Transcript
Page 1: Planet earth volcanism and plutonism_powerpoint_presentation

1

Volcanism and Plutonism

Page 2: Planet earth volcanism and plutonism_powerpoint_presentation

2

Extrusive Igneous ActivityVolcanism

Page 3: Planet earth volcanism and plutonism_powerpoint_presentation

3

Summary of Important Concepts

• Volcanism. Refers to the rise of magma which makes its way to the earth’s surface as LAVA cooling above the earth’s surface.

•Magma - a mixture of liquid rock, crystals, and dissolved gases beneath the earth’s surface (within the crust).

•Volcanoes are conical or dome shaped landforms built by the emission of magma and its contained gasses from a constricted vent onto the earth’s surface. Magma rises in a narrow, pipe like conduit from a magma reservoir beneath and flows at the earth’s surface as LAVA.

•Volcanoes form from sources of magma inside the earth. The main sources of magma are:

- Subduction at convergent plate boundaries.- Sea floor spreading at divergent plate boundaries. - Hot spots.

Page 4: Planet earth volcanism and plutonism_powerpoint_presentation

4

Locations of Volcanic Activity and Volcanic Hazards

Volcanic activity is controlled by plate tectonics, because plate movements relate to where sources of magma originate inside the earth. Nearly all active volcanoes are located in one of three plate tectonic settings:

•Subduction zones at convergent plate boundaries•Example: Volcanoes lining the trenches of the Pacific Ocean, forming the Pacific “Ring of Fire”.

•Rifting and sea floor spreading at divergent plate boundaries,

•Example: Volcanic eruptions at mid-ocean ridges, and in some rift zones on the continents, like the East African Rift Valley.

•Hot spots•Example: The Hawaiian Islands and the Galapagos Islands.

Page 5: Planet earth volcanism and plutonism_powerpoint_presentation

5

Map of the world’s active volcanoes, showing that the majority of active volcanoes (about 66%) occur in the Pacific Ring of Fire.

Page 6: Planet earth volcanism and plutonism_powerpoint_presentation

6

Because the western U.S.A. borders the Pacific Ring of Fire volcanic hazards exist primarily in the western states. On the map, regions with the greatest risk of volcanic hazards are shown in red, and those with the lowest risk are shown in blue. Areas that are uncolored are considered “risk free”.

Page 7: Planet earth volcanism and plutonism_powerpoint_presentation

7

Zooming in for a closer look at the western U.S., we see in this figure that the volcanoes of the Cascade Range, which extends from northern California north into Oregon and Washington, are related to the presence of the Cascadia Subduction Zone offshore.

The most recent major eruption in the Cascade Range was that of Mount Saint Helens in 1980. The Cascade Range is part of the Pacific Ring of Fire.

Page 8: Planet earth volcanism and plutonism_powerpoint_presentation

8

Magma and the Driving Force Behind Eruptions

• Magma may be ejected onto the earth’s surface as:

– Lava

– Pyroclastics or tephra- flour sized to boulder sized particles which are thrown in the air due to the built up pressure of gasses.

• The violence of a volcanic eruption depends on the magma’s viscosity and gas content. The more viscous (thick) and more gaseous the magma, the more explosive the eruption.

Above: Gigantic eruption cloud

Above: aa lavaRight: pahoehoe lava

Page 9: Planet earth volcanism and plutonism_powerpoint_presentation

9

Viscosity is a measure of a fluid’s resistance to flow. The main factor that determines the viscosity of magma is its silica (SiO2) content. The more silica in the magma, the more viscous it is. The more viscous (silica-rich) the magma, the more violent the eruption.

We are concerned, then, with three types determined by the chemical composition of the magma.

1. Mafic or Basaltic magma – low silicon and oxygen, high iron and magnesium. Simple silicate minerals. Dark magmas. Low Viscosity.

2. Intermediate or andesitic magma - intermediate silicon and oxygen, intermediate iron and magnesium. High Viscosity.

3. Felsic or rhyolitic magma - high silicon and oxygen, low iron and magnesium. Complex silicate minerals. Pale magmas. High Viscosity.

Left: The basic building block of all silicate minerals - the silica tetrahedron. Four oxygen atoms surrounding a single atom of silicon.

Page 10: Planet earth volcanism and plutonism_powerpoint_presentation

10

The gas content of a magma also relates to its behavior. A magma with low gas content will tend to flow out of a volcano as relatively quiet lava. A magma with high gas content will tend to blow apart violently upon erupting. The higher the gas content, the more violent the eruption.

The composition of the gases in magma are:

•Water vapor •Carbon Dioxide •Minor amounts of sulfur dioxide, hydrogen sulfide, chlorine, and flourine gases

But it is the amount of dissolved water that typically inspires a volcano to violence

Page 11: Planet earth volcanism and plutonism_powerpoint_presentation

11

Steps to a Volcanic Eruption

• Magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rock.

• As they rise they may encounter a depth (or pressure) where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase (makes bubbles).

• When a gas bubble forms, it will also continue to grow in size as pressure is reduced and more of the gas comes out of solution. In other words, the gas bubbles begin to expand.

Page 12: Planet earth volcanism and plutonism_powerpoint_presentation

12

Steps to a Volcanic Eruption:Two Possibilities

• If the magma has a low viscosity, the gas will easily expand to atmospheric pressure at the earth’s surface and simply burst, and a non-violent eruption will occur, usually as a lava flow.

• If the magma has a high viscosity the gas will not be able to expand very easily creating a high pressure inside which will cause them to burst explosively on reaching atmospheric pressure. This will cause and explosive volcanic eruption.

Page 13: Planet earth volcanism and plutonism_powerpoint_presentation

13

It turns out that viscosity and gas content of magma both relate to plate tectonic setting:

Divergent boundaries (mid-ocean ridges) and hot spots both draw their magmas from the upper mantle. This magma is called MAFIC MAGMA, and is characterized by relatively low silica and low gas content.

In contrast, the magma at convergent boundaries comes from melting of subducted oceanic plates. This magma is called FELSIC MAGMA, and is characterized by relatively high silica and high gas content.

Page 14: Planet earth volcanism and plutonism_powerpoint_presentation

14

Bottom line: The world’s most dangerous volcanoes are those at convergent plate boundaries!

Page 15: Planet earth volcanism and plutonism_powerpoint_presentation

15

Hazard Summary

The main HAZARDS associated with volcanic eruptions are:

Lava flows that burn and destroy what they overrun.

Ash falls that cover vast areas of landscape, creating respiratory problems, messy conditions, and potential lahars.

Pyroclastic flows: hot, fluid mixtures of rock particles and gas that travel at great speed down the flanks of a volcano; have caused thousands of fatalities.

Lahars: fast-moving mud flows caused by mixing volcanic ash with water (from rain or from eruptions melting snow and ice on the volcano); responsible for more death and destruction than any other volcanic hazard.

Gases emitted during eruptions that may be toxic and/or corrosive; the most common is CO2 gas - when present in large enough quantities it causes suffocation.

Page 16: Planet earth volcanism and plutonism_powerpoint_presentation

16

Lava Flows

• Lava is molten magma that flows out and onto the Earth’s surface.

• Lava flows are typically formed from low viscosity mafic magma that erupts at divergent boundaries and hot spots.

Fluid basalt lava flows are called Hawaiian type lava and can come in two varieties. Smooth, runny pahoehoe (top) and chunky aa (bottom).

Page 17: Planet earth volcanism and plutonism_powerpoint_presentation

17

The lower silica content (and therefore low viscosity) of mafic magma allows the lava to run down slopes easily.

Lava flow eruptions are fairly gentle and quiet. They may cause property damage, but rarely fatalities.

Page 18: Planet earth volcanism and plutonism_powerpoint_presentation

18

Ash Falls• Ash falls form when

an eruption column of tephra and gas is blown into the air by an explosive eruption. The eruption column can rise up more than 20 km into the atmosphere.

• Tephra is a general term for any size of fragmental material blown out of a volcano.

Page 19: Planet earth volcanism and plutonism_powerpoint_presentation

19

Shown right: Volcanic Bombs form from the rapid

cooling of lava thrown in the air. When the lava cools in mid-air, it

forms the characteristic ellipsoidal shape.

Shown left: Pele is the goddess of the volcano. Teeny tiny volcanic bombs are known as Pele’s tears.

Large-sized tephra typically falls back to the ground on or close to the volcano.

Page 20: Planet earth volcanism and plutonism_powerpoint_presentation

20

Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano.

1980 Eruption of Mt. St. Helen and map of ash fall distribution and thickness.

Page 21: Planet earth volcanism and plutonism_powerpoint_presentation

21

Ash spreads in upper atmosphere around the globe. The suspended ash can decrease the insolation from the sun and lower global temperatures!

Page 22: Planet earth volcanism and plutonism_powerpoint_presentation

22

Pyroclastic FlowsPyroclastic flows (Ash Flows) are avalanches of a very hot

(1300-1800F) mixture of hot rock particles and hot gas that are blown out of the vent of the volcano as an eruption column which subsequently collapses and moves very rapidly down the flanks of the volcano at speeds from 50 to over 200 km per hour and can travel for 10’s of kms burning, burying and suffocating everything in their path.

Above: 1997 eruption on island of Montserrat in the West Indies.

Right: 1986 eruption of Augustine Volcano, Alaska.

Page 23: Planet earth volcanism and plutonism_powerpoint_presentation

23

...destroy by direct impact.

Pyroclastic flows…..

...bury sites with hot rock debris.

...burn forests, crops, and buildings.

Page 24: Planet earth volcanism and plutonism_powerpoint_presentation

24

One of the greatest volcanic disasters in history was the destruction of the city of Saint Pierre, Martinique, by a pyroclastic flow from Mount Pelee in 1902.

Before the eruption right…...

….and after the eruption left.

An estimated 30,000 people died gruesomely by burning, burial, and suffocation.

Page 25: Planet earth volcanism and plutonism_powerpoint_presentation

25

Mudflows or LaharsLahars have caused more fatalities than any other volcanic

hazard because they are more common, and they can occur at any time.

Volcanic Mudflows or Lahars form by mixing water with loose volcanic ash and debris on the flanks of a volcano. As the mud moves downslope, it gathers rocks of all sizes accelerating as it goes.

The water can come in several ways including: A major rainstorm. An eruption melts large amounts of snow and ice on the

flanks of the volcano.

Page 26: Planet earth volcanism and plutonism_powerpoint_presentation

26

When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders. Most lahars travel much too fast for people to outrun.

Page 27: Planet earth volcanism and plutonism_powerpoint_presentation

27

Above and Below: Stands of dead trees killed by excess CO2 seeping from the ground from magma close to the surface.

Poisonous Gases

Gases emitted during volcanic eruptions may be toxic and/or corrosive.

The most common hazardous gas is CO2. Carbon Dioxide is deadly to people, animals, and trees in high concentrations.

Page 28: Planet earth volcanism and plutonism_powerpoint_presentation

28

These cattle, and about 1700 people, were victims of asphyxiation from a massive CO2 discharge from Lake Nyos (Cameroon, West Africa) in 1986. Lake Nyos lies within a volcanic crater.

Being heavier than oxygen, it can pour downslope and displace oxygen at the surface. Because it is colorless and odorless, it can suffocate without warning.

Page 29: Planet earth volcanism and plutonism_powerpoint_presentation

29

Types of VolcanoesShieldCompositeCinder Cone

They differ in igneous rock chemistry, eruption style, physical features and geographic location.

Note the size differences!

Page 30: Planet earth volcanism and plutonism_powerpoint_presentation

30

Shield Volcanoes

Shield Volcano Stats • Found along hotspots and divergent boundaries. • Mafic (basaltic) magma• Low viscosity• Low gas content (no water because no subduction). • GENTLE eruptions of lava flows. • Landform composed almost entirely of relatively thin lava flows. • Gently sloping due to the low viscosity of the magma which

allows lava to flow great distances before it cools.• The largest structures on earth! 9 km from seafloor to summit!• Basalt is the most common rock type.

Page 31: Planet earth volcanism and plutonism_powerpoint_presentation

31

The big island of Hawaii has three major volcanoes: Mauna Loa, Mauna Kea, and Kilauea.

Mauna Loa is Hawaii’s largest volcano but its activity is infrequent.

Kilauea is in the main growth stage having erupted almost continuously since 1983.

Page 32: Planet earth volcanism and plutonism_powerpoint_presentation

32

Because they form from relatively quiet eruptions of low viscosity, fluid magma, shield volcanoes are very large, with gently sloping sides and a convex shape.

Left: a shield volcano in the Galapagos Islands created over a hot spot.

Right: Iceland created where a mid-ocean ridge sticks up above sea

level.

Left: Mt. Etna, Sicily. The largest continental volcano on earth and the most active volcano in Europe, Etna erupts almost continuously, with an occasional violent episode.

Page 33: Planet earth volcanism and plutonism_powerpoint_presentation

33

Stratovolcano Stats • Found along convergent boundaries paralleling subduction zones.• Andesitic (intermediate) to rhyolitic (felsic) magma.• High viscosity• High gas content (subduction drags down water)• VIOLENT eruptions. Dangerous and Explosive. • Landform composed of alternating layers of tephra and lava.• Steeply sloping due to the piling up of tephra around the central vent

and the high viscosity of the lava that glues it together.• Can reach heights of roughly 3500 meters.• Andesite and rhyolite are the most common rock type.

Composite or Stratovolcanoes

Page 34: Planet earth volcanism and plutonism_powerpoint_presentation

34

This figure shows a composite of the main hazards associated with a typical stratovolcano.

The main hazards are:•pyroclastic flows•lahars•ash falls•Poisonous gas emissions

Page 35: Planet earth volcanism and plutonism_powerpoint_presentation

35

Map of the world’s active volcanoes, showing that the majority of above sea active volcanoes (about 66%) are stratovolcanoes produced by subduction in the Pacific Ring of Fire.

Page 36: Planet earth volcanism and plutonism_powerpoint_presentation

36

Because the western U.S.A. borders the Pacific Ring of Fire volcanic hazards exist primarily in the western states. On the map, regions with the greatest risk of volcanic hazards are shown in red.

*Yellowstone Caldera

The volcanoes of the Cascade Range, which extends from northern California north into Oregon and Washington, are related to the presence of the Cascadia Subduction Zone offshore.

Page 37: Planet earth volcanism and plutonism_powerpoint_presentation

37

Cinder Cone• Cinder cones form when fluid

basaltic magma rises along a fracture and encounters groundwater. The steam generated blows fragments of lava violently into the air which solidify and fall as cinders around the vent forming a circular or oval cone.

• Cinder cones are commonly found on the flanks of shield volcanoes and stratovolcanoes.

• Most cinder cones erupt for only a few months to a few years and rarely rise more than 300-500 m above their surroundings. Being unconsolidated they tend to erode rapidly.

Page 38: Planet earth volcanism and plutonism_powerpoint_presentation

38

Above: The photos the eruption of Paricutin Volcano, Mexico, a classic example of a cinder cone.

Above: View of cinder cones near the summit of Mauna Kea

Above: This cone is one of two cinder cones called the Red Cones located south of Long Valley Caldera in California. These basaltic cones were erupted about 5,000 years ago.

Above: This cinder cone on the flank of Mount Etna is surrounded by a younger basaltic lava flow.

Page 39: Planet earth volcanism and plutonism_powerpoint_presentation

39

Intrusive Igneous ActivityPlutonism

Page 40: Planet earth volcanism and plutonism_powerpoint_presentation

40

Intrusive Igneous Activity

Crystallize below the earth’s surface so what igneous texture would we see? Fast or slow cooling?

Intrusive bodies are classified based on shape or size.

Magma travels through (intrudes) into another rock body (commonly called country rock).

Common intrusive rocks are peridotite, granite and gabbro.

Page 41: Planet earth volcanism and plutonism_powerpoint_presentation

41

Intrusive Igneous Activity

Shallow intrusive bodies occur <2km depth.

Cool quicker than deep LARGE intrusive bodies so crystals are on the smaller side.

Dikes are vertical structures that cut across sedimentary rock layers while Sills are horizontal running between sedimentary layers. It can bulge up between layers and form a Laccolith.

If the country rock has numerous fractures magma will fill in the crackscalled veins.

Page 42: Planet earth volcanism and plutonism_powerpoint_presentation

42

Erosion of overlying country rock will expose the stable felsic

intrusions.

Page 43: Planet earth volcanism and plutonism_powerpoint_presentation

43

Sill (left) Dike (right): Surrounding rock may be weaker than the igneous

intrusion and therefore the dike remains vertical while the

surrounding rocks are weathered away. Recall felsic igneous rocks are high in silica so they are composed of more

stable minerals.

Page 44: Planet earth volcanism and plutonism_powerpoint_presentation

44

Deep Intrusive Bodies

BATHOLITHS are large (>100 square kilometers) bodies made primarily of felsic granite.

Because they are so large in size, rocks in the center of the batholith cool SLOWER than those at the periphery. How will the igneous textures vary?

Felsic intrusions move slowly through the crust due to their high viscosity.

Page 45: Planet earth volcanism and plutonism_powerpoint_presentation

45

Stone Mountain in Georgia is an example of a batholith (large igneous pluton 100’s of square miles) that formed below the earth’s surface (within the crust). Here, the felsic magma cooled (slowly).

Page 46: Planet earth volcanism and plutonism_powerpoint_presentation

46

Lithospheric plates are essentially ‘floating’ on top of the asthenosphere. Where plates are thicker (continental crust), it weighs more and is sunk into the asthenosphere in these places (like floating a block of wood in water).

When we remove overlying rock, the weight burden becomes less and continental crust is uplifted. Vertical movement of the crust is called isostacy.

Page 47: Planet earth volcanism and plutonism_powerpoint_presentation

47

Over time, weathering and erosion of the overlying rocks exposed Stone Mountain. What is actually seen of Stone Mountain is the ‘tip of the iceburg’.

Most of it remains buried. As surrounding country rock (weaker than stmt) weathers away, it ‘lightens’ the weight on top of stone

mountain. This causes stone mountain to uplift where it can become more susceptible to erosion, and too will weather into

sediments….one day.


Top Related