Edward J. Tarbuck & Frederick K. Lutgens Earth Science, 10e Edward J. Tarbuck & Frederick K. Lutgens

Volcanoes and Other Igneous Activity Chapter 8 Earth Science, 10e Stan Hatfield and Ken Pinzke Southwestern Illinois College

Volcanic eruptions Factors that determine the violence of an eruption Composition of the magma (% silica content) Temperature of the magma (Very hot and cool) Dissolved gases in the magma (Water vapor etc…) Viscosity of magma (function of silica content) Viscosity is a measure of a material's resistance to flow

Let’s Review What types of Magma are there?... How does temperature relate to a type of magma? How Does viscosity relate to a type of Magma?

Volcanic eruptions Viscosity of magma Factors affecting viscosity Temperature (hotter magmas are less viscous) Composition (silica content) High silica – high viscosity (e.g., rhyolitic lava) Low silica – more fluid (e.g., basaltic lava) Dissolved gases (volatiles) Mainly water vapor and carbon dioxide Gases expand near the surface Hotter magmas have less gases Cooler magmas trap more gases

Volcanic eruptions Viscosity of magma (continued…) Factors affecting viscosity Dissolved gases (volatiles) thicken the magma Provide the force to extrude lava Violence of an eruption is related to how easily gases escape from magma Easy escape from fluid magma Viscous magma produces a more violent eruption

Fluid Lava: The Columbia River basalts

Let’s summarize Viscous magma: Felsic/Andesitic or Mafic? Cooler magma Felsic/Andesitic or Mafic? Violent eruptions: Felsic/Andesitic or Mafic?

Materials associated with volcanic eruptions Types of Basaltic Lava flows Basaltic lavas are more fluid Types of basaltic lava Pahoehoe lava (resembles braids in ropes) Aa lava (rough, jagged blocks) Gases Represent one to 5 percent of magma by weight Mainly water vapor and carbon dioxide

A Pahoehoe lava flow

A typical aa flow

What is a Hot Spot? An intermittent stationary source of magma that originates deep inside the core or the mantle Plates riding over a hot spot form a chain of volcanoes. i.e. Hawaiian chain The chain changes direction as the plate changes its direction over the hot spot Only the volcano over the hot spot is active

A size comparison of the three types of volcanoes

Types of Volcanoes There are three types of volcanoes defined according to their shape and size: Shield volcano Broad, slightly domed Primarily made of basaltic (fluid) lava Generally large size e.g., Mauna Loa in Hawaii Mostly found on the Ocean floor and/or over hot spots

A shield volcano

Types of Volcanoes Types of volcanoes (continued…) Cinder cone Built from ejected lava fragments Steep slope angle Rather small size Frequently occur in groups

Sunset Crater – a cinder cone near Flagstaff, Arizona

Types of Volcanoes Types of volcanoes (continued…) Composite cone (or stratovolcano) Are made of Interbedded lavas and pyroclastics Most are adjacent to the Pacific Ocean also called Ring of Fire (e.g., Mt. St. Helens, Mt. Pinatubo,…) They are of large size They produce the most violent eruptions (i.e. Mt. St Helens May 18, 1980 eruption)

Volcanoes Types of volcanoes (continued…) Composite cone (or stratovolcano) Often produce nuée ardente (Pyroclastic flow) Fiery pyroclastic flow made of hot gases infused with ash Flows down sides of a volcano at speeds up to 200 km (125 miles) per hour May produce a Lahar – deadly volcanic mudflow

A composite volcano (stratovolcano)

Mt. St. Helens – a typical composite volcano

Pyroclastic flow on Mt. St. Helens

What’s on a Volcano’s arsenal? Pyroclastic flow (very hot ash) Lahar (deadly mud flow) Crater Lake’s deadly gases

A lahar along the Toutle River near Mt. St. Helens

Other volcanic landforms Calderas Steep walled depression at the summit Formed by collapse of the crater Nearly circular Size exceeds one kilometer in diameter When filled with water calderas form crater lakes

Crater Lake, Oregon is a good example of a caldera

Crater Lake in Oregon

Materials associated with volcanic eruptions Pyroclastic materials "Fire fragments" Types of pyroclastic material Ash and dust – fine, glassy fragments Pumice – from "frothy" lava Lapilli – "walnut" size Cinders – "pea-sized" Particles larger than lapilli Blocks – hardened lava Bombs – ejected as hot lava

A volcanic bomb Bomb is approximately 10 cm long

Volcanoes General features Conduit, or pipe caries gas-rich magma to the surface Vent, the surface opening (connected to the magma chamber via a pipe) Crater Steep-walled depression at the summit Caldera (a summit depression greater than 1 km diameter)

Mt. St. Helens following the 1980 eruption

Other volcanic landforms Volcanic pipes and necks Pipes are short conduits that connect a magma chamber to the surface Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone

Origin of magma Magma originates when essentially solid rock, located in the crust and upper mantle, melts Factors that influence the generation of magma from solid rock Role of heat Earth’s natural temperature increases with depth (geothermal gradient) is not sufficient to melt rock at the lower crust and upper mantle

Origin of magma Factors that influence the generation of magma from solid rock Role of heat Additional heat is generated by Friction in subduction zones Crustal rocks heated during subduction Rising, hot mantle rocks

Origin of magma Factors that influence the generation of magma from solid rock Role of pressure Increase in confining pressure causes an increase in melting temperature Drop in confining pressure can cause decompression melting Lowers the melting temperature Occurs when rock ascends

Origin of magma Factors that influence the generation of magma from solid rock Role of volatiles Primarily water Cause rock to melt at a lower temperature Play an important role in subducting ocean plates

Plate tectonics and igneous activity Global distribution of igneous activity is not random Most volcanoes are located on the margins of the ocean basins (intermediate, andesitic composition) called subduction zone Second group is confined to the deep ocean basins (basaltic lavas) called Mid Ocean ridges Third group includes those found in the interiors of continents at Continental rift valley

Locations of some of Earth’s major volcanoes

Plate tectonics and igneous activity Plate motions provide the mechanism by which mantle rocks melt to form magma Convergent plate boundaries Deep-ocean trenches are generated Descending plate partially melts Magma slowly rises upward Rising felsic to andesitic magma can form Volcanic island arcs in an ocean (Aleutian Islands) Continental volcanic arcs (Andes Mountains)

Plate tectonics and igneous activity Plate motions provide the mechanism by which mantle rocks melt to form magma Divergent plate boundaries The greatest volume of volcanic rock is produced along the oceanic ridge system Lithosphere pulls apart Less pressure on underlying rocks Partial melting occurs Large quantities of fluid basaltic magma are produced

Plate tectonics and igneous activity Plate motions provide the mechanism by which mantle rocks melt to form magma Intraplate igneous activity Activity within a rigid plate Plumes of hot basaltic magma rise Form localized volcanic regions called hot spots Examples include the Hawaiian Islands and the Columbia River Plateau in the northwestern United States

Tectonic Settings and volcanic activity Divergent margin: Has volcanic activity Convergent margin: Has volcanic activity Transform fault: Is a margin but has no volcanic activity Hot spot: Not a margin but has volcanic activity

Formation of a volcanic neck

Intrusive igneous activity Two types of igneous intrusive features Sills, Laccoliths, and dikes Lens shaped masses Batholiths and Stocks Largest intrusive bodies Often occur in groups Surface exposure 100+ square kilometers (smaller bodies are termed stocks) Frequently form the cores of mountains

Intrusive igneous structures exposed by erosion

A batholith exposed by erosion

Intrusive igneous activity Most magma is emplaced at depth An underground igneous body is called a pluton Plutons are classified according to Shape Tabular (sheetlike) Massive

Intrusive igneous activity Plutons are classified according to Orientation with respect to the host (surrounding) rock Discordant – cuts across existing structures Concordant – parallel to features such as sedimentary strata

Intrusive igneous activity Types of igneous intrusive features Dike, a tabular, discordant pluton Sill, a tabular, concordant pluton e.g., Palisades Sill, NY Resemble buried lava flows May exhibit columnar joints Laccolith Similar to a sill

A sill in the Salt River Canyon, Arizona

End of Chapter 8

1-Which Magma is more viscous? Basaltic Felsic/Andesitic Ultramafic None of these

2-which magma traps more gases? Mafic/Ultramafic Felsic/Andesitic None of these I am not sure

3-Which Magma is Hotter? Mafic/Ultramafic Felsic /Andesitic None of these I am not sure

4-Which magma produces more violent eruptions? Mafic/Ultramafic Felsic/Andesitic None of these I am not sure

5-Magma viscosity is a function of… Temperature Silica content Gases content All of these b only

6-Which volcano is over a hot spot? Mt St Helens (Washington State) Mt Rainier (Washington State) Mauna Loa (Hawaii) All of these a and c only

7-Which one is a Shield Volcano? Mt St Helens (Washington State) Mt Rainier (Washington State) Mauna Loa (Hawaii) All of these a and c only

8-Which one is a Composite cone? Mt St Helens (Washington State) Sunset Crater (Arizona) Mauna Loa (Hawaii) All of these a and c only

9-Which volcano type produces lava + pyroclastic flows Mt St Helens (Washington State) Kula (Hawaii) Mauna Loa (Hawaii) All of these a and c only

10-Volcanoes with steep cones have Fluid lava Viscous lava No relation whatsoever I am not sure

11-Volcanoes at divergent boundary erupt mainly…lava Felsic to Andesitic Mafic None of these I am not sure

12-Volcanoes at convergent margins produce mainly…lava Felsic to Andesitic Mafic None of these No relation whatsoever

13-Which volcanoes are more explosive? Shield volcanoes Composite cones Cinder cones None of these I am not sure

14-Volcano explosiveness is influenced by … Viscosity Temperature Amount of gases All of the above A and c only

15-A Pahoehoe lava is Gases-free with jagged surface lava Wrinkled, ropy, and gaseous lava Explosive ash and fragments flow None of these I am not sure

16 - Quartzite is what type of metamorphic rock? Regional metamorphism Contact metamorphism Not a metamorphic rock

17 – In what type of metamorphism do pressure and temperature work together? Contact metamorphism Regional metamorphism All of the above None of the above

18 - What is the parent rock of limestone? Quartzite Granite Marble Basalt This does not make sense

19 – Which one of these is a foliated metamorphic rock? Marble Granite Mica-schist Basalt Shale

20 - Which one is the metamorphic rock? Do not take SANDSTONE for GRANITE (granted)! Have a c. GNEISS (nice) day!

THE END THAT’S ALL FOLKS! THANK YOU!!!