinning globe. These are dominated by two main types of volcanoes, the gently-sloping shield volcanoes, and the steeper slopes of composite volcanoes. Most of those triangles near the middle of the oceans are probably shield volcanoes which typically form at hot spots and along oceanic and continental rift systems. (01:20) That line of green along the rim of the Pacific Ocean and along the southern boundary of the Eurasian plate represents chains of composite volcanoes that formed above subduction zones at convergent plate boundaries. We will describe shield volcanoes first. Shield volcanoes have a distinctive low angle profile that forms broad triangular landforms that are thought to resemble an overturned warrior’s shield. (01:41) These volcanoes are built up by a series of lava flows that can emerge from vents or from networks of fissures, or deep cracks, along the volcano’s flanks. These lava flows are thin and fluid and can flow easily. For example, let’s take a few moments to watch this time lapse video of lava emerging from fissures near the Hawaiian coast. (02:02) Notice how thin each flow is. Lava from shield volcanoes can travel long distances along a series of lava tubes or channels that can carry the hot lava tens of kilometers before it can cool off and solidify. The largest shield volcanoes make up the main island of Hawai’i. The biggest volcano, Mauna Loa, is more than 100 km across and it rises more than 9 kilometers from the sea floor making it taller than Mt. Everest. (02:37) That’s one impressive volcano. But keep in mind that many shield volcanoes are much more modest in size (a little more on that later). In contrast to shield volcanoes, composite volcanoes have a distinctive steep triangular profile. Eruptions of composite volcanoes are much more violent than those of shield volcanoes, and are characterized by the production of a mix of tephra and thick, viscous lava. (02:59) In addition, some of the magma will force its way into the interior of the volcano and provide support for the structure’s growth Volcanic rocks in composite volcanoes are more silica rich, resulting in more viscous lava and more violent eruptions often separated by hundreds or thousands of years. Many volcanoes look pretty big up close, but it is important to keep a sense of scale in mind when we are thinking about classifying volcanoes. (03:25) Shield and composite volcanoes are an order of magnitude larger than typical smaller types of volcanoes such as cinder cones and lava domes. These figures are approximate but we measure the width of shield and composite volcanoes in 10’s of kilometers and their heights in thousands of meters. In contrast, their smaller cousins have heights measured in hundreds of meters. (03:48) One way to distinguish these volcanoes from each other is to compare the size of the volcano to nearby vegetation. Images of larger volcanoes are often show them surrounded by forests. In contrast, you can often make out individual trees in images of cinder cones and lava domes. Let’s take a closer look at these volcanoes. (04:10) These small cones can be found on their own or on the flanks of larger volcanoes and are typically formed by a single eruption. They are composed of smaller pieces of tephra produced when lava is blasted into the air and cools as it falls to ground. The cooled lava fragments often contain air bubbles and are known as scoria. (04:28) Lava domes form when the most viscous lava oozes out at the surface and slowly crawls along and cools to form steep-walled, bulbous domes. The steep slopes along the dome flanks may collapse to create dangerous hazards such as pyroclastic flows. While lava domes often form in the craters of composite volcanoes, they can also be found in other volcanic environments where high-silica magmas are generated. (04:53) Take a look at these sped up time lapse images of the growth of a lava dome in the crater of Mount St. Helens over a period of nearly three years. Notice the steepness of the slopes. This looks fast because it all happens in less than 30 seconds but it actually represents very slow, gradual movements. Now, let’s see how well you have assimilated all this information. (05:19) We are going to show you five pictures of different types of volcanoes and you will have four seconds to classify the image as a shield or composite volcano or a cinder cone or lava dome. Jennifer will be playing along with you, she hasn’t seen these images before either, let’s see how you all do. Okay here comes the first image. (05:40) Composite Volcano Cinder cone Shield Volcano Lava Dome And... Shield Volcano Alright lets look and see how well you did Oh, I got one wrong! Not bad! We had two learning objectives for this lesson. How confident are you that you could accomplish each of these tasks? What are Volcanic Hazards? https://www.youtube.com/watch?v=BCm6xTZj-vk Transcript: (00:00) Volcanic eruptions are among the most spectacular natural hazards on Earth. A volcanic hazard is any volcanic process that threatens life or destroys land or infrastructure. In this lesson we will characterize the principle types of volcanic hazards using examples from a variety of composite cone volcanoes and a few shield volcanoes. (00:19) As we will learn, people who perish from volcanic eruptions are rarely consumed by glowing floods of lava Instead they are more likely to be buried by mudflows or engulfed in hot clouds of toxic gas. The residents of Portland, Seattle and Vancouver, Canada, might be interested in this discussion. They sit to the west of a chain of active volcanoes known as the cascade range. (00:41) More than 2000 flights travel over the Cascade Range each day. Cascade volcanoes have erupted on average of 1 to 2 times per century during the past 4000 years. And are guaranteed to erupt again in the not too distant future. Most active volcanoes around the world are composite volcanoes that are located along convergent plate boundaries. (01:01) The Cascades represent examples of these types of volcanoes. They form on the North American plate as it overrides the diminutive Juan de Fuca plate adjacent to Washington and Oregon. Most hazards associated with composite volcanoes are characterized by four eruption products. Tephra that is blasted into the air. (01:20) Lahars or mudflows that flow outward from the mountain. Pyroclastic flows or hot clouds of toxic gases that tumble down the volcanic slopes. And a limited volume of lava that tends to form localized flows or volcanic domes. Lava is a more abundant product of other types of volcanic eruptions. And we'll discuss in the context of eruptions on the Hawaiian Islands. (01:40) The term tephra is used to describe the rock fragments and other particles ejected from a volcano. Tephra is the most far flung product of a volcanic eruption. This material is blasted into the atmosphere and represents ash or lava bombs and even larger blocks of rocks rip from the cone of the volcano. The finest material can be blasted more than 20 kilometers into the atmosphere and carried much farther downwind than the larger blocks which fall to Earth on the volcano itself. (02:08) These images illustrate tephra clouds from recent eruptions from volcanoes in the Aleutian Islands of Alaska. Tephra can also result in significant economic losses in areas downwind from the volcanic eruption. These tiny shards of glass destroyed car and jet engines A recent eruption in Iceland shut down many European flights for weeks. (02:29) Tephra is slippery when wet and can block transportation routes or make travel hazardous. Ancient tephra deposits often form thick layers that can be tied back to their volcanos to analyze the size of past eruptions. Tephra that falls back to the ground or even tephra deposits from previous eruptions Can mix with water in streams from heavy rainfall or from melting glacial ice create mudflows. (02:51) The volcanic mudflows are termed lahars and can be a dangerous hazard for communities many miles down slope from the volcanoes. When Mount Saint Helens erupted in 1980 the lahars filled river valleys leaving a mudline several meters up trees and carrying away bridges and buildings. Remember those dead trees later. (03:10) A lahar resembles a river of wet cement and can vary in speed depending on the amount of water and the size of the debris carried in the flow. The most fluid lahars travel fast making them impossible to outrun. More than 20,000 people were killed in Columbia when the town of Armero was overrun by a lahar when an eruption melted snow and ice triggering a massive mudflow. (03:30) Many communities west of Mount Rainier are located on top of a giant 5000 year old lahar deposit. You can see the chaotic mix of debris including large boulders in this section of the lahar that is 50 kilometers downstream from Mount Rainier. Remember those dead trees from earlier? We can see whole tree trunks and branches preserved in this lahar deposit generated from an eruption at Mount Baker. (03:54) A pyroclastic flow may be the nastiest product of volcanic eruptions. These hot dense clouds of toxic gases mixed with tephra roll down the flanks of a volcano at a great speed. Anything in their way is incinerated. The greatest volcanic disaster in the last century occurred on the Caribbean island of Martinique when a pyroclastic flow overwhelmed the city of Saint-Pierre killing all its residents with the exception of one lone survivor deep in the cells of the local jail. (04:27) Pyroclastic flow deposits can be identified on the north side of Mount Saint Helens where the flow lost energy as the slope decreased. The blocks of light colored pumice were deposited at the toe of a pyroclastic flow. Elsewhere we can identify pyroclastic flow deposits over 100 meters thick that were formed after a single massive eruption of a volcano 7000 years ago in what is now Oregon. (04:50) The one hazard with the relatively modest impact on most active composite cone volcanoes is lava. Composite volcanoes have relatively sticky, viscous lava that doesn't flow very far and doesn't get much distance from the crater. At Mount Saint Helens lava has built up to form a small lava dome inside the crater left behind by the 1980 eruption. (05:13) In contrast fluid lava is a primary product of shield volcanoes such as those in Hawaii. The more fluid and magic lavas flow greater distances from the crater. Consequently they build up wide volcanic shapes. Lava flows are one of the least deadly of all the volcanic hazards. This is partly because lava flows generally don't move very fast. (05:35) Even the more runny lava flows typically only travel a few miles per hour. Having said that we wouldn't recommend walking up to a lava flow because temperatures can reach as much as 1400 degree Celsius. That is definitely hot enough to singe your eyebrows off. Lava flows can kill vegetation and destroy property by burning homes and destroying infrastructure such as bridges and roads. (05:59) While lava rarely kills anyone these flows are unstoppable and can flow through populated areas burying everything below several meters of hardened basalt. So for today we only had one learning objective for this lesson How confident are you that you could complete this task? Magma Viscosity, Gas Content & Milkshakes https://www.youtube.com/watch?v=2iaqE0xmsHI Transcript: (00:02) In this lesson we discuss why some volcanic eruptions are violent while others are mild mannered. As we will discover, the relationship between the gas content and magma viscosity is critical in determining just how explosive an eruption will be. We have three learning objectives:To define the term "viscosity"; to provide examples of common materials with different viscosities and to explain how gas content and magma viscosity influence the style of a volcanic eruption. (00:31) The story we want to tell is why composite and shield volcanoes erupt so differently. Composite volcanoes are known for their explosive eruptions that toss clouds of tephra far into the atmosphere. In contrast, shield volcanoes have runny lava that builds wide, gently sloping landforms. First, lets consider the gases in magma. (00:52) Opening a bottle or can of soda after shaking it causes the contents to overflow. In such cases, the carbonation in the soda represents gases dissolved in the liquid. Under normal conditions, we can't see any bubbles in the drink. However, if we shake the bottle, we can see some of the bubbles come out of solution. (01:10) And when the bottle is opened, pressure decreases and the escaping gas causes the bottle to overflow. The same kind of thing happens as magma rises toward Earth's surface. At depth in the crust, volcanic gases such as sulfur dioxide and carbon dioxide are dissolved in the magma. As the magma rises, it is under less pressure and gases start to come out of solution causing the volume to increase. (01:35) As the gases come out of solution, it forces the magma upward and under appropriate conditions has the potential to propel the magma out of the volcano. So gases drive volcanic eruptions, but gases don't escape from all magma in the same way. Depending on the viscosity of the magma, gases can either get trapped in the rising magma or escape readily from the magma. (01:57) Viscosity, or the resistance of the material to flow, depends on the temperature and composition of the magma. Everyday materials have a range of viscosities. Liquids that flow easily like water and milk have a low viscosity. Oils, things like olive oil or canola oil, typically have more resistance to flow and will take longer to flow a similar distance. (02:30) Thicker substances, such as syrup or honey, have higher viscosities. They will exhibit more resistance to flow and will take a much longer time to travel the same distance. To consider how magma might influence the violence of a volcanic eruption, lets do a simple demonstration. Imagine blowing air through a straw into a glass of water. (02:55) The low viscosity water will bubble readily as the air escapes without much effort. Now, try doing the same thing again but this time replace the water with a milkshake. First, it will be more challenging to blow air through the mixture. Second, if you succeed, it will react more vigorously and splatter the milkshake over anyone standing nearby. (03:18) The higher viscosity milkshake makes it difficult for air to escape, causing the pressure to build up, and producing bigger bubbles. Back to our consideration of the gases and viscosity in magma. So, gases drive volcanic eruptions, but gases don't escape from all magmas in the same way. Depending on the magma viscosity, gases can either be trapped in the rising magma or escape easily as the magma approaches the surface. (03:47) Low viscosity magmas of mafic composition allow these gases to escape. Eruptions of these magmas are commonly associated with shield volcanoes and are characterized by flowing lava and occasional lava fountains. In contrast, higher viscosity magmas of intermediate or felsic composition trap gases causing pressure to build up internally and eventually producing a violent eruption. (04:16) Eruptions of these magmas are commonly associated with composite volcanoes that are characterized by tephra blasted high into the air. So viscosity is the resistance to flow and low viscosity magmas that allow gas to escape produce mild eruptions and high viscosity magmas that trap gases produce explosive eruptions. (04:39) That basically sums up our lesson for today. These are our three learning objectives. How confident are you that you could complete these tasks. How to Melt Rocks https://www.youtube.com/watch?v=6ZKIXrksQM8 Transcript: (00:02) This video will explain how rocks melt to produce magma in a variety of plate tectonic settings. We have a companion video on partial melting that might help you understand these processes more fully. When we think about melting, things like ice or butter or chocolate might come to mind. These common items melt as they warm up, turning from a solid into in to a runny liquid. (00:26) You probably know that Earth’s interior is very hot. The rate at which temperature increases with depth is represented by the geothermal gradient. Earth’s internal temperature increases downward, and is estimated to be greater than 6000 degrees Celsius in the core. So, there you go. Things melt when they warm up and Earth gets hotter with increasing depth. (00:48) Surely, there must be a point below the surface where it just gets so hot that rocks start to melt. Not so fast, Sherlock. It turns out we are overlooking another key property of Earth’s interior, and that’s pressure. Just like temperature, pressure increases with depth. And that’s going to prevent rocks from melting except under very specific types of conditions. (01:11) We are going to focus in on the upper few hundred kilometers of Earth. This is where most of the melting is going to take place. We are going to use this graph to show how temperature varies with depth. On this figure, the oceanic crust would be up here, the continental crust would be about this thick, and the rest of the graph would represent rocks in the upper part of the mantle. (01:33) Including parts of the lithosphere and asthenosphere. The first thing to know is that this graph is drawn to represent melting conditions for peridotite, the typical composition of the mantle. There are several lines on this graph, lets explain what they represent before we go any further. This line, known as the solidus, indicates the temperature necessary to start melting peridotite at different depths. (01:57) Remember that pressure increases with depth, so it takes much higher temperatures to melt rocks the deeper we go. The second red line is the liquidus, this represents the temperature needed to completely melt peridotite at different depths. Anything between the two lines represents conditions where partial melting will take place. (02:16) Technically, any area to the left of the solidus curve represents conditions where the rocks would remain solid. However, many of these conditions are actually unrealistic. For example, we aren’t going to find any deeply buried at relatively low temperatures like these. Let’s add another curve, this one indicates a typical geothermal gradient for the crust and mantle. (02:41) This is the temperature of the rocks at a given depth. For example, a rock at 100 kilometers would lie near the top of the asthenosphere below oceanic basins, and would be solid and around 1300 degrees Celsius. One final point, this graph is drawn for an ultramafic mantle rock with relatively high melting temperatures. (03:02) If, instead, we were to generate a similar graph for the continental crust (more about that later) the relative positions of the liquidus and solidus lines would shift to the left. OK, enough of the set up. Let’s use this figure to demonstrate how rocks melt at plate boundaries. First, let’s remember what happens at divergent plate boundaries that are mostly represented by oceanic ridges. (03:26) Plates move apart and the lithosphere is stretched and it thins. The hot rocks of the asthenosphere rise from deeper levels to fill this growing gap. The key here is that these mantle rocks start out at around 100 kilometers depth where pressures are greater, and move upward to a point just below the surface where pressures are much lower. (03:47) This change in pressure is sufficient to prompt partial melting of the rising asthenosphere, a process known as decompression melting. Let’s see what that looks like on our graph. So, imagine if the hot asthenosphere was to move upward fast enough so that it doesn’t lose much of its initial heat. Somewhere above 50 kilometers it would cross the peridotite solidus line and partial melting would begin to generate a mafic magma. (04:14) As rising continued, a greater proportion of the original rock would undergo melting but it would never be sufficient to completely melt the peridotite. A similar process occurs below hot spots like Hawai’i as mantle rocks rise in hot plumes that begin near the core/mantle boundary. As they neared the surface, the hot mantle material in these plumes would undergo decompression melting to form mafic magma. (04:40) Next, let’s turn our attention to convergent plate boundaries where a plate descends into the mantle along a subduction zone. We find volcanoes on the over-riding plates, typically in the form or island arcs or volcanic arcs on land. Let’s see where this magma comes from. Water-rich minerals in the descending oceanic plate are compressed in the subduction zone, and the water is squeezed out of the minerals and into the much hotter mantle rocks immediately above. (05:08) The addition of this water causes these rocks to melt. This process is known as flux melting. The presence of water completely realigns the position of the solidus and liquidus for the mantle rocks. Rocks that would have been solid under normal conditions at around 100 kilometers, are now well inside the partial melting field. (05:30) As this mafic magma rises through the plate some of it may get trapped at the base of the continental crust, heating it up and causing it to undergo partial melting to form a felsic magma by a process known as heat transfer melting. OK, let’s see what that looks like on the graph. We are dealing with continental crust now, which starts to melt at lower temperatures. (05:52) The base of the crust would be around 35 to 40 kilometers depth, and these rocks would need to be heated by another 500 degrees or so to start partially melting the crust and generate felsic magma. So, to summarize, we described three different ways to create magma by melting rocks in Earth’s mantle and crust. (06:12) As arranged here, they are in order of the volume of magma produced. Decompression melting is the dominant mechanism at oceanic ridges and hot spots, and also contributes magma to continental rifts. Flux melting is the main source of magma for island arcs and volcanic arcs associated with convergent boundaries. (06:31) However, some of the magma produced in these settings is also produced by heat transfer melting. Finally, this latter process is also associated with continental rifts. Here is our learning objective for this video. How confident are you that you could successfully respond to this prompt? Partial Melting of Igneous Rocks https://www.youtube.com/watch?v=btSxjIpsjXU Transcript: (00:03) This video will consider how magma is made and how the different types of magma are linked to specific plate tectonic settings. We're going to assume that you have a working knowledge of the classification of igneous rocks if not you might want to take a quick look at this video. Lava flowing on earth's surface is an example of a molten rock, kind of mesmerizing isn't it. (00:31) However our own experiences with melting things may give us a simplified view about how this lava originally formed. When we think of melting, we often think of substances that melt to produce a liquid version of the solid material. In these situations, the composition of the material doesn't change as a result of the melting process. (00:52) For example, this water and ice have the same chemical formula. However, most rocks are made up of several minerals, each with different compositions and properties. In particular, these minerals often melt at different temperatures. Some at around six to seven hundred degrees Celsius and others at over a thousand degrees. (01:15) Let's try a different analogy. If we were to leave some vanilla chocolate chip ice cream out on the counter, the vanilla ice cream gradually melts, but the chocolate chips remain solid. What we've just managed to do is to partially melt the material. Part of it is melted while part of it has stayed solid. (01:37) If we had way too much time on our hands, we could separate the melted ice cream and the chocolate chips. If we were to re-freeze the ice cream it would still taste good but it would be different than the original. So this partial melting process has essentially changed the composition of the ice cream. (01:53) This is the gist of how partial melting occurs in igneous rocks. We're going to melt some of the minerals to generate a new magma with a different composition The minerals that melt first have the highest silica content and are the lighter colored minerals in this sample of granite. Partial melting will form a new magma with these minerals. (02:14) while the darker minerals with a lower silica content will remain solid. Keep in mind that this is all relative, in comparison to other types of rock, this granite has a lot of high silica minerals with low melting temperatures. If instead we were to start with a rock like peridotite, an ultramafic rock that has a composition similar to the mantle, the only minerals present would be olivine, pyroxene and amphibole. (02:43) In this situation, some varieties of the amphibole would have a relatively higher silica content than the rest of the minerals present and therefore would melt first. So, to summarize, partial melting generates a magma that has a higher silica content than the original source rock. Now, let's look to see what this means for different plate tectonic settings. Let's take that sample of ultramafic peridotite we had earlier. (03:10) Partial melting would generate a mafic magma that would solidify to form either a volcanic or plutonic igneous rock. This process is characteristic of how magma is generated at oceanic ridges and hot spots. Most of the magma generated on earth is formed below the oceanic ridges as a result of partial melting of the mantle. (03:33) The magma source of hot spots comes from the mantle too, but from much deeper depths and therefore exhibits some secondary compositional differences to the magmas we find at the oceanic ridge. Regardless of whether the magma comes from the ridges or from the hot spots, it will form the volcanic igneous rock basalt at the surface, and it will form its plutonic equivalent, gabbro, at depth. (03:53) We can identify a third location, where partial melting of mantle rocks is occurring in the plate overlying subduction zones. If this magma were to rise undisturbed to the surface it would also solidify to form mafic rocks. However, as we'll see, these magmas will undergo a variety of changes on their path toward the surface. (04:14) As this magma rises upward its composition may change. Mafic minerals with higher melting temperatures will solidify first, and may separate out of the magma to leave a more silica-rich melt behind. Likewise, the rising magma forces its way upward through fractures in the crust and may incorporate and melt enough chunks of these rocks to alter its composition. (04:38) Finally, different magmas can essentially bump into each other on their travels and end up forming a new mixture. Some, or all of these processes, may alter the original mafic magma to yield a more intermediate composition that forms the volcanic rock andesite and its plutonic equivalent diorite. The composition of these rocks is actually pretty close to the average composition of the continental crust. (05:04) So, let's now take a look at what happens if the continental crust were to undergo partial melting. If we melt this intermediate composition rock it would result in a felsic magma that would create the plutonic rock granite and the volcanic version, rhyolite. Some of the mafic magma that forms above the subduction zone may get trapped at the base of the crust causing, the intermediate crustal rocks to heat up and undergo partial melting to form a felsic magma. (05:38) These magmas are pretty viscous and most won't rise far enough to reach the surface but instead will solidify underground to form large granitic plutons. So to summarize, most of the igneous rocks begin by partially melting ultramafic mantle rocks At the oceanic ridge, partial melting produces a mafic magma that generates new oceanic crust of basalt near the surface and gabbro at depth. (06:07) A similar combination of mafic rocks is characteristic of hot spots like Hawai'i. Things get a bit fussier in association with volcanic arcs overlying subduction zones. First, the mafic magma travels at greater distance through the crust and undergoes a variety of compositional changes that may create a melt with an intermediate composition. (06:27) This can produce eruptions of andesite and plutons of diorite. Second, the mafic magma may get trapped at the base of the crust, heating it up sufficiently to create a new felsic magma that solidifies to form either rhyolite or granite. OK, that's it for today how confident are you that you could complete the two learning objectives associated with this video. Earth's Elements and an Introduction to the Silicate minerals https://www.youtube.com/watch?v=mqXUytwB3uQ Transcript: (00:01) There's an old saying in mining, if it's not grown, it must come must come out of the Earth. Each year the US produces 75 billion dollars worth of mineral resources that include exotic metals such as gold and silver, and more basic materials like sand and gravel. (00:19) As geologists we interpret minerals and rocks to unravel the history of our planet and find and manage mineral resources to support economic growth We have to learning objectives for this lesson we are going to identify the surprisingly short list earth's most popular elements and introduce you to our friends the silicates, the most common mineral group in the rocks below our feet. (00:40) We know that rocks are made minerals and minerals or composed of elements. Elements are the primary ingredient so all earth materials. We consume elements in the food we eat, the air we breathe and the water we drink. Elements can't be separated into simpler substances by chemical means. There are 92 naturally occurring element on Earth. (01:00) How many of these do you recognize? Rocks and minerals are composed of combinations of these elements, but don't get too worried about learning all of the names. Just 8 elements make up most of the rocks in the Earth's crust and several these are also key ingredients in Earth's core and mantle Remember that earth is differentiated into three major compositional layers. (01:35) Most of these layers are made up of just four elements, iron, oxygen, silicon and magnesium in some combination. Dense elements such as iron and nickel sank to Earth's core during the early history of the planet. The core and the mantle do contain silicon and oxygen, but these lighter elements are much more abundant in the outer layers of Earth, and especially in the crust. (01:57) Scientists have discovered over four thousand different minerals and more are found each year. However, only a few dozen minerals are relatively abundant in the Earth, that's a lot easier to wrap your mind around. We live on the planet surface, so we are most interested in the rocks below our feet that we can use for resources and that break down to form our soils. (02:23) When we look more closely at a characteristic rock on Earth's crust we can see that it is composed of different colored minerals. Analyzing the elements in these minerals shows that oxygen and silicon make up 75 percent of the elements present. In addition to oxygen and silicon there are only six other elements that make up the majority of the Earth's crust. (02:46) All six of these may be familiar to you. We take in iron calcium, potassium and magnesium in our diet. Foods like spinach are rich in all of these elements. We consume sodium as salt and we use aluminum in a variety of forms every day In combination with a handful of other elements, oxygen and silicon combine to form the most common mineral group, the silicates. (03:11) So, silicate minerals contain oxygen and silicon, often with one or more of the other six elements we just mentioned. Let's consider the rock granite, a common rock in Earth's crust. Granite contains several silicate minerals. Some, like quartz, have a relatively simple composition and are composed exclusively of oxygen and silicon. (03:35) Granites also contain one or more varieties of the mineral feldspar that will have additional elements such as potassium, sodium and aluminum. Most of the minerals in granite are light-colored and may be white, pink, tan or clear. However, some are darker. We see a difference in the elements that are present in these minerals. They contain iron and magnesium and lower proportions of silicon and oxygen. (03:57) However, all of these minerals are silicates, as indicated by the presence of silicon and oxygen in their chemical formulae. The silicates are therefore composed of two nonmetallic elements. These non metallic elements often give mineral group their names. These elements may form bonds with metallic elements such as potassium and aluminum to create specific varieties of minerals such as feldspar or muscovite mica. (04:28) Many silicate minerals begin life as elements in magma. Silicates form as the magma cools and crystallizes to form different types of igneous rocks. The original composition of the magma and the characteristics to the physical and chemical environment will determine the type of igneous rocks that form. Magmas with higher percentages of silicon and oxygen will erupt more violently than those where these two elements make up a smaller proportion the total elements present. (04:54) Being able to identify the minerals present in igneous rocks allows geologists to unravel the volcanic history of ancient mountain ranges, as well as evaluate the potential hazards of modern active volcanoes. These igneous rocks can be broken down into their constituent minerals by weathering processes at Earth's surface. (05:15) For example we can see large feldspar crystals exposed here at the Black Canyon the Gunnison National Park in Colorado. These and other silicate crystals weather out of the outcrop and are then washed away into the river and carried downstream toward the coast where they may become part of the beach. Eventually these mineral grains and fragments may be cemented together to form a sedimentary rock such as sandstone. (05:37) Sand and gravel deposits are dominated by resistant silicate minerals like quartz and feldspar. The US mines about seven billion dollars of sand and gravel each year, most of which goes to make concrete or is used for road construction. We to principal learning objectives for today. (05:57) How confident are you that you complete both of these tasks successfully? Okay, that's it for us. Jen, how about we go and replenish our elements with some spinach? Naming Igneous Rocks https://www.youtube.com/watch?v=Zbz4e-9pjY4 Transcript: (00:01) This presentation will explain how geologists figure out the conditions responsible for turning hot magma into solid rocks and how we can make some simple observations to identify and classify some common igneous rock types. There are three specific things we want you to learn. (00:18) The first is to figure out if a rock formed near Earth’s surface or deep underground. Next, we want to further classify these rocks on the basis of their chemical composition, and finally, we would like you to be able to name some common types of igneous rocks We can divide all igneous rocks into two groups. (00:37) Volcanic igneous rocks form on or near Earth’s surface. An example would be a lava flow. In contrast, when magma cools and solidifies below ground, it forms a plutonic igneous rock. Both volcanic and plutonic igneous rocks form when magma solidifies. The big difference is in how rapidly that process takes place. (00:56) Magma that starts at temperatures in excess of a thousand degrees Celsius will cool rapidly in the relatively cold conditions of Earth’s surface. This rapid cooling produces millions of tiny crystals of different minerals that can only be observed using a microscope. In contrast, plutonic igneous rocks cool slowly under much hotter conditions, allowing some minerals to grow into relatively large crystals that can be readily distinguished by the naked eye. (01:20) The same type of magma can produce either plutonic or volcanic rock, depending upon where it comes to rest. Geologists can use the texture of an igneous rock - that is the size of the crystals or minerals within the rock - to determine if it had a plutonic or volcanic origin. Visible grains indicate a plutonic igneous rock, but if you can’t see any individual minerals, it must be a volcanic igneous rock. (01:44) For example, is this rock volcanic or plutonic? How about this one? We can’t see any individual crystals or mineral grains in the first rock, so we would classify it as volcanic. We can clearly see light and dark minerals in the other sample, so we would identify it as plutonic. How about these examples? We can differentiate volcanic and plutonic rocks on the basis of their texture, but if we want to classify igneous rocks further, we have to divide them up by composition. (02:36) We will use the silica-content of the igneous rocks to divide them into three groups. Fortunately, we don’t have to do any fancy chemical analysis to determine composition as the color of the igneous rocks serves as a proxy for its silica content. Light-colored rocks are mainly composed of silica-rich minerals. In contrast, dark-colored rocks are made up of silica-poor minerals. (02:59) We use the terms felsic, mafic, and intermediate to label igneous rocks with high, low or medium silica contents. Felsic igneous rocks are silica-rich and are dominated by light-colored minerals. Mafic rocks are darker, and intermediate igneous rocks contain roughly equal measures of light and dark minerals. (03:19) The chemical composition of these rocks ranges from about half-silica for the mafic rocks to more than two-thirds silica in the felsic rocks. Let’s try classifying a few igneous rocks by composition. What about these examples, can you identify the felsic, mafic and intermediate varieties? Here are the answers. (03:53) Now that we know how to identify the texture and composition of igneous rocks, we are ready to learn how to apply this information and match it with the names of some common igneous rocks. We can separate igneous rocks into volcanic and plutonic varieties on the basis of texture. And there is a felsic, mafic and intermediate example of each, giving us six possible rock names. (04:17) Felsic volcanic and plutonic rocks are known as Rhyolite and Granite respectively. Their intermediate equivalents are Andesite and Diorite, and the darker mafic volcanic igneous rock is Basalt, and the plutonic version is Gabbro. Try creating this table on your own as a useful way of remembering the classification scheme. (04:37) Now, let’s see if you can apply this new-found knowledge to identify some igneous rock samples. We see visible light-colored grains, so this must be a plutonic, felsic igneous rock, and that would be Granite. You are on your own now for the rest. Good luck! So, to summarize, we can use the texture of igneous rocks to tell us something about where they formed relative to Earth’s surface and their color to indicate their chemical (05:41) composition. In a future lesson we will discuss how composition is linked to the plate tectonic setting where the magma originally formed. On the basis of texture and color, geologists can classify igneous rocks and use this information to decipher the geologic history of the region where the rocks were found. (06:00) We had three learning objectives for this lesson. How confident are you that you could readily complete these tasks?