unit 4_v2

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Unit 4 The Rock Cycle & Igneous Rocks

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The Rock Cycle

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When we study rocks, we are interested in two things: 1. rock properties, and 2. the processes that produce the rock

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Knowledge of rock properties can help determine things like where it is dangerous to build whether a groundwater contaminant will migrate off site what is the best material for making buildings or highways

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where to expect earthquakes where to dig for gold or oil how to make synthetic rubies and diamonds why a lake in Africa discharged a CO 2 cloud that killed hundreds Understanding earth processes can help determine things like

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Igneous : formed by solidification from magma There are three basic rock types. Sedimentary : formed by accumulation and lithification of existing mineral grains OR precipitation of dissolved ions from water Metamorphic : formed by alteration of existing rocks under high temperature and/or pressure Note how the three rock types line up with the three ways in which minerals are formed!

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Lithification : the process of turning sediments into rock A few additional terms to be familiar with before moving on: Weathering : physical breakage and chemical decomposition of rocks (including dissolution) Erosion : transport of weathered rock away from point of origin

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We tend to think of rocks as being eternal. The thought of changing a rock from one form into another seems far fetched, but in reality, it is going on continuously. Rocks can be quite literally “recycled”, where one rock can be broken down or altered and turned into a different type of rock. This process is called the Rock Cycle.

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Igneous Rock Cycle We’ll arbitrarily start with an igneous rock (formed when magma solidifies).

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weather erode Igneous uplift Rock Cycle After solidification, earth process result in uplift where the igneous rock is exposed to wind and water. Weathering breaks the rock down and erosion transports the material away.

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weather erode Sedimentary Igneous uplift Rock Cycle deposition burial lithification The materials are deposited, become buried and lithified (compacted and cemented) which turns the material into a sedimentary rock. Alternatively, weathering may have dissolved portions of the igneous rock. The ions are transported and later precipitated (the other way sedimentary rocks are formed).

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weather erode Sedimentary Igneous uplift Rock Cycle deposition burial lithification Newly formed sedimentary rocks can be uplifted and exposed to wind and water where they are broken down again. Eroded materials may form yet another sedimentary rock after deposition. uplift

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weather erode Sedimentary Igneous Metamorphic uplift heat pressure Rock Cycle deposition burial lithification If our sediments are exposed to high temperatures or pressures without melting, the texture and chemical makeup of the rock can be altered, resulting in a metamorphic rock. uplift

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weather erode Sedimentary Igneous Metamorphic uplift heat pressure Rock Cycle deposition burial lithification If the metamorphic rock is uplifted, it can weather and erode to provide materials for yet another sedimentary rock. uplift uplift

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weather erode Sedimentary Igneous Metamorphic uplift heat pressure Rock Cycle melt cool solidify deposition burial lithification If temperatures increase to the point of melting, an igneous rock will form when the magma solidifies again. uplift uplift

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weather erode Sedimentary Igneous Metamorphic uplift heat pressure Rock Cycle melt cool solidify deposition burial lithification heat pressure Finally, the igneous rock can be reheated or repressurized to the point of altering the igneous mineral texture or composition which generates another metamorphic rock. uplift uplift

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So which of the transitions below cannot happen directly without an intermediate rock type occurring first? Sedimentary  Metamorphic Sedimentary  Igneous Igneous  Sedimentary Igneous  Metamorphic Metamorphic  Sedimentary Metamorphic  Igneous ?

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Answer: All transitions are possible except one. Sedimentary  Metamorphic Sedimentary  Igneous Igneous  Sedimentary Igneous  Metamorphic Metamorphic  Sedimentary Metamorphic  Igneous On the way to becoming an igneous rock, a sedimentary rock will form a metamorphic rock prior to melting.

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5. weather, erode away quartz 7. Sandstone 3. Granite 9. Quartzite 1. magma 4. plate collision / uplift 8. heat & pressure Example Cycle 2. cool / solidify 6. quartz sand deposits on beach - buried / cemented This example is very similar to the previous rock cycle diagram, but specific rock names and events are used to illustrate what can happen.

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Igneous Rocks

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Over the next few weeks, we will focus on each of the three rock types (Igneous, Sedimentary and Metamorphic) individually. This week will be spent on igneous rocks, followed by a second week focusing on volcanism.

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Igneous rocks come in many varieties, some with very large crystals (sometimes bigger than a person!), and some with tiny or non-existent crystals. The crystal size, or texture or an igneous rock, was used to classify igneous rocks before it was really understood why crystal sizes varied so much. The primary clues to what causes crystal size to vary came from studying volcanic lava flows. These flows obviously started as magma, and solidified to form igneous rocks. If a section of the cooled lava was broken open, several observations could be made: Large crystals are never observed in lava. Of the small crystals present, the larger crystals are always found near the base or middle, never at the top.

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Since the top of the lava flow cools the fastest, and the interior cools slowly, this suggested that time must play a role in crystal size. The slower the cooling rate, the more time atoms have to align themselves to grow larger crystals. Since lava flows always produce small crystals, igneous rocks with large crystals must have formed beneath the surface where cooling rates could be very slow. It has since been discovered that the presence of water in a magma also facilitates larger crystal growth.

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Igneous rocks are now divided into two categories depending on where they formed: Intrusive igneous rock : magma intruded into existing rock and solidified beneath the surface Extrusive igneous rock : magma extruded to the surface (lava) and solidified on the ground.

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Igneous rocks are classified using two different systems based on either the occurrence of the rock (where found and in what form), or based on the composition of the rock (what minerals it is made from). We will look first as classification based on occurrence, starting with Intrusive Igneous Rocks

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Intrusive Igneous Rocks pluton : large igneous body formed at depth batholith : large pluton, typcially > 100 km 2 (square 10 km wide on each side) stock : smaller pluton

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Plutons form in different ways. All ways start, however, with magma that begins to rise. Magma rises because it is less dense than solid rock. The simplest way a pluton forms is by simply melting its way upward. As it rises, the pluton grows larger and gets cooler. Once the temperature drops low enough, the magma solidifies and no longer rises.

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Rock breaks & melts or falls to bottom Hot magma swells, bows and fractures overlying rock Magma moves into fractures A second way a pluton will form is by stressing overlying rock and flowing upward into fractures. Part of the overlying rock will melt, and weakened blocks will break and fall to the bottom.

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Intrusive Igneous Rocks sill : tabular intrusion between existing layers (usually forms horizontally because most layers are horizontal) more types - dike : roughly planer intrusion that cuts across existing layers sills are also known as concordant intrusives dikes are also known as discordant intrusives Memory tip – think of someone who causes discord – they don’t go with the flow, they run contrary to everyone else (cut across).

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limestone pluton sill dike shale shale sandstone

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Intrusive Igneous Rocks vein : deposit of foreign minerals within a fracture last type - Veins can be either igneous or sedimentary. If minerals precipitate out in a fracture from water heated by a magma body, the vein is considered igneous. If minerals precipitate out in a fracture at lower temperatures not associated with magma, the vein is considered sedimentary.

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The Sierra Nevada Mountains in California are made from a massive, uplifted batholith (covers over 30,000 square miles)!

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This close up shows the original rock capping this mountain, and the contact with the roof of the pluton (batholith).

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Yosemite National Park, California, is also a massive, uplifted batholith. El Capitan, shown here, is a 3,000 ft wall cut by a glacier. It is now a frequent destination for rock climbers.

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Most take two or three days to climb El Capitan, but a few daring (and insane) free climbers* have done it in one day by climbing all day and all night. In 2004, two German brothers, Alexander and Thomas Huber, free climbed the entire face in less than 2 hours! * Free climbing means climbing without any safety ropes or harnesses (in other words, old age bothers you).

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Devil’s Tower, Wyoming, is an example of a stock, possibly beneath an ancient volcano. Following uplift, erosion wore down the surrounding sedimentary rocks much more easily, leaving behind a plug of igneous rock standing up in the air.

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This close up shows the columns commonly formed in basalt. As the magma cools, it shrinks. The rock tends to form cracks in an octagonal pattern which results in these columns. Local Indian tribes explained the long parallel lines as scratch marks made by a giant bear trying to reach men who had climbed the mountain to escape the bear.

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A dike crossing other layers near the bottom of the Grand Canyon, Arizona.

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A dike and veins cutting through pre-existing igneous rock. The dike intruded first, and was later fractured and minerals precipitated into the fractures to form the veins. How can we tell the order? The veins cut through the dike, so the dike must have been there first!

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Ship Rock, New Mexico. This is similar to Devil’s Tower – probably formed beneath an ancient volcano. Pressure from the magma chamber intruded magma into fractures radiating outward from the chamber. Following uplift, erosion washed away the surrounding sedimentary rock more easily, leaving the harder igneous rock jutting up above the ground.

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Some of these radiating arms are over 5 miles long!

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Thick sill in Glacier National Park, Montana.

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Extrusive igneous rocks are divided into two categories: Extrusive Igneous Rocks Lava : magma that flows over the surface of the ground and solidifies Pyroclastic material (also called tephra ): magma and solidified fragments thrown into the air during an explosive eruption We’ll spend more time on extrusive igneous rocks when we cover volcanoes in Chapter 6.

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If we unearthed a horizontal layer of igneous rock with sedimentary rock above and below, how could we tell if it was a sill, or an ancient lava flow that was later buried by sediments?

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sedimentary rock x igneous rock Suppose we have two rock faces exposed in two different road cuts (shown below) that look very similar, but one igneous layer is a sill and the other is a lava flow. There are two things we will look at – the size of the crystals in the igneous rock, and evidence of metamorphism above the rock (at the “x”) If magma intruded between two existing layers, the upper layer should show some signs of alteration (metamorphism)

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sedimentary rock x small crystals Outcrop 1 - the crystals in the igneous rock are small, and there is no metamorphism at “x”. This igneous rock must have cooled quickly, and the overlying layer must have been deposited after the igneous rock cooled – it must be an ancient lava flow. no metamorphism lava

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Outcrop 2 - the crystals in the igneous rock are large, and there is metamorphism at “x”. This igneous rock must have cooled slowly, and the overlying layer must have already been in place as the magma cooled – it must be a sill. sedimentary rock x large crystals metamorphism sill

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We are now ready to consider classification based on the composition – what minerals make up the rock. Note that all the rocks previously assigned names based on their occurrence, are also given names based on their composition. There are many different minerals found in igneous rocks, but the vast majority of igneous rocks are made up of just 8 different minerals. These 8 minerals are divided into two groups. felsic minerals – high silica content (SiO 2 ), which makes them less dense, and often lighter in color mafic minerals – low in silica, higher in Fe & Mg, which makes them more dense, and often darker in color

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One type is found more in continental crust, and more of the other is found in oceanic crust. So where do you think mafic minerals are more abundant, continental or oceanic crust? Answer: oceanic

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On this figure, we won’t worry about the rock names just yet – let’s look at the minerals.

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Minerals found in the mafic box include olivine pyroxence amphibole Ca-rich plagioclase (Note the only one missing under ultramafic is amphibole.)

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Minerals found in the felsic box include quartz muscovite potassium feldspar biotite amphibole (Note amphibole appears in both mafic and felsic lists – so it is most common in intermediate rocks.)

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Know these eight minerals !

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OK – now we’ll pay attention to the rock types on this figure. For each group, there are two names. The first is intrusive. The second is extrusive.

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If it helps, this table contains the same information in a different format. Intrusive Extrusive Felsic / Mafic Minerals (most abundant) Granite Rhyolite Felsic quartz, K-feldspar, Na-plagioclase ~70% silica Diorite Andesite Intermediate plagioclase & amphibole Gabbro Basalt Mafic pyroxene & Ca -plagioclase Peridotite Komatiite Ultra-mafic olivine & pyroxene ~40% silica The next figure is designed to help remember names and where these rocks tend to form.

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Mafic, felsic and mixed magmas result from the location and way in which the magmas form. Magma rising through oceanic crust will tend to be made of mafic minerals. Magma rising through continental crust will tend to be mixed if not much continental crust is melted. Magma that melts lots of continental crust will tend to be felsic. basalt gabbro rhyolite granite andesite diorite

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granite dacite gabbro Note that not all the felsic or mafic minerals are light or dark, but the overall appearance is light or dark. 1 cm 1 cm

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rhyolite andesite basalt These have the same mineral composition as the extrusive rocks, but much smaller crystals. 1 cm 1 cm

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If a magma cools very fast, the resulting rock will have no crystals. These extrusive igneous rocks are called volcanic glass . There are two types of volcanic glass. Obsidian : volcanic glass with no air bubbles Pumice : volcanic glass with lots of air bubbles, called vesicles If either of these samples are ground to a powder, they look like volcanic ash.

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Additional terms Tuff : pyroclastic material fused into a solid rock As tephra settles, partially solidified particles will weld together as they cool. Volcanic ash : dust size tephra Porphyry : a rock with large crystals (called phenocrysts ) in a matrix of fine grained crystals A slowly cooling magma begins forming crystals. If suddenly released to the surface, the remaining magma will cool quickly. Large crystals seem to float in a sea of fine grained crystals.

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large crystals in fine grained matrix

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Solidification As a magma begins to solidify, not all minerals crystallize at the same time. Each mineral has its own melting temperature. As a magma cools, some minerals start to crystallize before others. A few points on melting (and solidification) temperature: 1. Mafic minerals generally melt at higher temperatures than felsic minerals. 2. At higher pressure, higher temperatures are needed to melt the rock. 3. If water is present, the rock will melt at a lower temperature. This is illustrated in a textbook figure on the next slide.

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A rock will melt more easily at higher temperature lower pressure higher water content higher felsic mineral conten t

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Solidification One of the first to conduct laboratory studies of magma and solidification was a man named Bowen, who discovered strange behavior that is now known as Bowen’s Reaction Series . Bowen’s Reaction Series refers to the sequence of minerals that crystallize from a magma as it cools, and reactions that take place along the way.

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high temperature low temperature olivine pyroxene Bowen’s Reaction Series There are two typical reaction pathways. We’ll start with a pathway where the first mineral to start crystalizing is olivine. As the magma cools further, something odd happens. Rather than starting to crystallize pyroxene, the magma reacts with the olivine, and the olivine is converted into pyroxene.

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high temperature low temperature pyroxene amphibole biotite This stepwise reaction sequence led to calling this pathway the discontinuous reaction series . Bowen’s Reaction Series With more cooling, the magma reacts with the pyroxene to produce amphibole. Finally, the remaining magma reacts with the amphibole to produce biotite.

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high temp low temp Bowen’s Reaction Series Now for the second typical pathway. This one starts with Ca-rich plagioclase as the first mineral to crystalize. As the magma cools further, the magma reacts with the Ca-plagioclase to gradually increase the Na content. Ca-plagioclase Na-plagioclase The gradual transition from Ca to Na rich plagioclase is called the continuous reaction series .

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high temp low temp Bowen’s Reaction Series At lower temperature, crystals begin to form in a manner we are more accustomed to expect – K-feldspar begins to crystalize, then muscovite, and finally quartz, without reacting with the residual magma. K-feldspar muscovite quartz

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high temperature low temperature olivine pyroxene amphibole biotite Ca-plagioclase Na-plagioclase discontinuous reaction series continuous reaction series Putting it all together K-feldspar muscovite quartz

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The same idea using a figure from the book.

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If magma reacts with minerals like olivine to convert them into other minerals as the magma cools, how is it possible to ever end up with olivine in an igneous rock? There are several possible means by which minerals early in Bowen’s Reaction Series might be preserved and someday appear in a rock outcrop. First, as minerals crystallize and settle to the bottom of a magma chamber, they may become covered and isolated from the magma. This would result in a layering effect where, in a mafic magma, olivine would be found at the bottom, then pyroxene, then amphibole, and finally biotite at the top. This layering is found in some plutons and sills, but not in all.

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A second possibility is that after a magma chamber has partially solidified, the magma migrates into a new region leaving behind the crystallized minerals. Click through the next 8 slides to see this phenomenon illustrated. (don’t click too fast or you will miss some animation affects)

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Solidifying magma in this region is not in direct contact with minerals back in the original chamber

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The final possibility is slow cooling followed by faster cooling where there is insufficient time for the magma and existing crystals to react, so they are preserved in the final rock.