Igneous and Sediment

7/23/2019 Igneous and Sediment

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Schedule and Midterm Info

Week 5 Oct-05 Topic 4:Magma, Igneous Rocks and Volcanic Eruptions

Topic 5: Sediment and Sedimentary Rocks

Oct-07 Topic 5: Sediment and Sedimentary Rocks

Oct-09 Topic 5: Sediment and Sedimentary Rocks

Week 6 Oct-12 Thanksgivingno lecture

Oct-14 In Class Review Activity(to be handed in at the end of class- 1% of final grade)

Oct-16 Midterm 1: In Class

(~40 multiple choice questions, 20% of final grade)

The midterm will cover the material we learn up until the end of lecture onOctober 9th.

Office Hours: 09:00-09:50 Mo, We, Fr, or by appointment

Office: ES 530 (open door policy)


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Topic 4: Magma, Igneous Rocks and Volcanic

Eruptions(Chapters 4 & 5 from the textbook)

October 5th, 2015


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By the end of this lecture you should know:

How eruptions can be predicted and hazards mitigated

How volcanoes alter climate

Where igneous activity takes place in the context of

plate tectonic theory

That Earth is not unique in possessing volcanoes

Objectives

Intrusive sill


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Predicting Eruptions

Volcano status:

Activeerupting, recently erupted or likely to erupt Dormanthasnt erupted in hundreds to thousands of year

Extinctnot capable of erupting

Status determined by historical records, age of erupted rocks, if the

volcano still lies within a tectonically active area and the landscape

character of the volcano

Fig. 5.20 Long-term predictions (years)

Constrained by recurrence

interval (average time between

eruptions in the geologic record)

Large error

Short-term predictions (days

months)

Can be very accurate

Some volcanoes send out distinctwarning signs


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Predicting Eruptions

Warning signs indicate that an eruption is imminent.

Earthquake activitymagma flow increases seismicity. Heat flowmagma causes volcanoes to heat up.

Changes in shapemagma causes expansion.

Gas and steam emission

increasesmagma degassingor groundwater heated by

magma

These signs cannot predict

the exact timing or the style.

Fig. 5.21a


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Danger assessment maps

Used for planning, zoning Delineate danger areas

Pyroclastic flows, lahars

and landslides

Mitigating Volcanic Hazards

Fig. 5.21b

Evacuation

Moving those at high risk

saves lives

Diverting lava flows

Using explosives, heavy

equipment (to build dams)

or seawater


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Volcanoes and Climate

Volcanic eruptions can be large enough to alter climate.

Ash and aerosols injected high into atmosphere rapidlycircle the globe.

Particles can remain in higher layers of the atmosphere for

months to years.

This reflects solar radiation back into space, causingatmospheric cooling.

Fig. 5.23


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Top Hat


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Where Does Igneous Activity Occur?

Igneous activity occurs in four plate-tectonic settings.

Isolated hot spots Volcanic arcs bordering subduction zones

Mid-ocean ridges

Continental rifts

Igneous activity occurs at established or newly formedtectonic plate boundaries

Except: hot spots, which are independent of plates

Fig. 4.15


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Hot Spots

About 50100 mantle-plume hot-spot volcanoes exist.

Independent of tectonic plate boundaries.

May erupt through oceanic or continental crust.

Oceanicmostly mafic magma (basalt)

Continentalmafic and felsic (basalt and rhyolite)

Burn a volcano chain through overriding tectonic plate.

Creates a hot-spot track

Fig. 4.15


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Oceanic Hot Spots

Oceanic hot spotmafic magma (e.g. Hawaii)

Submarine eruptions form irregular mounds of pillow lava. Builds above sea level, basalt can flow long distances.

Thousands of thin basalt flows build up through time and

the island grows.

Shield volcano.

Fig. 5.13


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Continental Hot Spots

Continental hot spotmafic and felsic magmas

Yellowstoneeruption ~630 Ka created a 72-km caldera.A thousand times more powerful than Mt. St. Helens

Blanketed 2500 sq. km. in pyroclastic debris

Magma beneath the caldera continues to fuel geysers

Fig. 5.14


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Flood basalts are huge thicknesses of low-viscosity basalt that spread as flows over large

geographic areas. One hypothesis suggests that when a mantle plume intersects the base

of rifting continental lithosphere enormous volumes of mafic magma are created. TheColumbia River basalt is one example.

Flood Basalts


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Subduction Zones

A chain of volcanoes (volcanic arc) forms on an overriding

plate adjacent to deep ocean trenches at convergentboundaries.

Subducting oceanic lithosphere adds volatiles (water)

Ultramafic rocks of the asthenosphere partially melt producing

mafic magma

Magma may undergo fractional crystallization producingintermediate or felsic magma

Magma rises and creates volcanoes on overriding plate

Fig. 4.15

Subducting oceanic

lithosphere adds

volatiles (water), which

facilitates partial melting

of the asthenosphere.

Magma rises and

creates volcanoeson overriding plate.


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Subduction Zones

Continental arcs (e.g. Andean arc, Cascade arc)

Heat transfer into continental crust causes partial meltingLeads to intermediate to felsic magmas

Island arc (e.g. Aleutian arc, Mariana arc)

Fig. 4.15


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Subduction Zones

Beneath volcanic arcsintrusions develop

Plutons, batholiths sills,

dikes etc.

Exposed later when erosionremoves the volcanic

overburden

Plutons are large blobs of magma

that cool deep underground. In

subduction zones, plutons

aggregate to form batholiths

made of immense volumes of

intrusive rock. Batholiths often

remain as evidence of fossil

subduction zones.


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Mid-Ocean Ridges

Most igneous activity takes place at mid-ocean ridges.

MOR-generated oceanic crust covers 70% of Earth Rifting spreads plates leading to decompression melting.

Basaltic magma wells up and fills magma chambers.

Solidifies as gabbro at depth

Moves upward to extrude as pillow basalt

Fig. 4.15

Fig. 2.17c


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Continental Rifts

Continental lithosphere is being stretched and thinned

(e.g. East African Rift Valley) Decompressional melting in asthenosphereproduces mafic

magma

Heat transfer melts crustproduces felsic magma

Local magma mixing produces intermediate magmas

Effusive and explosive eruptions

e.g. Mount Kilimanjaro,Tanzania

Stratovolcano

Fig. 4.17


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Geologic Settings of Volcanism

The Ring of Fire (blue line) dominates Pacific margins.

Fig. 5.12


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Top Hat


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Volcanoes on Other Planets

Volcanic activity evident on the Moon and other planets.

Lunar maria (dark seas) are regions of flood basalts.

Olympus Monsextinct shield volcano on Mars.

Active volcanoes on Io, a moon of Jupiter.

Fig. 5.24a, c, d


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Topic 5: Sediments and Sedimentary Rocks

(Interlude B and Chapter 6)


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Sedimentloose fragments of rocks or minerals broken

off bedrock, mineral crystals that precipitate directly outof water, and shells or shell fragments.

Sediments are produced by weathering of preexisting

rock

Sediment


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Weatheringthe processes that break up and corrode

solid rock, eventually transforming it into sediment. Two types of weathering: physical or chemical

Physical weatheringthe process in which intact rock

breaks into unconnected clasts (grains)

Jointing, frost wedging, salt wedging, root wedging,

thermal expansion, animal attack

Each size range of clast (grain) has a different name:

Weathering


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Chemical weatheringthe

process in which chemicalreactions alter or destroy

minerals when rock comes

into contact with water

solutions and/or air.

Dissolution, Hydrolysis,

Oxidation, Hydration

Physical and chemical

weathering happen together,aiding one another in

disintegrating rock to form

sediment.

Weathering

Suggested reading: section B 2 in textbook (p 150-153)

Igneous and Sediment

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