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UNIT 12 IGNEOUS ACTIVITY (Chapters 4 & 5) Study Guide (Revised 7/12) UNIT 12 HOMEWORK WEB HIT HOMEWORK - part 1: ONE WRITTEN PARAGRAPH from any selected unit web hit site VIDEO WEB HIT HOMEWORK – part 2: ONE WRITTEN PARAGRAPH from any selected unit video site For any Unit Web Hits and Unit Web Videos, go to the “DMC HOME” website; in Search box –type “Geology”, select “Vernon Kramer”, scroll down to GEOL 1303, select “Syllabus”, select “Web Hit Links”, later then the select “Video Web Hits”, click on icon of interest for web sites OR: go to DMC Home website, select “Degrees, Certificates, Courses”, scroll down to Natural Sciences and select “Geology”, select “Faculty Listings”, select “Walter Vernon Kramer”, find “Geol 1303”, select “Syllabus”, and there you can find the” web hit links” click on icon of interest for web sites and then to “Web video hits”, click on icon of interest for video site [IF NONE OF THE WEB SITES COME UP, YOUR COMPUTER PROBABLY NEEDS TO BE REBOOTED (RESTARTED) [Please note that our book gives a much more detailed and complex presentation of igneous rocks and igneous activity than I will with this presentation. Also, this book interchanges the two terms silicic and felsic. The term felsic is derived from feldspar and silica( quartz).]

Earth’s Hot Interior - The original Earth became very hot during it’s formation. This heat was generated by:

1. Impacts of meteorite 2. Compression caused by the gravity of thousands of miles of overlying rock layers 3. Heat generated by radioactive decay of elements in Earth’s mantle and core.

- All of this heat, over time, allowed the Earth to differentiate (form concentric layers) resulting in a rocky crust and mantle and an iron core.

- As Earth’s molten surface began to cool, gas and water vapor escaped from within the Earth.

Rocks found of Earth’s Surface - Our moon is the product of a glancing collision between a Mars-sized planet and early Earth. - At one time all of the Earth’s surface was covered by igneous rocks (like that of the early moon). - The erosion of Earth’s surface by the hydrologic system produced sedimentary rocks that now cover

most of these igneous rock surfaces. -- Although there are still areas of exposed igneous rocks on continents, most of today’s “exposed”

igneous rocks are found on ocean floors as oceanic crust.

Continual Outpouring of Igneous Rocks on Earth -Unlike our moon, Earth’s interior is still very hot and includes an inner core of hot, liquid iron and hot

solid iron. - Because of Earth’s remaining heat, we should expect to see continual generation of igneous rocks for

billions of years to come.

Some General Igneous Terms (Review) - Igneous rock is formed by the cooling of molten minerals (remember that minerals make up rocks). - Magma: is molten rock below the Earth’s surface that contains suspended crystals and dissolved

gases. - Lava is molten rock that has reached the Earth’s surface. - Extrusive igneous rocks form from magma that solidifies after reaching the surface of the Earth. Gas

bubbles are commonly found within this type of rock. - Intrusive rocks form from magma that solidified below the Earth’s surface. Slow cooling produces

large mineral crystals while quicker cooling forms smaller crystals.

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- Magma is less dense (lighter) than its solid rock phase because its molecules are further apart. This lower density will cause the magma to rise toward the surface (like the salt domes).

Geologic Temperatures and Gradients - The Earth is relatively cool on its surface but gets warmer as you move inward toward the Earth’s

center. - Geothermal gradient: represents the increase in temperature with depth into the Earth - Assuming a 750F Earth’s surface , the South Texas geothermal gradient is 150F/ 1,000 ft depth. - At 15,000 feet, the South Texas rocks reach temperatures of +3000 F - So the geothermal gradient depends upon your location. In many parts of Earth, the temperature will

increase at a rate of 1230F /mile depth - Not surprisingly, the higher geothermal gradients are found closer to volcanoes and magma

intrusions. The closer to an active or recent volcano or magma, the higher the surrounding temperatures will become. This is why we have geysers and hot springs.

Geothermal Temperatures vs. Earth’s Crust - The higher geothermal temperatures are found closer (shallower), under the oceanic crust, because

the oceanic crust is a thinner crust. - But in regards to continental crust, the higher geothermal temperatures are generally much deeper

because the continental crust is thicker.

Geothermal temperatures - The coldest and deepest geothermal temperatures in rocks are those found within an oceanic plate as

it goes down in the subduction (convergent) zone. Geologic Pressures and Gradients - Not only does temperature increase as you go deeper into the Earth, the surrounding rock pressure

also increases. This is caused by the weight of the overlying rocks and water. - Geopressure gradient: increase of pressure with depth into the Earth Geopressure and Temperatures - An increase in geopressure can increase a rock’s melting point (higher pressure = higher melting

temperature or lower pressure = lower melting point). Thus high geopressure can prevent rocks from melting, even at very high temperatures.

- The state of the rocks (molten or solid) found within Earth’s interior depends upon Earth’s heat and pressure. This is why Earth has liquid, plastic and solid internal layers.

Many of us spend half our time wishing for things we could have if we didn't spend half our time wishing. Alexander Woollcott

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FYI: Seismic studies show that Earth has a mantle of rocks and minerals. We now know that from 50-300 kilometers depth, olivine, pyroxene and some garnets are the most common minerals. From 300-660 kilometers, garnets and spinel become common. Below 600 kilometers wustite and perovskite become common. The mineral types change because of different temperatures and pressures. Diamonds Come From Volcanoes - Diamonds (which are crystallized carbon) are created within the Earth’s asthenosphere, more than

180 kilometers below Earth’s continental crust.

Kimberlite pipes - Diamonds are brought to the surface by “kimberlite pipes” (a special type of explosive volcano). - It is not uncommon to find a garnet crustal inside a diamond crystal. Depths of Melting - Magmas can actually form at depths of thirty (30) miles under an oceanic crust - Under continental crust magmas may form at depths of a hundred miles Magma Composition and Melting - You do not need to melt 100% of a rock to get a magma - Partial melting: only 10%-20% of minerals melting are needed to produce a fluid magma. - Thus most magmas are a mixture of solid minerals, liquid minerals and gases. FYI: Geologists use a chart called the “Bowens Reaction Series” that defines which minerals in a

magma crystallize first to last, depending upon the magma and temperatures. Every morning you are handed 24 golden hours. They are one of the few things in this world that you get free of charge. If you had all the money in the world, you couldn't buy an extra hour. What will you do with this priceless treasure?

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Decreasing Melting Temperatures

fracture melting - Rock-melting temperatures can be decreased in a convergent (subduction) zone by: 1) Decreasing the pressure on hot solid rock BY FRACTURES (pressure relief melting) will

lower the normal melting point of a mineral which can result in a magma. (Used a pressure cooker as an example.)

2) Adding water to hot solid rock can lower the normal melting point of a rock which can result in a magma.

- Rock-melting temperatures can be decreased in a divergent or rift zone by: 1) The hot crust will rise and fracture. 2) This fracturing decreases the pressure on the hot rock which in turn will lower the normal

melting point of a mineral which finally produces a magma (pressure relief melting) Gases and Magmas - Gases within magmas can equal 5% of volume of magma and can cause huge explosions:

1) Water vapor (a green house gas) forms about 70% of normal magma gases. - This water can be juvenile water from the Earth’s formation or as connate water trapped in a

subducted plate as it penetrates the mantle

black smoker - Black smokers are deep ocean vents of hot water and gases associated within divergent zones,

but this water is too shallow to produce a magma. - Recently, cooler white smokers have also been discovered

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2) Carbon dioxide (a green-house gas) forms about 20% of magma gases. - Carbon dioxide can also be deadly when first accumulated at the bottom of volcanic lakes –

then expelled. Gave an example of carbon dioxide flowing upward from Lake Nygos, a dormant volcano in Cameroon, West Africa. This event in 1986 resulted in 1,700 deaths.

3) The remaining 10% of magmatic gases include acids and sulfur gas. - Acid can “alter” black scoria rocks into red scoria rocks. - Sulfur gas can cool and form sulfur deposits. Showed an example of a sulfur mine off a

volcano in Kiwah Ijen, Java, Indonesia Environments Associated with Magmas - Magmas are formed in three geologic environments:

1) Tectonic divergent plate boundaries (mid-oceanic ridges and continental rifts). 2) Tectonic convergent plate boundaries 3) Mantle plumes or hotspots that rise from Earth’s liquid iron core

- Earth’s internal heat is the prime driving force for igneous activity Types of Minerals Associated With Tectonic Environments - Convergent Zone Minerals and Magmas

- Quartz and orthoclase (which are silicic or felsic minerals rich in silica) are directly associated with areas of continental crust and are available for melting.

- Sand and ooze (silicic or felsic minerals of quartz and orthoclase) are often found on top of subducted oceanic crust which are also available for melting.

- Divergent Zone and Hotspot Minerals and Magmas - At divergent zones and hot spots, only mantle minerals (mafic minerals rich in iron and magnesium

and low silica) are available for melting within the mantle. - Mafic magma creates oceanic crust (and basalt flows on land). Tectonic Environments and Magma Temperatures - “Lower temperature” convergent zones will usually produce a different magma than the “higher

temperature” divergent zones and hot spots. Igneous rocks with quartz are termed silicic (felsic) rocks. These rocks melt at lower temperatures

and are associated with convergent (subduction) zones. Felsic or silicic magma is produced by partial melting of a continental or oceanic crust [Felsic = lower temperatures @ convergent zone – which produces continental crust rocks] Igneous rocks without quartz are termed mafic rocks. These rocks melt at higher temperatures and

are associated with divergent zones and hot spots. - Divergent zones and hot spots produce mafic magmas or magmas from the mantle - (Mafic is from Ma = magnesium and fic – ferric or iron – thus rich in magnesium and iron) - Repeat - mafic magma is produced by the partial melting of the Earth’s mantle. Factors Affecting the Creation of Igneous Rocks (a review): 1) Heat and density 2) Depth of melting 3) Pressure at melting zone 4) Presence of water and gases 5) Mineral content of source rock 6) Tectonic environments

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Review of Felsic Magmas - Silicic (felsic) magmas

1) Silicic (felsic) rocks form at convergent plate boundaries. 2) Contain 65%-70% silica (quartz rich) and melt at 800 degrees C 3) Associated with intense, explosive volcanic activity 4) Silicic (felsic) rocks such as pumice, obsidian, rhyolite, granite are less dense (lighter) than

the dark-colored mafic rocks. 5) Quartz and orthoclase are the most common minerals thus silicic (felsic) rocks are lighter in

color. 6) Silicic (felsic) magmas can form very viscous lavas.

- Viscosity: the resistance of a liquid to flowing. The greater the silica (felsic) content, the greater the

viscosity of a lava, the slower the lava will flow. Used cold honey as an example of high viscosity. Review of Mafic Magmas -Mafic magmas 1) Mafic magma from the mantle is found at divergent plate boundaries, rifts and hotspots. 2) Contain 50% silica (quartz poor) and melt at 9000-12000 C 3) Volcanic activity is relatively quiet compared to felsic volcanoes. 4) Form very dark-colored igneous rocks 5) Form igneous rocks such as scoria, basalt (lava) and gabbro that is more dense than the light-

colored felsic rocks 6) Form very liquid-like lavas which can flow almost like water (very low viscosity) 7. Again, mafic volcanic activity is relatively quiet Geologic Features that Produce Extrusive Igneous Rock - We will spend most of our time discussing extrusive igneous rocks since they are the most visible. - Magma can reach the Earth’s surface by two means: a) via volcanoes or b) via fissures. - Volcano: cone-shaped hill formed usually by extrusive igneous rocks - Fissure: a long open fracture or fault that allows magma to pour out onto the Earth’s surface in large

volumes - Some igneous rocks are the result of being exploded from volcanoes and are called pyroclastics

(pyro = fire & clastic = rock Pyroclastic Rocks - Pyroclastic: solid material (igneous rock) blown from a volcano during an explosive eruption A) Types of pyroclastics fragments: 1) Volcanic ash: volcanic dust is represented by tiny, sharp-edged dust particles violently ejected

from volcanoes (dangerous to inhale in large quantities) - The wind can carry volcanic ash long distances to form thick ash falls. - When water is added to this ash, the ash can become very hard. - Suspended volcanic ash in the atmosphere provides beautiful red sunsets. - 30 MY uranium-enriched volcanic ash from West Texas is responsible for the South Texas uranium deposits Talent is like electricity – we don’t understand electricity. We just use it. – Maya Angelow

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Pele’s hair 2) Volcanic fibers: (Pele’s hair) long, glassy fibers of igneous rock produced from volcanic

explosions. Pele’s tears are small tear-shaped particles associated with volcanic fibers. 3) Volcanic Cinders: sand to gravel size explosive igneous rocks (scoria and pumice)

Volcanic bombs 4) Volcanic Bombs: volcanic rocks that are larger than gravel size and are tapered from passage

through air (while still molten) Lava Flow Rocks B) Type of rocks will depend upon type of lava flows: - 1) Lava flows onto land: - Discussed the “sport” of lava walking

Pahoehoe Aa a) Pahoehoe: rock formed by “fast-flowing” lava (typically has a ropy crust) b) Aa: jagged, sharp-edged rock formed by relatively slow-moving, very viscous lava; aa is

also a famous crossword puzzle name for lava

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-Although aa flows can be somewhat slow moving, they are almost impossible to “steer” or stop c) Obsidian: a black, glassy silicic lava; famous for making arrowheads - 2) Lava flows into water:

Pillow lava Pillow lava (pillow basalt): a rock formed by lava that is extruded under water and forms an

ellipsoidal mass; most common along mid-oceanic rift zones - 3) Pyroclastic (lava) flows (through the air):

Pyroclastic cloud Ash flow or Pyroclastic Flow: rock formed from a hot “cloud” of volcanic ash and superheated

gases and rock that flows rapidly down the side of a volcano. Also called tuff and welded tuff: Volcanoes and Volcanic Activity Classified by the Shapes of Volcanoes (see figure) - There have been recorded more than 550 active volcanoes during historic time.

Volcano shapes - Volcano shapes: 1. Shield volcano: typical mafic volcano that is shaped as a very broad dome.

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- Basalt lavas can flow like water down the volcano slope, making it almost impossible to build up a steep cone; shaped like a very broad dome - The volcanoes of Hawaii provide many excellent examples of shield volcanoes.

Volcano near Uvalde TX - Local examples include relatively small, 78 MY old basalt volcanoes are found in a belt that

extends from Uvalde TX to Austin TX - Shield volcanoes can erupt many times. 2. Cinder cone (spatter cone): mafic, small, (less than 600 feet high) cone-shaped hills - These cones are built up from mostly cinders of scoria that erupts from a volcanic vent. - Very little lava is associated with these cones. - These cones are generally produced within a single eruption event - Don’t forget to go “volcano boarding” on their slopes 3. Composite volcano: silicic, typical steep-sided cone with a central crater composed of alternating layers of lava and pyroclastic rock (also called a stratovolcano)

Composite volcano with Ash Flow or pyroclastic flows Pyroclastic flows through the air again rock formed from a hot “cloud” of volcanic ash and

superheated gases and rock that flows rapidly down the side of a volcano. This type of eruption can occur many times throughout the life of the volcano..

- Example includes the 1996 eruption on Montserrat. A pyroclastic flow formed from a hot expelled “cloud” of volcanic ash and superheated gases and flowed rapidly down (60 – 120 m/hr) the side of a volcano. As the flow moves down the valley, it will “coat” everything with a layer of hot lava rock. Also played a song about this volcano.

- Mt. Pinatubo of the Philippines is an example of a composite volcano that caused

considerable destruction. The ash from this cloud almost brought down two commercial airplanes.

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Pompeii - An extreme example was the 70 AD eruption of Mt. Vesuvius, Pompeii in Italy. Here a

large pyroclastic cloud swept over the ancient city. Today, we find human molds of victims that were caught up and enveloped by the flow.

That which a person does not understand, he tends to reject. - Mt. St. Helens in Washington State is a composite volcano that erupted violently in 1980

which resulted in numerous deaths. More than a dozen dormant volcanoes like Mt. St. Helen lie along the CA, OR and WA coast, because of an underlying subducted plate.

- The active composite volcano Popocatepetl is adjacent to Mexico City

4. Caldera: (mafic or silicic) a very large, circular depression caused when the summit of a volcano subsides or explodes away.

Caldera formations - A caldera can form as a result of a magma exiting the side of the volcano (mafic) like Hawaii.

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Super volcanoes - A caldera can also form as a result of a large, explosive eruption (silicic) often referred to as a

super-volcano. - Examples of super volcanoes include Mt. Toba, Mt. Tambora and Krakatoa – all of

Indonesia. - The explosion of Mt. Toba in Indonesia almost destroyed the human race 75,000 years ago. It

ejected more than 670 cubic miles of volcanic ash around the world. The resultant caldera is 65 miles long and 25 miles wide.

- The explosion and resultant caldera of Mt. Tambora produced the 1816 “year without

summer” in the Eastern US, which lead to massive crop failures. - The Krakatoa eruption of 1883 was the “last super volcano explosion. It destroyed an entire

island. This explosion also produced that loudest sound ever heard on Earth (heard 2,000 miles away)

- There have been several super volcano explosions in the US, including Yellowstone.

Yellowstone have had three massive eruptions, each about 200,000 years apart. The last one occurred more than 200,000 years ago. Yellowstone sits over a large pool of deep magma that is causing the area to rise and fall. Some are predicting another super volcano explosion here “soon”.

FYI: The US atomic secrets and working labs are located at Los Alamos, NM. Los Alamos is

sited on the lip of the Valles Caldera super volcano that had a minor eruption 130,000 years ago.

5. Volcanic dome (resurgent dome): silicic, volcanic rock pushed up from the bottom of a

caldera - Examples include Crater Lake in Oregon and Mt. St. Helens Flood Basalts 6. Fissures (flood basalts): mafic, continuous massive flows of basalt from fault zones - Mafic magma can be extruded from very long cracks or fissures to “flood vast areas with basalt” thus the term flood basalts.

Columbia River flood basalts - The Columbia River flood basalt of Washington could have covered the entire US under 36 feet of basalt. - The flood basalt flow in Siberia has over a half-million cubic miles of basalt rock.

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- The correlation between some flood-basalt episodes and extinctions may implicate volcanism for the mass extinctions of plants and animals in the pre-historic past. - Flood basalts would emit vast quantities of poisonous gases and greenhouse gases into the atmosphere. Features associated with Older Igneous Activity

Shiprock Devil’s Pile Post - Volcanic neck: an eroded volcanic cone with the remaining intrusive exposed. - Examples would include the felsic Ship Rock of New Mexico and the mafic Devil’s Pile Post

of the Dakotas. - Columnar jointing: columns of rock created by polygonal cracks (contraction by cooling) found in mafic volcanic necks and in many mafic flows (sometimes can be found with silicic flows)

Columnar Jointing A man who has never gone to school may steal from a freight car, but if he has a university education he may “steal” the whole railroad. --Franklin Delano Roosevelt

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Intrusive Rock Terms - Plutons: igneous rock bodies that were formed below the Earth’s surface - Pluton classification is based is based on size, shape and relationship to rocks that they are intruding. 1) Batholith: largest intrusive, must cover a surface area greater than 36 sq. mi. with an even

larger mass below the surface 2) Stock: smaller intrusive body whose surface area is less than 36 sq. mi. 3) Dikes are narrow, tabular intrusions that intrude and “cut across” older rocks. 4) Sills are narrow tabular intrusions that are intruded parallel to older overlying rocks

(generally follows bedding planes).

Types of intrusives - A famous Texas batholith is the 1.1 billion-year old intrusive called the Enchanted Rock, west of

Austin, near Fredrickburg. Solar System Volcanoes - Largest known volcano is Olympus Mons on Mars, at more than 66,000 feet high. - Jupiter’s moon Io has many, currently active volcanoes. - Saturn’s tiny moon of Encelades has active volcanoes. - Neptune’s moon of Triton also has volcanoes. - It is now believed that Venus may have active volcanoes, with one more than 30,000 feet tall. Mexico - In 1943, the volcano Paricutin grew from a crack in a cornfield to a large killer volcano in Mexico. - In 1949, a pyroclastic cloud from that volcano killed more than 1,000 locals attending a fiesta. Taft Volcano? - The volcano-like structure near Taft, TX is the result of a gas well blowout that caught fire in 1939.