Geology test #3

What are 3 ways to melt the mantle?

• decrease temperature

• Increase temperature

• add violates

What is the geotherm?

The change in temperature with depth(red/ green line in diagrams)

Lithospheric Geotherm

Conductive heat transvers

• temperature changes significantly with depth

Asthenosphere geotherm

Convection heat transvers

• Adiabatic Geotherm : pressure changes a lot, temperature changes very little

On What side of the curve does it remain solid or melt?

• A mantle curves that resides at T and P to the left of the curve with remin solide

• Where as to the right it will be at a high enough T and P to melt

Where does hot spot volcanism most occur

• occurs within plates

• away from plate boundaries

What is decomposition melting?

• Thinning of the lithosphere

• which allows partial melting to occur where geotherm crosses the solidus

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How is decomposition melting accomplished

• Extension at midocean ridges

• continental rifts

• divergent boundaries

Adding volatics: Flux melting

• changes the melting curve/ solidus/ liquidus such that it cross the geotherm

• The blue lines is the wet solius

• Cause by convergent margins

What is an example of adding volatiles

Ocean-continetial and ocean-ocean convergent plate boudries

• The deceding slab contains wet sediment and hydrate minerals formed by alteration of the basalt on the seafloor

• As the plate subducts is driven off the slub and into the overlaying mantle wedge

• The hyrdate mantle melts at lower temp than dry mantle

How can melts be created

• Plates seperate and mantle rises at mid-ocean ridges ( decompostion melting)

• Hot spot- localized plumes of melt

• Fluid fluxing

Where is the only part of earth where the mantle can melt?

Outermost 200km of earth

How does the geotherm slope change?

the slope is a function of geotherm thickness

• lithopsheric geotherm increase as you move the curve up

• astherospheric remains constant

Mantle plumes

increase the geotherm by the addition of heat from the mantle

• allow melting to occur in the triangular region

• happens at hot spots in Hawaii

1.      What is the name given to the region surrounding the Pacific Ocean where most subaerial (land-based) volcanoes occur?

• The ring of fire

• cotnains 75% of planets voclanoes

1.      Why do volcanoes occur here—what is the tectonic setting of this region?

Is is where multiple tectonic plates meet

What is the Volcanic Explosivity Index (VEI)

It measure and classify eruptions. It starts at zero and is a log scale. Does not have a upperlimit

1.      What was the most explosive (higher VEI) volcano recorded?

VEI 8 which ocured thosands of millions of years ago

The most explosive volcanoe recorded was in Indonesia in 1815 which was a VEI of 7

1.      List 2 benefits of volcanic eruptions

• volcanic ash provide nutriest to nearby soil making the land fertile

• when lava hardens into rock and creates new landforms

what characterists of an eruption are used to classify volcanoes according to this Index?

volume of lava gas and other emmsions from the volcanoe

height of the eruption cloud above the summit

the degree of fragmentation

What produces the contrast in explosivity

1. melting viscosity (the main control)

   Composition and temperature: felsic vs mafic

2. Volcaltiles- context of gasses

3. Interaction with external water

Melting Viscosity

The higher the silica content and the lower content of FE and MG, the more viscous the melt

Which of the compositions would have the higher viscosity

• this mean that these compsotions tend to be more explosive

What type of characteristic have less viscous (less explosive)

• more mafic/ bastlic melts

• higher temerature

• more fluid

Where do you get partial melts of the matnle reaching the surface of the earth without getting throguh the crust?

• Oceanic divergent plate boudnries

• oceanic hot spots

Where are more felsic melts produced

• contiential divergent plate boundries

• contienal hot spots

• convergent plate boundries

What is the role of expanding gas on volcanic erruptions

• they drive the volcanic erruptions

• dissolved magmas under pressure

• come out of solution and expand as magma rise

What are the mian magetic gas that are associated with expanding gases

• H20, C02, so2

What is an example of expanding gases?

Mount St. Helens

• magma rises in the chamber

• at lower pressures the gases began to exsolve and the chamber expands

• creates a bulge in the side of the mountain

What is the role of external water ?

If rising magma contacts water the water can vaporize

Intermingle with melt and cause an erruption

What is the classification of volcanic explosive index based of?

The volume of ejecta

What was the largest recored on the VEI

Tambora 1815

Do volcanoes always have the same classification during their life time ?

No, depends on composition and volatiles which change through the life of a volcano

Hawaiian

• basaltic ( low viscosity magmas)

• volatile bubble out quietly

• not very explosive

Strombolian

• bit more explosive

• erruption chacterized by short term out put of pasty lava

• results in scoria cones

• example

• MT etna Italy

• Particun mexico

Plinian

• much stronger

• eruption columns reach 45 km into stratosphere

• Examples

  Naples, Italy

  Vesuvius ( one of the most dangerous volcanoes in the world)

Surtseyan

• results from basaltic eruption interacting with groundwater or surface water

• wet equivalent of Strombolian eruption

• water cause to be more explosive

Vulcianian

• more explosive that surtseyan

• discrete canon like explosions

• followed by sustained eruption

Phreatophian

What is a glacier

Mass of compressed ice on land that moves under its own wight

What are the different types of glacier

Alpine, conteinal ice sheets

Where are continental ice sheets only found

Antarctica and Greenland

How do alphine glacier move ?

downhill

How do continental icesheets?

move outward from the center where mass of ice is the greatest

How do glaciers move?

plastic flow

basalt sliding

What happen when more snow is add than lost?

accumulation> ablation

Glacier advances (grows)

What happens when more snow is lost than gained

accumulation> ablation

Glacier retreats shrinks

What are the two forms of weathering for glacier?

Abrasion and frost wedging

Glacial deposits

angular poorly sorted grains

Glacial erratic

isolated large-grain remnants of glacial deposits

Moraine

piles of sediment dropped or spit out by glacier

Which landromed is produced through meltwater dissoposits from the toe of a glacier

outwash plain

The bigger the glacier, the bigger the valley and …

bigger the cirque

Glacier cover blank% of land surve todya and blank% 18,000 years ago(iceage)

10 and 30

Clastic

fragment of pre-existing rack (sandstone)

Chemical

precepted from solution Carbonate)

Biochemical

organisms play a role (shell material)

Organic

comprising large amounts of organic material

Weathering

The decomposition and disintegration of rocks and minerals at the earth’s surface by machinal and chemical processes

• converts rock to gravel, sand, clay and soil

Erosion

The removal of weathered rocks and minerals for the place they were found

• water

• wind

• glaciers

• gravidity

Mechanical weathering

The physical disintegration rock into smaller pieces each retaining there original characteristics

Chemical weathering

decomposition of rock and minerals and as a result f chemical reactions (removal and/or addition of elements)

What are the main processes of chemical weather?

• frost wedging

• salt cracking

• abrasion

• biological activity

• thermal expansion and contracting

• pressure release fracturing

Frost wedging

when water freezes it expands and water migrates into cracks in rocks. Ice crystals growth puts tremendous pressure on surrounding rocks

Most effective in mountain areas where daily freeze/ thaw. Dangers to hikers

Salt cracking

salts crystallize in cracks puts pressure on surrounding rock

Important in

dry climate:

costal areas

Abrasion

breakdown of rock by fiction and impact

• gilders(effective)

• wind

• running water(effective)

• waves(effective)

Biological activity

• plants growing in cracks in rocks

• burrowing animals

• human blasting for roads, development exploration

Pressure release fracturing

• buried rocks are under confining pressure

• when exposed they expand due to releasing

• problem for miners

What is the main agent of chemical weathering

water

Dissolution

produces ions ( not minerals) and are reversable is the solvent is removed

Ex dissolving soluble materials in water, the salt is re-participated if the water evaporates

Salt deposits are creating with the drying of ancient seas

Example of dissolution

Natural rain water fall through the soil and reacts with C02 to form carbonic acid.

Carbonic acid reacts with calcite (Limestone) to produce it

Hydrolosis

where dissolution involves the release of ions from a mineral

• is a reaction with water to form a hydrated mineral

Oxidation

reaction with oxygen

ex iron bearing minerals oxdize to form rust

What are the control on rates and types of weathering

• climate

• Oxygen and C02 in the atmosphere and soil

• Minerology

• Surface area- how fragmented the rock is

What are characteristics of sedimentary rocks?

Clast textures:

Size

Shape

Sorting

What is the majour control on size, shape, and sorting?

Agents of transportation:

• gravity

• wind

• water

• ice

What is an example of chemical; and biochemical sediment

Limestone

• formed from shell fragments and test (shells of microfossils)

What are the four general ways that produce volcanic rocks of different compositions