Chapter 6: Volcanoes and Igneous Rocks

Volcanoes and Volcanic Hazards

Volcano –A vent through which lava, solid rock debris, volcanic ash, and gases erupt from Earth’s crust to its surface.

Lava –Molten rock that reaches Earth’s surface.

Magma –Molten rock, which may include fragment of rock, volcanic glass and ash, or gas.

Explosive and non-explosive volcanoes depend of the physical properties of their magma.

Types of eruptions

NON – EXPLOSIVE

Flood basalts or basalt plateaus –Fissures that can produce vast, flat lava plains.

Hawaiian eruptions –consists of very runny lava that flows easily from a volcanic vent.

EXPLOSIVE

Strombolian eruptions –are more explosive than Hawaiian eruptions, ejecting showers of lava and rock hundreds of feet in the air.

Vulcanian eruptions –much more explosive. The showers of lava can reach from hundreds of feet to about 20km.

Plinian eruptions - The most violent eruptions (called Plinian, after Pliny the Elder a Roman scholar that die during the eruption of Mt. Vesuvius). They produce ash columns that reach into the stratosphere (20 km high or more).

TYPES OF VOLCANOES

Shield volcano –A broad, flat volcano with gently sloping sides built of successive lava flow. (Associated with Hawaiian eruptions)

Stratovolcano –volcano composed of solidified lava flows interlayered with pyroclastic material. Such volcanoes usually have steep sides that curve upward. (Associated with Strombolian, Vulcanian, and Plinian eruptions).

Pyroclastic flow – (pyro = fire, clast = pieces) Hot volcanic fragments (tephra) that are buoyed by heat and volcanic gases, and flow very rapidly.

VISCOSITY AND GAS CONTENT

What causes the diversity of eruption types? The answer lies in the type of magma. Two factors affect the magma: viscosity and gas content.

Viscosity –The degree to which a substance resists flow; a less viscous liquid is runny, whereas a more viscous liquid is thick.

If the magma is less viscous gases could escape easily, if the magma is more viscous gases cannot escape easily therefore we will have violent explosions.

MAFIC MAGMA – Dark in color. Less viscous (behaves like water) – Gases escape easily; magma is NON-EXPLOSIVE. MAFIC (Mainly Mg Magnesium and Iron, 50% Silica). Low viscosity, low gas content. Temperature of the magma 1100 Celsius.

FELSIC MAGMA – Light in color. More viscous (behaves like syrup) – Gases are trapped; magma is EXPLOSIVE. FELSIC (Mainly Feldspar Al Aluminum and K Potassium, 70% Silica). High viscosity, high gas content. Temperature of the magma 800 Celsius.

Pyroclasts - (From the Greek that means “fire broken”) Fragments of rock ejected during a volcanic eruption.

Tephra –All the ejecta from a volcano. They can range from a car-sized rocks to ultrafine volcanic ash.

Volcanic ash the smallest tephra.

Other volcanic features –

Crater –A funnel shape depression from which gas, tephra, and lava are ejected. Hundreds of meters in size.

Caldera –A large depression roughly circular, steep walled basin that could be several kilometers in diameter.

Crater Caldera

Volcanoes do not necessarily become inactive after a major eruption. So we have: active volcanoes (showing some sort of activity), dormant volcanoes (magma chamber is still hot, but there are no signs of activity on the surface), and extinct volcanoes (magma chamber has completely cooled off).

A small crater inside a caldera

Other volcanic features –

Geysers –intermittent eruptions of water and steam.

Fumaroles –Volcanic vents that emit only gas – usually water that is sometimes mixed with foul smelling sulfur compounds.

Thermal springs –when groundwater comes in contact with hot rock.

Hazards and prediction –

Primary effects –directly caused by the volcanic eruption.

Secondary effects –indirectly triggered by the eruption. Tsunamies, tremors, mudslides (lahar), debris avalanche.

Tertiary effects –long lasting or even permanent changes brought about by the eruption. Block river water, diversion of river water, climatic change. Climatic change due to the albedo effect.

Particles from the eruption go up onto the sky and remain there for some time (sometimes years). These particles will reflect the sun rays producing the Albedo Effect, which is the reflection of sun light away from the Earth, decreasing the temperature on Earth. Some volcanic explosions have decreased the Earth’s temperature by as much

of 5 degrees globally. Also as there is less sunlight coming towards the Earth, photosynthesis of plants also decreases. Remember the collision impact that killed the dinosaurs 65 million years ago? The impact of the meteorite cracked the other side of the Earth opening a big gap from where millions of cubic tons of pyroclastic material went into the Earth’s atmosphere causing the Albedo Effect. As the Earth became colder and less photosynthesis was being produced, millions of plants died and so did the herbivores and as the herbivores went so did the carnivores. The extinction of the dinosaur did not happen directly from the collision of the meteorite, but for the Albedo Effect that was caused by this collision and the dinosaur’s demise took maybe thousands of years.

Hazards and prediction –

Beneficial effects -Volcanic ash supplies the soil of beneficial nutrients.

Predicting eruptions

Signs –change on the shape of the volcano, vibrations, bulging. Gas emissions, changes in water temperature in crater lakes.

Volcanoes and climate –Eruption of Laki -Iceland

This eruption is closely tied in with the French Revolution. The eruption of this volcano in Iceland produced the Albedo Effect, that did three things: lowered the temperature in Europe in general, decreased the amount of sunlight going into the Earth, and also the increase of acidity in the rainfall (acid rain). These three processes initiated by the volcano Laki lead to diminish of food all around Europe. The lack of food

produced one the biggest political upheavals in history: The French Revolution.

Which kinds of eruption pose the greatest risk to humans? - Strombolian, Vulcanian, and Plinian eruptions.

Which pose comparatively low risk? - Flood basalts and Hawaiian eruptions.

Why do pyroclastic flows travel faster and kill more people than lava flows? – Because gases (ash) can travel faster than solids (lava).

What is the difference between a crater and a caldera? - Crater –A funnel shape depression from which gas, tephra, and lava are ejected. Hundreds of meters in size. Caldera -A large depression roughly circular, steep walled basin that could be several kilometers in diameter.

Why do explosive eruptions occasionally have global effects on the weather? – Explosive eruptions produce the Albedo Effect. Ashes and small particles can do did three things: lower the temperature around the world, decrease the amount of sunlight (diminishing photosynthesis), and also the gases from the volcano increase of acidity in the rainfall (acid rain). Large volcanoes have great effects that are felt all over the world.

Identify several different categories of volcanic eruptionsNON – EXPLOSIVEFlood basalts or basalt plateaus –Fissures that can produce vast, flat lava plains.Hawaiian eruptions –consists of very runny lava that flows easily from a volcanic vent.EXPLOSIVEStrombolian eruptions –are more explosive than Hawaiian eruptions, ejecting showers of lava and rock hundreds of feet in the air.

Vulcanian eruptions –much more explosive. The showers of lava can reach from hundreds of feet to about 20km.Plinian eruptions - The most violent eruptions (called Plinian, after Pliny the Elder a Roman scholar that die during the eruption of Mt. Vesuvius). They produce ash columns that reach into the stratosphere (20 km high or more).

Explain why stratovolcanoes tend to erupt explosively, whereas shield volcanoes tend to explode nonexplosively. –Strato volcanoes are fed by FELSIC magma and shield volcanoes are fed by MAFIC magma.

Describe volcanic features such as calderas, geysers, and fumaroles. - Caldera -A large depression roughly circular, steep walled basin that could be several kilometers in diameter. - Geysers –intermittent eruptions of water and steam. - Fumaroles –Volcanic vents that emit only gas – usually water that is sometimes mixed with foul smelling sulfur compounds.

Identify volcanic hazards and in which way are volcanoes beneficial - Primary effects –directly caused by the volcanic eruption. - Secondary effects –indirectly triggered by the eruption. Tsunamies, tremors, mudslides (lahar), debris avalanche. -Tertiary effects –long lasting or even permanent changes brought about by the eruption. Block river water, diversion of river water, climatic change. Climatic change due to the albedo effect.

Beneficial effects -Volcanic ash supplies the soil of beneficial nutrients.

What are the possible signs of a volcanic eruption? - Signs –change on the shape of the volcano, vibrations, bulging. Gas emissions, changes in water temperature in crater lakes.

Why and how rocks melt

Underneath every active volcano lies a reservoir of magma, called a magma chamber.

Understanding volcanism includes understanding how rocks melt to become magma. Fortunately, rocks can be melted artificially as well as naturally.

Rocks begin to melt between 800 C and 1000 C, but since rocks are composed of many minerals we could say that all of them will melt at 1200 C.

Two factors that affect the melting temperature are: pressure and the presence of water in the rock.

If you descend to a mine, it becomes apparent that the farther you go down, the hotter it gets.

How is it possible that the upper mantle is solid (plastic) at a temperature of 1450 C? The answer is that as pressure increases the melting temperature becomes greater.

The presence of water (or water vapor) also reduces the melting temperature of the rock.

Fractional melt –A mixture of molten and solid rock.

Fractionation –Separation of melted materials from the remaining solid material during the course of melting.

Magma and lava –Molten rock below the ground is called magma, when rock reaches the surface is called lava. A lot of this magma never reaches the surface, instead remains underground until it crystallizes and hardness to become an igneous rock.

Composition –Most magma is dominated by silicon, aluminum, iron, calcium, magnesium, sodium, potassium, hydrogen, and oxygen –Earth’s most abundant element. Silica (SiO 2 ) accounts for 45 to 75%. Between 0.2 to 3% of the magma is composed by gases (H 2 O, CO 2 ) despite their low abundance, gases, greatly influence the property of the magma.

Viscosity –Some lavas are fluid, like water, whereas others are very viscous like molasses. High silica content tend to make very viscous lavas and viscous lavas tend to erupt explosively.

Describe how temperature, pressure, and water content affect a rock’s melting point. – Rocks melt anywhere from 800C to 1000C, at 1200C rocks are completely melt.

Define fractional melting. – Separation of minerals during melting.

Identify three properties that distinguishes one magma from another. – Chemical composition (mafic vs. Felsic). Silica content (50% vs. 70%). Gas content (Low vs. High). Explosiveness (Low vs. High). Viscosity (Low vs. High). Magma temperature (1000C vs. 800C).

Why is not possible to assign a single melting temperature to rock? – Rocks are made of many minerals and each mineral has its own melting temperature.

How does water affect the melting temperature of a rock? – The more water its present, the less temperature its needed to melt the rock.

When can rock melt without an increase in temperature? – Decrease in pressure.

How does fractional melting separate different minerals from one another? All minerals have different melting temperature.

Cooling and crystallization

Whereas melting influences the properties of magma, cooling and crystallization influence the properties of igneous rock.

The rate of cooling determines how large the individual mineral grains in the rock will grow. Grain size affects the appearance or texture of the rock.

The composition of the magma determines the final mineral assemblage in the solidified rock.

Rate of cooling

Looking at igneous rocks you will see that some rocks have easily visible mineral grains, and others do not.

Rate of cooling divides igneous rocks into two main groups:

Volcanic rocks and Plutonic rocks.

Volcanic rocks – Extrusive rocks - form at Earth’s surface where lava contacts cooler air or water. Volcanic rocks solidify quickly so that large mineral grains have no time to form.

Plutonic rocks – Intrusive rocks - Form when magma crystallizes deep underground. This process is slow and therefore it gives the mineral grains to grow larger.

Crystallization –The process whereby mineral grains form and grow in a cooling magma (or lava).

Volcanic rock –An igneous rock formed from lava.

Plutonic rock –An igneous rock formed underground from magma.

Rapid cooling: volcanic rocks and their textures – Sometimes lava cools so rapidly that mineral grains do not have a change to form at all.

The resulting volcanic rock is not crystalline but glassy. More often mineral grains can be so small that they can only be seen under magnification. Fine grained texture is said to be aphanitic. Other rocks have another type of texture called porphyritic that happens when magma starts to crystallize at depth and grow large grains before it erupts, and these grains were erupted along with the liquid lava.

When erupting lava bubbles and cools quickly it forms pumice and scoria (gas/air holes are called vesicles). In basaltic lava the same process makes another rock called vesicular basalt.

Slow cooling: plutonic rocks and their textures – Magma cools slowly where minerals form large crystal that can be seen by the naked eye.

The coarse grained texture where grains are easily seen is called phaneritic. The vapor facilitates the growth of large crystals.

Chemical composition –

Felsic –Rocks that contain large amounts of silica (about 70% SiO 2 by weight). Felsic comes from feldspar and silica. (light colored rocks)

Mafic –Rocks that contain large amounts of dark colored minerals rich in Mg and Fe (about 50% SiO 2 ). Mafic comes from Mg and Fe. (dark colored rocks)

Intermediate –Rocks that contain about 60% SiO 2 . (intermediate in color).

What is the relationship between crystal size and the rate of crystallization? – Rapid cooling small crystals – Slow cooling large crystals.

Distinguish between volcanic and plutonic rocks. - Volcanic rocks – Extrusive rocks – Igneous rocks formed from lava. Form at Earth’s surface where lava contacts cooler air or water. Volcanic rocks solidify quickly so that large mineral grains have no time to form.Plutonic rocks – Intrusive rocks - Igneous rocks formed underground from magma. Form when magma crystallizes deep underground. This process is slow and therefore it gives the mineral grains to grow larger.What are the different textures of igneous rocks and how are they produced? - Rapid cooling: volcanic rocks and their textures –The resulting volcanic rock is not crystalline but glassy. More often mineral grains can be so small that they can only be seen under magnification. Fine grained texture is said to be aphanitic. Other rocks have another type of texture called porphyritic that happens when magma starts to crystallize at depth and grow large grains before it erupts, and these grains were erupted along with the liquid lava.When erupting lava bubbles and cools quickly it forms pumice and scoria (gas/air holes are called vesicles). In basaltic lava the same process makes another rock called vesicular basalt. Slow cooling: plutonic rocks and their textures – Magma cools slowly where minerals form large crystal that can be seen by the naked eye.The coarse grained texture where grains are easily seen is called phaneritic. The vapor facilitates the growth of large crystals.

Name two volcanic rocks and their plutonic equivalents. MAFIC Basalt (volcanic) Gabbro (plutonic). FELSIC Rhyolite (volcanic Granite (plutonic)

What is fractional crystallization? – Fractional crystallization is the separation of minerals during cooling/crystallization.

How do rocks with high silica content differ in appearance from those with low silica content? – Rocks with high silica content are lighter in color, whereas rocks with low silica content are darker in color.

What is the difference between fractional crystallization and fractional melting? – Fractional crystallization is the separation of minerals during cooling and fractional melting is the separation of minerals during melting.

Plutons and plutonism

Plutons –Any body of intrusive igneous rocks, regardless of size or shape. They always originate as magma in the underground, but they are exposed as plutons at the surface by erosion.

Batholith –A large irregularly shaped pluton that cuts across the layering of the rock into which intrudes.

Stock –smaller version of a batholith.

Laccolith -is an igneous intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. Lens shaped.

Dikes and sills

Dikes –Vertical magmatic intrusions

Sills –Horizontal magmatic intrusions

Volcanic pipe is the remnant of a channel that original fed magma to the volcanic vent; when exposed by erosion, it is called a volcanic neck.

What are some of the common types of plutons?Plutons –Any body of intrusive igneous rocks, regardless of size or shape. They always originate as magma in the underground, but they are exposed as plutons at the surface by erosion.Batholith –A large irregularly shaped pluton that cuts across the layering of the rock into which intrudes.Stock –smaller version of a batholith.Laccolith -is an igneous intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. Lens shaped.Dikes and sillsDikes –Vertical magmatic intrusionsSills –Horizontal magmatic intrusions

How do plutons become visible at Earth’s surface, if they are formed underground? - Erosion

Describe the most common plutonic formations.Plutons –Any body of intrusive igneous rocks, regardless of size or shape. They always originate as magma in the underground, but they are exposed as plutons at the surface by erosion.Batholith –A large irregularly shaped pluton that cuts across the layering of the rock into which intrudes.Stock –smaller version of a batholith.Laccolith -is an igneous intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. Lens shaped.Dikes and sillsDikes –Vertical magmatic intrusionsSills –Horizontal magmatic intrusions

Explain why volcanoes also create plutonic rocks and plutons. – Volcanoes have a mama chamber associated with them, as magma comes up starts to create all the plutons described above. Once these magma cools down, it creates all the plutonic rocks: gabbro, granite, and pegmatite.

Classify these eruptions according to their pyroclastic flows and ash plumes that extend into the stratosphere. – Most strongest Plinian, Vulcanian, Strombolian, Hawaiian Least strongest

Classify these eruptions according to their viscosity.- Most viscous Plinian, Vulcanian, Strombolian, Hawaiian Least viscous

What is the difference between a crater and a caldera? - Crater –A funnel shape depression from which gas, tephra, and lava are ejected. Hundreds of meters in size. Caldera -A large depression roughly circular, steep walled basin that could be several kilometers in diameter.

How geologists monitor volcanoes? - Signs –change on the shape of the volcano, vibrations, bulging. Gas emissions, changes in water temperature in crater lakes.

Melting of rocks increases by the presence of water.

What is fractional crystallization? – Separation of minerals during cooling/crystallization.

A-Mauna Loa and Mauna Kea

B –Mount Fuji

Which volcano has greater potential for an explosive eruption? B

Which volcano is being fed by high viscosity lava? B

Which rock A or B was cooled slowly, followed by rapid cooling B

Phaneritic texture Phorphyritic texturePlutonic rock Volcanic rock

Identify the following rocks. Which were cooled rapidly, which were cooled slowly, which are mafic, which are felsic?

1 GRANITE- FELSIC PLUTONIC ---- 2 RHYOLITE - FELSIC VOLCANIC ----- 3 and 4 Intermediate (didn’t study those rocks in the lab) ------ 5 GABBRO –MAFIC PLUTONIC ---- 6 BASALT – MAFIC VOLCANIC

Label this diagram: (dike, sill, batholith, volcanic neck, stock, xenoliths, laccolith)

How all these plutonic features will become exposed? Erosion of the units (softer material) around the plutonic features (harder material)

The Hawaiian Islands are composed of what type of volcanoes? – shield volcano

The degree to which a substance resists flow is known as _______. – viscosity

The composition of most magma is 40%-75% ________ by weight.

A) silicates

_______ igneous rocks are composed of dark-colored minerals rich in magnesium and iron.

B) Mafic

The smallest type of tephra is known as _______.

B) volcanic ash

In the figure below, which of the igneous intrusive bodies is parallel to pre-existing bedding?

D) sill

Which of the following terms best describes the lava in the figure below?

A) low viscosity

Molten rock that reaches the Earth's surface is known as _______.-lava

Define the following terms associated with igneous rocks:

Aphanitic phaneritic porpyritic glassy vesicular - Rapid cooling: Fine grained texture is said to be aphanitic. Other rocks have another type of texture called porphyritic that happens when magma starts to crystallize at depth and grow large grains before it erupts, and these grains were erupted along with the liquid lava. Glassy like obsidian.When erupting lava bubbles and cools quickly it forms pumice and scoria (gas/air holes are called vesicles). In basaltic lava the same process makes another rock called vesicular basalt. Slow cooling: plutonic rocks and their textures – Magma cools slowly where minerals form large crystal that can be seen by the naked eye.The coarse grained texture where grains are easily seen is called phaneritic. The vapor facilitates the growth of large crystals.

Igneous rocks that cool underground from a magma are known as _____.

C) plutonic rocks

True/False

Beneath the surface, rocks completely melt at about 1000ºC. False

The presence of higher amounts of water usually results in the lower of the melting temperature. True

Hawaiian eruptions are extremely violent and produce large pyroclastic flows. True

Igneous rocks high in feldspars and silicates are known as felsic rocks. False

When groundwater comes in contact with hot rock, it heats up and creates a resurgent dome. False

Pyroclastic flows are classified as a primary effect. True

Vulcanian eruptions are not very explosive and only release ash 5km into the air. False

Basalt magma has a high amount of dissolved gases and is very explosive. False

The most violent volcanic eruptions in history are classified as _______ eruptions. Plinian

When magma is injected horizontally between layers and solidifies, a _______ forms. Sill

_______ are volcanic vents that emit only gas. Fumaroles

_______ is the separation of melted materials from the remaining solid material during the course of melting. Fractional Melting

The magma from erupting volcanoes has a temperature ranging between ________. 800C to 1000C

Igneous rocks with small crystals undergo _______ cooling. Rapid

The mass of bubbles found in pumice are more commonly known as _______. Vesicles

The separation of liquids from crystals during crystallization is known as _______. Fractional Crystallization

The texture of igneous rocks that contain large crystals set in a finer matrix is known as _______. Porphyritic

Igneous rocks with about 60% silica by weight have a(n) _______ composition. Intermediate

Other than lava flows, please list three hazards associated with volcanoes. Climate change, tsunamis, acid rain, tremors, mudslides (lahar), debris avalanche.

Contrast the igneous rock textures associated with those rocks formed internally and externally. – rocks formed internally will have large crystals, rocks formed externally will have small crystals.

How does the presence of water affect the melting temperature of a rock? - It lowers the melting temperature of the rock.

Discuss the difference between a sill and a dike. - Sill is a horizontal magmatic intrusion and dike is vertical magmatic intrusion.

What is the correlation between volcanoes and plate boundaries? – Volcanoes are produced at subduction zones in convergent plate boundaries (collision between continental-oceanic plates).

Discuss how large eruptions like Mt. Pinatubo in 1991 and Tambora in 1815 affected the Earth's climate.- When those explosion occurred the particles that were ejected from the volcanoes reached the stratosphere and those particles stayed there for a long time producing the Albedo Effect. This reflection outwards of sunlight produced a decrease in temperature around the world and less photosynthesis. The gases from these volcanoes caused massive acid rain.

Please discuss how flood basalts may have had a link to mass extinctions. - Particles from the eruption go up onto the sky and remain there for some time (sometimes years). These particles will reflect the sun rays producing the Albedo Effect, which is the reflection of sun light away from the Earth, decreasing the temperature on Earth. Also as there is less sunlight coming towards the Earth, photosynthesis of plants also decreases. The collision impact that killed the dinosaurs 65 million years ago. The impact of the meteorite cracked the other side of the Earth

opening a big gap from where millions of cubic tons of pyroclastic material went into the Earth’s atmosphere causing the Albedo Effect. As the Earth became colder and less photosynthesis was being produced, millions of plants died and so did the herbivores and as the herbivores went so did the carnivores. The extinction of the dinosaur did not happen directly from the collision of the meteorite, but for the Albedo Effect that was caused by this collision and the dinosaur’s demise took maybe thousands of years.

Discuss the relationships between gabbro and basalt and granite and rhyolite. - GABBRO –MAFIC PLUTONIC and BASALT – MAFIC VOLCANIC GRANITE- FELSIC PLUTONIC and RHYOLITE - FELSIC VOLCANIC