Volcanoes Chapter 8

Volcanic Eruptions – Objections  Distinguish between non-explosive & explosive volcanic eruptions.  Explain how the composition of magma determines the type of volcanic eruption that will occur.  Classify the main types of lave & volcanic debris. Section 1

Volcanic Eruptions  Think about the force of the explosion produced by the first atomic bomb used in World War II. Now imagine an explosion 10,000 times stronger, & you get an idea of how powerful a volcanic eruption can be. This was the size of the explosion that occurred in Indonesia when the Krakatau volcano erupted in The eruption of Krakatau was so powerful that it was heard 4,000 km away & killed 36,000 people.  Few volcanoes give rise to explosive eruptions, Most eruptions are of a non-explosive variety. Section 1

Non-explosive Eruptions  Lava flows come from non-explosive eruptions. Lava flows – rivers of red-hot lava. Relatively calm outpourings of lava can release a huge amount of molten rock.  In non-explosive eruptions Lava can flow in continuous stream quietly from a crater. It can flow many kilometers before it finally cools & hardens.  Sometimes non-explosive eruptions can spray lava into the air. Section 1

Explosive Eruptions In an explosive volcanic eruption:  clouds of hot debris & gases shoot out from the volcano, often at supersonic speeds. molten rock is blown into millions of pieces that harden in the air. The dust-sized particles can circle the globe for years in the upper atmosphere, while larger pieces of debris fall closer to the volcano.  NO lava flows  can blast millions of tons of solid rock from a volcano. A volcano may actually shrink in size. Section 1

Cross Section of a Volcano Explosive or non-explosive, all volcanoes share the same basic features.  Magma = the hot liquid that forms when rock partially of completely melts, the driving force that creates volcanoes.  Vents = a hole or crack in the Earth’s crust through which magma rises to the surface.  Lava = magma that erupts onto the Earth’s surface Section 1

Cross Section of a Volcano: Cont.  Pyroclastic material = magma & fragments of rock that are ejected into the atmosphere during a violent volcanic eruption.  Volcano = a mountain that forms when lava or pyroclastic material builds up around a volcanic vent Section 1

Magma  The composition of the magma determines whether a volcanic eruption is non-explosive, explosive, or somewhere in between. Water  A volcano is more likely to erupt explosively if its magma has a high water content. As water increases pressure increases the greater the chances are that a violent explosion will occur. Section 1

Magma: Cont. Silica   % of silica = explosive eruptions. Silica-rich magma has a thick, stiff consistency. It flows slowly & tends to harden in the volcano’s vent. This plugs the vent, resulting in a buildup of pressure as magma pushes up from below. Thick magma also prevents water vapor & other gases from easily escaping.   % of silica = thinner, runnier consistency. Gases escape this magma more easily, making it less likely that explosive pressure will build up. Section 1

What Erupts from a Volcano?  Non-explosive eruptions produce mostly lava.  Explosive eruptions produce mostly pyroclastic material.  Over many years, a volcano may alternate between eruptions of lava & eruptions of pyroclastic material.  Eruptions of lava & pyroclastic material may also occur as separate stages of single eruption event. Section 1

What Erupts from a Volcano? Cont. Lava  Blocky Lava – is cool, stiff lava that cannot travel far from the erupting vent. It forms jumbled heaps of sharp-edged chunks.  Pahoehoe – flows slowly, like wax dripping from a candle, forming a glassy surface with rounded wrinkles.  Aa – This slightly stiffer lava pours out quickly & forms a brittle crust. The crust is torn into jagged pieces as the molten lava underneath continues to move.  Pillow Lava – forms when lava erupts underwater. Section 1

What Erupts from a Volcano? Cont. Pyroclastic Material  Volcanic Blocks – largest; consist of solid rock.  Volcanic Bombs – large blobs of magma that harden in the air. More than 64 mm across. Could be red hot when it hits the ground.  Lapili – pebble-like bits between 2 to 64 mm in diameter. Could be red hot when it hits the ground.  Volcanic Ash – particles less than 2 mm across. It forms when the gases in stiff magma expand rapidly & the walls of the gas bubbles explode in tiny glasslike slivers. Section 1

Volcanic Eruptions - REVIEW 1. Is a non-explosive volcanic eruption more likely to produce lava or pyroclastic material? Explain. 2. If a volcano contained magma with small proportions of water & silica, would you predict a non-explosive eruption or an explosive one? Why? 3. Pyroclastic material is classified primarily by the size of the particles. What is the basis for classifying lava? Section 1

Volcanoes’ Effects on Earth - Objectives  Describe the effects that volcanoes have on Earth.  Compare the different types of volcanoes. Section 2

Volcanoes’ Effects on Earth  The effects of volcanic eruptions can be seen both on land & in the air. Heavier pyroclastic materials fall to the ground, causing great destruction, while ash & escaping gases affect global climatic patterns. Volcanoes also build mountains & plateaus that lasting additions to the landscape An Explosive Impact  Ash can block out the sun for days over thousands of km 2. It can blow down trees & buildings & can blanket nearby towns with a fine powder. Section 2

An Explosive Impact: Cont. Flows  Clouds of hot ash can flow rapidly downhill like an avalanche, choking & searing every living thing in their path.  Large deposits of ash can mix with rainwater or the water from melted glacier during an eruption. With the consistency of wet cement, the mixture flows downhill, picking up boulders, trees, & buildings along the way. More powerful than ordinary rivers, these mudflows move swiftly & cause immense damage. Section 2

An Explosive Impact: Cont. Fallout  As volcanic ash fails to the ground, the effects can be devastating. Buildings may collapse under the weight of the ash Ash can dam up rivers, resulting in massive floods. Ash can smother crops, causing food shortages & loss of livestock. Section 2

An Explosive Impact: Cont. Climatic Fluctuation  In eruptions, volcanic ash, along with sulfur-rich gases, can reach the upper atmosphere. As the ash & gases spread around the globe, they can block out enough sunlight to cause the average global surface temperature to drop noticeably. The lower average temperatures may last for several years, bring wetter, milder summers & longer, harsher winters. Such changes in climate can cause worldwide food shortages that result in starvation & disease. Section 2

Different Types of Volcanoes Three basic types of volcanoes:  Shield Volcanoes = built out of layers of lava from repeated non-explosive eruptions. Because the lava is very runny, it spreads out over a wide area.  Cinder Cone Volcanoes = small volcanic cones made from moderately explosive eruptions.  Composite Volcanoes = They form by explosive eruptions of pyroclastic material followed by quieter outpourings of lava. The combination of both types of eruptions from alternating layers of pyroclastic material & lava. Section 2

Craters & Calderas  Crater = a funnel-shaped pit at the top of the central vent in most volcanoes  A crater’s funnel shape results from explosions of material out of the vent as well as the collapse of material from the crater's rim back into the vent.  Caldera = forms when a magma chamber that supplies material to a volcano empties & its roof collapses. This causes the ground to sink, leaving a large circular depression. Section 2

Lava Plateaus  The most massive outpouring of lava do not come from individual volcanoes.  Most of the lava on Earth’s continents erupts from long cracks, or fissures, in the crust.  In this non-explosive type of eruption, runny lava pours form a series of fissures & may spread evenly over thousands of square kilometers.  The resulting land form is known as lava plateau. Section 2

Volcanoes’ Effects on Earth – REVIEW 1. Briefly explain why the ash from a volcanic eruption can be hazardous. 2. Why do cinder cone volcanoes have narrower bases & steeper sides than shield volcanoes? 3. Briefly describe the difference between a crater & a caldera. Section 2

What Causes Volcanoes? Objectives  Describe the formation & movement of magma  Identify the places where magma forms  Explain the relationship between volcanoes & plate tectonics  Summarize the methods scientists use to predict volcanic eruptions. Section 3

What Causes Volcanoes?  Scientist have learned a great deal over the years about what happens when a volcano erupts. Many of the results are dramatic & immediately visible.  Unfortunately, understanding what causes a volcano to erupt in the first place is much more difficult. Scientists have no way of seeing firsthand what is going on deep within the Earth. They must rely on models based on rock samples & other data that provide insight into volcanic processes. Section 3

The Formation of Magma  The key to understanding why volcanoes erupt is understanding how magma forms.  All volcanoes begin when magma collects in pockets in the deeper regions of the Earth’s crust & in the uppermost layers of the mantle.  Although hot & pliable, the rock of the mantle is considered a solid. But the temperature of the mantle is high enough to melt almost any rock, so why doesn’t it melt? The answer has to do with pressure. Section 3

The Formation of Magma: Cont.  Rock melts & forms magma when the temperature of the rock increases or when the pressure no the rock decreases. Because the temperature of the mantel is relatively constant, a decrease in pressure is usually what causes magma to form.  Once formed, the magma rises toward the surface of the Earth because it is less dense than the surrounding rock.  Not all magma makes it all the way to the Earth’s surface to form a volcano. Stiff magma often cools & solidifies while still in the Earth’s crust. Section 3

Where Volcanoes Form  The locations of volcanoes around the globe provide clues to how volcanoes form. A large number of the active volcanoes lie on tectonic plate boundaries. Why is that do you think?  These boundaries are where the plates either collide with one another or separate from one another. It is easier for magma to travel upward through the crust at these boundaries.  The plate boundaries surrounding the Pacific Ocean have so many volcanoes that these boundaries together are called the Ring of Fire. Section 3

Where Volcanoes Form: Cont. When Tectonic Plates Separate  divergent boundary – when two tectonic plates separate & move away from each other  Rift = a zone of thin, fractured lithosphere that forms between tectonic plate as they separates  Mantle material then rises to fill in the gap. Because the mantle material is closer to the surface, the pressure on it becomes less. This decrease in pressure causes the mantle rock below the rift to partially melt & become magma. Section 3

Where Volcanoes Form: Cont. When Tectonic Plates Collide  Convergent boundary – the place where two tectonic plate collide  Subduction – the movement of one tectonic plate under another.  Ocean crust contains water, which lowers the melting point of rocks it comes in contact with. As the descending oceanic crust scrapes past the continental crust, it sinks deeper into the mantle, getting hotter & hotter. As it does so, the pressure on the oceanic crust increases as well. Section 3

Hot Spots  Not all magma develops along tectonic plate boundaries. For example, the Hawaiian Islands, some of the most well-known volcanoes on Earth, are nowhere near a plate boundary. The volcanoes of Hawaii & several other places on Earth are caused by hot spots.  Hot Spots = places within tectonic plates that are directly above columns of rising magma, called mantle plumes, that begin deep in the Earth, possibly at the boundary between the mantle & core. Section 3

Predicting Volcanic Eruptions  Scientists classify volcanoes based on their eruption histories & on how likely it is that they will erupt again.  Extinct volcanoes = those that have not erupted in recorded history & probably never will again.  Dormant volcanoes = those that are not currently erupting but have erupted in recorded history.  Active volcanoes = those that are erupting or that show signs of erupting in the very near future.  When it comes to predicting eruptions, the dormant & active keep scientists guessing Section 3

Predicting Volcanic Eruptions: Cont.  However, scientists have found certain clues that reveal when a volcano is likely to erupt. Small earthquakes as the magma within them moves upward & causes the surrounding rock to shift. Measurements of a volcano’s slope. The outflow of volcanic gases from a volcano. Some of the newest methods scientists are using rely on satellite images. Many of these images record infrared radiation, which allows scientists to measure changes in temperature over time. Section 3

What Causes Volcanoes? REVIEW  How does pressure determine whether the mantle is solid or liquid?  Describe a technology scientist use to predict volcanic eruptions. Section 3