Volcanoes and PT

POWERPOINTS:

VOLCANOES & PLATE TECTONICS

Review: rocks • Three types of rocks:

① _igneous ________: Formed when molten rock solidifies and cools sedimentary Formed when ② _______________: sediments compact/cement ③ _metamorphic ______________: Formed under extreme pressure & temp.

• Rock cycle is powered by heat from Earth’s interior & energy from the sun

REVIEW

• Alfred Wegener developed _Continental ________________ Drift ______________ theory & proposed that a supercontinent called _Pangaea ____________ existed • Evidence that supported his theory includes: Puzzle Pieces • Continental _____________________________________ fossils • Matching _____________________________________ Matching rock types • _____________________________________ ancient climates • Matching _____________________________________

REVIEW

• Plate tectonics included a mechanism for plate movement that involved _convection _____________ currents in the mantle • Continental crust: Is older, _thicker __________, & dense than oceanic crust less _____________ • Oceanic crust: Is found under the ocean dense rocks; is still >loor; is made up of ___________ formed @ mid-­‐ocean ridges being ____________

PLATES REVIEW

• Earth's crust is divided into 7 major tectonic plates & a few smaller ones

Review: Boundaries • Divergent: Also called spreading centers; occurs when two plates move _______ apart • Convergent: Where two plates move __________ together (3 types) • Transform Fault: Margins where two plates grind past each other. Example?

San Andreas Fault ___________________

Plates: Overview

divergent

convergent

transform

Review: Types of Plate Boundaries floor spreading is the process by • Sea ________ which plate tectonics produces new oceanic lithosphere. Average about 5 cm/ year

transform

convergent

divergent

Divergent Boundaries: Overview • Spreading ridges • As plates move apart, new hot molten material is erupted to fill the gap • Forms on a conPnent? Called a con>nental ri?

• Forms under the ocean? Called an ocean ridge

Continental Rift & Oceanic Ridge

Age of Oceanic Crust

• Ocean Ridges: This map shows the age of the oceanic crust • Divergent boundaries are in the middle of the red areas (Iceland)

Iceland • Iceland offers scienPsts a natural laboratory for studying -­‐ on land -­‐ the processes that occur along submerged parts of a divergent boundary • Iceland is spliWng along the Mid-­‐Atlan*c Ridge -­‐ a divergent boundary between the North American & Eurasian Plate • As North America moves westward & Eurasia eastward, new crust is created on both sides of the diverging boundary

Iceland • While the creaPon of new crust adds mass to Iceland on both sides of the boundary, it also creates a ri? along the boundary

Iceland • Iceland will inevitably break apart into 2 separate land masses @ some point in the future, as the AtlanPc waters eventually rush in to fill the widening and deepening space between

Review: Plates • 1969: Plate Tectonic theory -­‐ Hess o Mapping of ocean floor, earthquake technology; SONAR • Evidence for Plate Tectonic theory o PaleomagnePsm o Earthquake locaPon o Ocean Drilling o Hot Spots

Evidence for Plate Tectonics: Paleomagnetism • Paleomagne>sm: Rocks that formed millions of years ago show the locaPon of the magnePc poles at the Pme of their formaPon

Evidence for Plate Tectonics: Paleomagnetism • ScienPsts don’t know for certain why magnePc reversals occur, but there is hard evidence that they have for hundreds of millions of years

Evidence for Plate Tectonics: Paleomagnetism

• As lava cools, it becomes magnePzed parallel to the magnePc field present at that Pme • When the polarity randomly reverses, a record of the paleomagne>sm is preserved in the sequence of lava flows

Magnetometer

• Magnetometer: A sensing device that detects magnePc fields, helping to confirm seafloor spreading

MAGNETIC REVERSAL

• Rock samples from many places on the ocean floor show that the north & south magne>c poles reversed hundreds of >mes over the last 330 million years • The last reversal was less than a million years ago

Paleomagnetism • The discovery of strips of alternaPng polarity, which lie as mirror images across the ocean ridges, is among the strongest evidence for seafloor spreading

Evidence for Plate Tectonics: Earthquakes

• ScienPsts found a close link between deep-­‐ focus earthquakes & ocean trenches

– Most shallow-­‐focus Earthquakes occur within or adjacent a trench – Intermediate & deep-­‐focus earthquakes occur toward the mainland

• Deep-­‐ocean trenches are where cool, dense slabs of oceanic lithosphere plunge into the mantle

Evidence for Plate Tectonics: Earthquakes • Shallow-­‐focus earthquakes are produced as the descending plate interacts with the lithosphere above it • As the slab descends farther into the mantle, deeper-­‐focus earthquakes are produced • No earthquakes have been recorded below 700km WHY? • At this depth, the slab

has been heated enough to soften

Evidence for Plate Tectonics: Ocean Drilling • The Deep Sea Drilling Project from 1968-­‐1983 used the drilling ship Glomar Challenger to drill hundreds of meters into the sediments and underlying crust • Data revealed that the age of the sediment increased with increasing distance from the ridge

Evidence for Plate Tectonics: Ocean Drilling • This data confirmed the seafloor spreading hypothesis that the youngest oceanic crust is at the ridge crest & the oldest oceanic crust is at the conPnental margins

Ocean Drilling (continued) • Discovered how youthful the ocean floor is in comparison to Earth's geologic history • ScienPsts concluded that the ocean floor is probably no older than 200 million years • Compared to the 4.5 billion years of our Earth • The Glomar Challenger launched on March 23rd ‘68 from Orange, TX • The Deep Sea Drilling Project accepted the ship on August 11, 1968

Evidence for Plate Tectonics: Hotspots • The Hawaiian Islands were created as the Pacific plate drined over a hotspot @ an average of 3-­‐4 inches a year

Evidence for Plate Tectonics: Hotspots

• The volcanoes increase in age with increasing distance from Hawaii (Hawaii = youngest) • A rising plume of mantle material is located below the island of Hawaii

Evidence for Plate Tectonics: Hotspots • The hotspot, which geologists esPmate began producing the Hawaiian Islands 30 million years ago, is a plume of molten rock that rises through the mantle, the mostly solid layer between the crust & core

Evidence for Plate Tectonics: Hotspots • The islands don’t last forever • As the Pacific plate moves Hawaii’s volcanoes farther from the hotspot, they erupt less frequently, then no longer tap into the upwelling of molten rock & die • The island erodes & the crust beneath it cools, shrinks & sinks, & the island is again submerged

Hotspots

• Hot spot evidence supports the idea that the plates move over the Earth’s surface

Intraplate Igneous Activity • Kilauea is Earth’s most acPve volcano, but it is in the middle of the Pacific Plate

Intraplate Igneous Activity • Intraplate volcanism: occurs within a plate, not at a plate boundary • Most intraplate volcanism occurs where a mass of horer than normal mantle called mantle plume rises toward the surface forming a hot spot, like the Hawaiian islands

• Did you know that Yellowstone NaPonal Park is actually an acPve supervolcano?

The Yellowstone volcano is another example of an intraplate volcano

Yellowstone • Yellowstone was designated as a NaPonal Park in 1872 to preserve and protect its more than 10,000 unique thermal features, the largest collecPon on the planet, spread throughout the park’s 2.2 million acres

Yellowstone Caldera

• Yellowstone is America’s first naPonal park (1872) • Located mostly in Wyoming, with edges that peak in to Montana & Idaho • A caldera is a large depression in a volcano (can form through the collapse of the top of a volcano or aner an explosive erupPon)

Volcanoes

• Volcanoes are located on or near plate boundaries • Most of the volcanic erupPons that make the news, such as the 1980 Mount St. Helens erupPon, take place near subduc>on zones

Mount St. Helens

• The May 18 erupPon sent volcanic ash, steam, water, & debris to a height of 60,000 feet

Mount St. Helens

• Erupted May 18, 1980 • Located at the convergent boundary between the Juan de Fuca plate & North American plate • Within 15 to 20 seconds of a magnitude 5.1 earthquake at 8:32 a.m., the volcano's bulge and summit slid away in a huge landslide-­‐the largest largest on Earth in recorded history

Mount St. Helens Eruption

• Rocks, ash, volcanic gas, & steam were blasted upward & outward • This lateral blast of hot material accelerated to at least 300 mph • Produced a column of ash & gas that rose more than 15 miles into the atmosphere in only 15 minutes

Mount St. Helens Eruption • The hot rocks & gas quickly melted some of the snow & ice capping the volcano • Created surges of water that eroded & mixed with loose rock debris to form volcanic mudflows (lahars) • A lahar is volcanic mudflow; these lethal mixtures of water & tephra have the consistency of wet concrete, but can flow down the slopes of volcanoes @ rapid speeds

Lahar - mudflow

Volcanoes

• These devastaPng, explosive erupPons reflect the composiPon of the magma: • It is extremely viscous & results in tall, steep-­‐sided volcanoes • Cinder cone

Volcanoes

• In contrast, the volcanic erup>ons that occur along spreading ridges are much gentler...

• Because most of these erupPons occur under 2 to 3 km of water • Because the magma is far less viscous

Volcanoes • Volcanoes form at places where large quan>>es of heat escape @ the surface— somePmes quite dramaPcally • Earth's volcanoes vary widely in size, form, and explosivity • Some erupt violently, others pour out rivers of lava • This diversity is largely related to their plate-­‐ tectonic environments

Factors that affect eruptions • The primary factors that determine whether a volcano erupts violently or quietly include: • magma composiPon • magma temperature • the amount of dissolved gases in the magma

Viscosity & Composition • Viscosity: A substance’s resistance to flow • What is a substance with a high viscosity? honey

• How does temperature affect viscosity?

As a substance is heated, it becomes less viscous • As lava flow cools & begins to harden, its viscosity increases, its mobility decreases, and eventually the flow halts • The more silica in lava, the more viscous • BasalPc lava tends to be more fluid

Pyroclastic Material

• Pyroclas>c material is the name given to parPcles produced in volcanic erupPons • The fragments ejected can range in size from fine dust & ash to pieces that weight several tons • The gases are mostly water vapor & CO2 • Very fluid basalPc magma allow gases to bubble & escape more gently (Hawaii) • More viscous magma holds bubbles unPl they must escape & explode (Mount St. Helens)

Pyroclastic Material • Individual erupPve fragments are called pyroclasts ("fire fragments”). Tephra (Greek, for ash) is a generic term for any airborne pyroclasPc accumulaPon

Ash – very fine-­‐ Lapilli – pea to Blocks and bombs grained fragments, walnut sized – are semi-­‐molten generally pyroclasts; may be when airborne, dominated by rounded or cinder making broken glass like aerodynamic shards shapes

Magma Vs. Lava • Magma: A body of molten rock found at depth, including any dissolved gases and crystals • Lava: Magma that reaches the Earth’s surface • When magma cools & hardens beneath the surface or as the result of a volcanic erupPon, ________ igneous rock forms

Igneous Rocks: Review • Intrusive igneous rocks: Rocks that form when magma hardens beneath the Earth’s surface (Granite is an example • Extrusive igneous rocks: When lava hardens, the rocks that form are called extrusive igneous rocks; they are extruded onto the surface

Connection to plate tectonics • More than 800 ac>ve volcanoes have been ID worldwide • Most of them are located in the margins of the ocean basins (Ring of Fire) • Others can be found on Hawaii & Iceland • And others can be found on the interiors of the conPnents

VOLCANOES & PLATES