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GEO 1001

SS-4 EARTH MATERIALS

VOLCANOES&SOCffiTY

Just the Basics ...

Earth Materials • How elements are bonded together can be as important as which elements are bonded together.

• Ionic bonds can react more easily with water than covalent bonds.

• Silicate ions link together (polymerize) more readily in silica-rich systems.

Minerals:

solids, with a specific chemical composition & a characteristic crystal structure

minerals do NOT equal rocks

rocks

are aggregates of minerals, other materials, or minerals & other material

not all solids are minerals!

>98% f Earth'~ Cnlst is composed of eight elements

but over 3000 different minerals Orf- ~"'r'lr

~ so how elements are bonded together can be as important as which elements are present

Earth's Crust

Element % by Weight Oxygen (0) 45.2%

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? /h/?()()h

Silicon (Si) 27.2% Aluminum (AI) 8.0% Iron (Fe) 5.8%

Calcium (Ca) 5.1% Magnesium (Mg) 2.8% Sodium (Na) 2.3% Potassium (K) 1.7%

differences in crystal structure (chemical bonds)

affect physical and chemical properties ofminerals

for example:

Polymorphism:

minerals with same chemical composition but different crystal structures

Covalent Bonds:

shared electrons

Ionic Bonds:

transfer electrons, + & - ions then attracted by charge

Both can be equally strong, but ionic reacts more readily with polar water molecules

So What?

-t,~~ J{f...~ -"'D1 \- Sr~ , ngly bound,

';\ ~ "I - ~~ l(.~ CArt~~ ~ ,\ combined i ns can ac

v (c\-y)a-?

M ~f'

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Silicate minerals make up most of the earth's crust

oxygen atoms in silicate ion still need electrons

can achieve more stability by either: io~n~i.:::.c~b:.:::o~n~d..........~~o::;:,:s:::.:i..::..:tic.:..v.:::.e...:..io:::.;n:.::s::.!...,

or

Polymerizatiob

two Si atoms share covalent bonds with an 0 atom to build much larger molecules

PolYmerization of silicate ions creates many of the most common mineral groups

But polYmerization of silicate ions also occurs in magma and leads to important differen~'!WL behavior

Igneous Rocks:

Rocks that form from magma:

a mixture of liquid, mineral crystals and gas (mostly water vapor)

95% of earth's crust is composed of igneous or metamorphosed igneous rock

usually divided into two broad genetic groups:

Intrusive (plutonic)

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rocks that form from magma cooling beneath the earth's surface

Extrusive (volcanic)

rocks th~rm from magma ~oling at the ea~s surfaGe)

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but how can you t II where an igneous rock originally formed?

Texture is sed as a proxy for cooling rate: \' &;T1 k ) ~ \f)1 k. ~ ~61......c &-- coarse- ained (phaneritic) cooled slowly J 1 G4.Y:J ~

Ql)t-~ 11 . d' k"a coarse-grame Igneous roc s a~ mtrusDLe-~

includes all volcanic roc}u; A some intrusive rocks V

(for example: those that formed when magma squeezed into other rocks along fracture planes)

a mixture of coarse and fine crystals can reflect two stages of cooling

and sometimes coolin,wtan be so rapid that gas bubbles are trapped ve . les, umi e

or that mineral crystals don't even have time to form volcanic lass) ~ oh~tL~

Just the Basics ...

Volcanoes • A volcano is any vent that allows magma to reach surface

• Magmas usually form as a result of changes in pressure or water content, NOT increased temperature! - .

• More silica-rich magmas tend to erupt more violently

• The type of magma and volcano ties closely with plate tectonic settin0

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• Volcanic activity & human society interact on different time scales.

Volcanoes

any opening that allows magma to reach the surface is a volcano, J

most are simple fissures, not tall mountains v

Society uses terms like:

active dormant extinct} ~~ em ~rrl""l. ~ .s:~k~ But these are often misconceptions, reflecting a human perspective and timeframe ·11--7 _DorJr

-ex Ut-

How do we get magma to begin with?

Magmas can be generated by:

, an mcreas temperature m

or a decrease in pressure V or introduction of water into rock V

increase in temperature easiest to conceptualize, least important here

Pressure Decrease

Melting points are combination of pressure & temperature

Why?

Liquid phase is less dense than solid, so has more volume.

High pressure - more energy required to go to liquid state

Low pressure -less energy required, so melting points drop

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Water introduced into hot rock will also lower melting points.

Why?

spreading centers/rifts

pressure decreases as mantle rock convects upwards

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subduction zones

water driven off subducting plate into overlying warm mantle rock, lowers mineral melting points to gener a- ,

collision zones

as ~-we~ rock crumples down into areas of high teT!2P~ratl!r.e and pressure

deep seated plumes of rising hot mantle rock

pres~:e decrease generates magma

-- hot spots

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\1 how can magma composition vary from that of its source rock?

magmatic differentiation can result from

Partial Meltin~ &/or Mixin~

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Partial Melting:

Rocks are aggregates of minerals each with different melting points, so melting is not uniform

Melt (less dense) tends to separate from remnant rock.

Partial melting can produce magmas that are quite different than their parent rock.

presence or absence of water has a profound influence on melting points - hence on magma composition

Mixing

Once a magma forms, it can melt and assimilate some of the rock through which it rises

depending on rock type, this process will usually increase silica content

- fW S -b ~,,-),u.hl. Oc::nic settings: ~-=

~~I~ ()

i~ ~l ~ h Continental settings: - - Y -= I .,J I___~~ m-m~

Magma

Magmas are commonly divided into three groups:

Felsic Intermediate. Mafic ""'r Rhyolitic Andesitic Basaltic j

v~ l-n~h

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(low viscosity = high fluidity),

it varies with temperature (high temp = low viscosity),

and gas content

but polymerization (linking of silica tetrahedra to form large molecules) is a crucial factor

Different types of volcanic eruption do tend to characterize certain plate tectonic settings

but realize that the following are generalizations!

a single volcano can expel a variety of magmas during an eruption cycle

Mafic (basaltic) magmas

Dominate at Spreading zones -;r -e..y. Mid-Ocean Ridge Basalts are derived from dry partial melting (~1-15%)

due to pressure drop as upper mantle material rises

Basalts also form over oceanic hot spots and during early stages of subduction

Basaltic magmas:

tend to flow as lava (little ash)

low silica content - low viscosity

relatively little gas - easily escapes

. hield1volcanoes dominate

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--ss3 Page 9 of 11 // hield volcano -1 w slopes «100 , built by multiple lava flows LL ~ - ---- e~ h' W' (lPlvV'\A..

stable, relatively safe features If-l1--t- Ft'\.J... l-""" ..,J.J.. \£.~ ~lrv, rJ.. ~ !.-)"{ ~,~

Because basaltic magma is so fluid, some eruptions can result in

Flood BaSaltsJ

~~~hL

Int'e. mediate (andesitic) magmas°n°'Z/ L-;high viscosity""--~-'---

composite volcanoes ,gominate J

composite volcano (stratovolcano) - high slopes (t

built of combined lava flow and ash fall/flows ~ -

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Andesitic & Rhyolitic eruptions are ash-rich:

Pyroclastic Flow = nuee ardente ('glowing cloud')

a mixture of ash and air, can flow up to 200 km/hr

Lahar - remobilized flow of deposited volcanic ash as rainwater or meltwater mixes with old ash (~k 1A.."~ deposits r \J\-l'l i) ({\.J1, P1(ivW'l~ )-,,,u...

Felsic (rhyolitic) magmas

typically form from wet partial m g.........-"='........'"'=_~c:....:n=lst as two continents collide ~

or as .:---....:;.::;.;:.;:.:very high viscosity

Magma can hold more water at high pressures than at low

More water = lower melting points

as magma rises->pressure drops->water lost->melting points rise

So most rhyolitic magmas form granite rather than rhyolite. If rhyolitic magma does reach surface, it tends to slowly pile up over vents as thick bulbous domes

but if gas pressure is high enough,

rhyolitic magmas can erupt explosively to form large cald r

ash » lava flows

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caldera - large collapse features over emptied magma chambers

Short Term Effects: m h

c.\

U. V I \ \ \' ~ l-lvr . 'volcanic eruption ~ OJ J c::. I C1 u u" v U.J ..,A ':SlyJ'(modem and ancient) I .

I ) 3() .J ) J. 'f 10" Ou J L ~ I 60 '-'v \

eruption aftennath ,

~_o_---,-climatic change

Long Term Effects:

_ atmosphere

crustal recycling' (mountain building)

mineral resources I

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