Evolution of Landscapes and Magma

marine transgression

water gets deeper → deep water sediments (finer grains), are deposited over shallow water sediments (coarser grains)

marine regression

water gets shallower → opposite pattern, presence of ripple marks and other patterns

hydrothermal metamorphism

metamorphic rocks

coastal, marine sediments

sedimentary sequence

igneous rocks

mafic → new ocean crust

accretion

material is added to a tectonic plate at a subduction zone, most often at an O-C convergence

igneous rocks at O-C convergence

intrusive and extrusive, ranges from mafic to felsic = intermediate

metamorphic rocks at O-C convergence

surround magma chambers, accretionary wedge

sedimentary rocks at O-C convergence

marine and continental sediments

orogeny

mountain building event

igneous rocks at C-C convergence

intrusive only (no volcanoes), felsic composition

metamorphic rocks at C-C convergence

regional metamorphism

sedimentary rocks at O-C convergence

new source of sediments for low elevation areas next to mountains, marine sedimentary rocks left over from O-C convergence could be pushed up tectonically

orogeny

Accretion of landmasses causes ____.

Reverse/Thrust faults, intrusive igneous and regional metamorphic rocks

What type of faults and rocks are associated with orogeny events?

Taconic → Acadian → Alleghenian orogenies

Appalachian formation orogenies names examples

600-550 MYA

supercontinent rodinia breaks up to form lapetus ocean

470-450 MYA

plate motion reverses, causing taconic accretion/orogeny

400-375 MYA

Acadian accretion/orogeny with Avalon microcontinent, Gondwana approaches

330 MYA

North America, now part of Laurasia, collides with Gondwana to form Pangaea

180 MYA

Pangaea starts rifting near the collision zone to form the Atlantic

mafic only

What composition of magma would you expect at a mid-ocean ridge?

We can correlate magma chemistry to mechanism of melting and the layer of the Earth in which the magma formed.

How do we make magma in tectonic settings?

increasing temperature

increasing kinetic energy and phase changes

increasing pressure

packs molecules tighter, making phase change more difficult

easier

If pressure decreases, ____ for molecules to move.

higher, increase

• High pressure makes rock melt at a _____ temp.

• Both temperature and pressure _____ as you move deeper into earth.

melting temp increases with increasing pressure

What does the slope of the line tell us?

increasing, decreasing

Magma forms by _____ temp and _____ pressure.

high temperature melting

increase temp of rocks to pass melting point

depressurization melting

decrease pressure to pass melting point

at depth, mantle & asthenosphere

high pressure keeps hot rock solid; limited processes that cause sufficient increase in temp → any increase in temp also results in decrease in density/rise

near surface, upper lithosphere

few processes that cause decrease in pressure needed to melt rock → no way to depressurize brittle solids quickly → uplift is slow enough that rock equilibrates with geotherm

ductile asthenosphere

decompression melting

lithosphere

high temp melting due to rising magma and/or changes in chemistry that decrease the melting temp

lower

Felsic minerals melt at ____ temps than mafic minerals.

partial melting

some components of the rock melt but others are still solid

highest

Mafic minerals melt/solidify at the _____ temp.

lowest

Felsic minerals melt/solidify at the _____ temp.

rigid structure

High water content makes it harder for magma to form ____ ____. → lowers solidification temp

melts, does not

When subducting plate is at depth/temp “E'“, high water content rock ____ and dry rock ____.

lowering, melt

Subducting crust releases water, _____ melting temp of overlying mantle and causing ____.

divergence/hot spots

decompression melting of mantle allows formation of magma

subduction

introduction of water at depth changes solidus and allows partial melting

continental crust melting

• felsic minerals easy to melt

• water release during metamorphism increases partial melting

• water and felsic minerals makes for viscous, slow-moving magma

geothermal gradient, underground

Magma forms and slowly moves towards surface, cooling via _____ ____ and solidifying _____.

melt

Felsic rocks ____ more easily.

high temperature melting

Mafic magma moving through felsic continental crust can cause ____ _____ _____.

subduction

_____ → increase in temperature

oceanic hot spots

_______ → decrease in pressure

Pyroclastic flows and/or eruption columns are likely to occur

An eruption occurs at a subduction zone! Which inference listed is most likely to be accurate?

B

Why do calderas form at continental hot spots but not oceanic hot spots?

a. Because the only source of magma is mantle decomporession at continental hot spots

b. Because partial melting of continental crust makes magma at continental hot spots more felsic and thus more explosive

c. Because magma generated at continental hot spots is more likely to be low viscosity 

d. Because continental hot spots are more likely to form shield volcanoes than oceanic hot spots

rhyolitic/felsic magma

high viscosity magma

basaltic/mafic magma

low viscosity

andesitic/intermediate magma

magma associated with subduction zones

basaltic/mafic magma

magma associated with divergence and hot spots

rhyolitic/felsic magma

magma associated with boundaries with continental crust

shield volcano

volcanoes/features associated with basaltic/mafic magma

stratovolcanoes

volcanoes/features associated with andesitic/intermediate magma

intrusive igneous bodies

volcanoes/features associated with rhyolitic/felsic magma

silica content, volatile content and temp

Magma viscosity is dependent on:

viscosity

resistance to flow

felsic magma

highest viscosity

intermediate magma

intermediate viscosity

basaltic magma

lowest viscosity

highest silica content, lowest temp

Why does felsic magma have the highest viscosity?

lowest silica content, highest temp

Why does basaltic magma have the highest viscosity?

stratovolcanoes

• range of compositions and viscosities

• convergent subduction zone boundaries

• continental divergence and hot spots

shield volcanoes

• basaltic, low-viscosity

• divergent boundaries

• oceanic hotspots

mafic magma

relatively high temp and low silica/solids, low viscosity

• low dissolved gases

• low-sloped shield volcanoes

• release of gases and aerosols

lava flow, release of gases

What are the hazards of mafic magma?

intermediate to felsic magma

relatively low temp, high viscosity and silica/solids

• high dissolved gases

• steep stratovolcanoes

solids → eruption columns, pyroclastic flow, tephra, lahars

What are the hazards of intermediate to felsic magma?

how explosive the magma will be and what hazards are run into

What does viscosity determine?

silica content

the higher this is, the more felsic it is

volatile content

the higher this is makes it more viscous

temperature

the higher this is the more easy it flows