Lecture 9 - Volcanic Evolution

Question 1

What does the Melting Point of a Rock Depend on?

Answer 1

-Temperature
-Pressure
-Volatiles (water and gases)
-Composition

Question 2

What does the Melt Composition depend on?

Answer 2

-What type of rock is melted
-How much of the rock is melted
-Mantle (peridotite) melts at 1200-1300C
-Basalt melts at 900-1100C
-Granite melts at 650-850C

Question 3

What is the Liquidus?

Answer 3

-The point where a whole rock (all minerals) is molten

Question 4

What is the Solidus?

Answer 4

-The point where a whole rock (all minerals) is molten

Question 5

How does the Source Rock affect Magma Composition?

Answer 5

-Mantle is ultramafic
-Oceanic crust is mafic (basalt)
-Continental curst is intermediate/ felsic (granite)
-Most the time, not all of the source rock is melted; only 2-30%

Question 6

How does Partial Melting affect Magma Composition?

Answer 6

-Rocks are made of different minerals
-Different minerals have different melting points
-Depending on the temperature, only some of the minerals will melt to produce magma
-The lower the % the partial melt, the higher the silica content (felsic minerals have a lower melting point)

Question 7

How does Magma Mixing affect Magma Composition?

Answer 7

-Two bodies of molten rock mix together
-This changes the melt composition, producing an average of the two
-Injection of new magma from depth tends to make the melt more mafic (higher melting point)

Question 8

How does Assimilation affect Magma Composition?

Answer 8

-Hot molten rock is injected into cold crustal rock
-Heat transfer from magma to solid means the crustal rock is melted
-This changes the melt composition (tends to make the magma more felsic)
-Xenoliths = blocks of the crustal rock that fall into the magma chamber

Question 9

How does Fractional Crystalisation affect Magma Composition?

Answer 9

-As magma cools, minerals crystalise faster than others
-This removes elements from the liquid
-The crystals sink/rise taking these elements with them
-The remaining liquid is a different composition (more felsic and volatile-rich)

Question 10

Why does Magma Explode?

Answer 10

-The lower the temperature, the more viscous and therefore explosive it is
-Presence of water and gas means they expand which causes explosions
-Felsic magmas are more viscous than mafic magmas (because Si bonds together in long chains which are very difficult to break)

Question 11

Describe the Characteristics of Felsic Melts

Answer 11

-Gasses have low melting points
-Partial melting, assimilation and fractional crystalisation concentrate gasses in the melt (making the melt more felsic and viscous)
-If no new magma is injected into the volcano, it naturally becomes more felsic over time (more gas-rich and more explosive!)

Question 12

Why do Explosive Eruptions not happen often at the Same Volcano?

Answer 12

-Large explosive eruptions release a lot of gas and pressure from the magma chamber
-Takes time for this to build back up
-Volcano therefore becomes less explosive for some time following an explosive eruption

Question 13

Describe the Santorini Volcanic Cycle

Answer 13

-Initially mafic intermediate melts producing effusive eruptions
-Fractional crystalisation and assimilation make melt more felsic and gassy
-Explosive Minoan eruption (c1600BC) releases gas
-Santorini returns to effusive behaviour and the cycle begins again!
-Strombolian eruptions are potentially restarting again as cinder cones are forming on the central island as magma is continuing to evolve

Question 14

Give a Background on Santorini

Answer 14

-Island in Greece
-Active stratovolcano but with elements of others
-Constantly changing

Question 15

What is the Present-Day Activity on Santorini?

Answer 15

-Basaltic/dacitic composition
-Low gas content and runny lava
-Shield volcano (Hawaiian-type volcanism)

Question 16

What was the Past Activity on Santorini?

Answer 16

-Red Beach (old scoria cone):
-intermediate magma
-more gas-rich
-Strombolian-type eruptions

-Pumice Cliffs:
-Felsic magma
-Very gas-rich
-Plinean eruption and caldera
formation (one of the largest in
recorded history)

Question 17

Describe the Minoan Eruption on Santorini

Answer 17

-Covered the whole island in ash
-Centre of the island sank beneath the waves
-Large tsunamis swept across the Agean, destroying many towns on the coast of Crete

Question 18

Where does Santorini’s Volcanism come from?

Answer 18

-Subduction zone
-Last remnant of the oceanic crust in the Mediterranean