Lecture 7 - During the Eruption

Question 1

What are the Products of Effusive Eruptions?

Answer 1

-Lava flows (basaltic or rhyolitic)
-Basaltic comes in two forms; ah ah and pahoehoe (ah ah is hotter and faster flowing, pahoehoe slower and forms a smooth crust)
-Rhyolitic flows are very rare, highly viscous and don’t travel far

Question 2

What are the Products of Explosive Eruptions?

Answer 2

-Pele’s tears (volcanic glass droplets) low velocity lava
-Pele’s hair (fine strands of volcanic glass) high velocity lava
-Tephra (bombs, scoria, pumice, ash)

Question 3

What are Volcanic Bombs?

Answer 3

-Large chunks (>64mm) of lava that have been blown into the air
-Big version of Pele’s tears
-Relatively light as they are full of gas
-Pumice: -floats (lots of vesicles)
-felsic composition
-not very dense

Question 4

What are Lapilli?

Answer 4

-Smaller (2-64mm) fragments of rock
-Basaltic lapilli is called scoria

Question 5

What are Lithic Fragments and why are they Useful?

Answer 5

-Fragments of bedrock around volcano or within volcanic conduit
-Appearance of them in tuff tells us that the volcano became unstable at this stage during the eruption
-Volcano was torn apart and incorporated these fragments into the ash column

Question 6

What is the Eruption Column?

Answer 6

-Super-heated cloud of tephra and gas
-Can rise 10s of kilometres into the air
-Rise is generated by heat and pressure from below, generating giant convection currents
-These inject gasses into the stratosphere

Question 7

What are the Different Types of Lava Structures?

Answer 7

-Pillow lava
-Lava tubes
-Lava tubes

Question 8

How do Pillow Lavas Form?

Answer 8

-Each pillow forms a glassy crust and expands like a balloon
-This ruptures to form a new pillow
-Top layer of the oceanic crust is made of these
-Piles of them make new landforms

Question 9

How do Lava Tubes Form?

Answer 9

-Hollow tunnel where lava used to flow
-Form when the top of a low-viscosity lava flow cools forming a thick crust
-Lava moves underneath, draining out and leaving a tunnel

Question 10

How do Lava Trees Form?

Answer 10

-Lava flows travel through forests at a high level
-When lava meets a tree, the sap creates a protective barrier which preserves and almost fossilises the tree
-Lava eventually drains to a lower level which leaves the cooled margins as fossil remnants of the trees

Question 11

What are Pyroclastic Density Currents?

Answer 11

-Mixture of super-heated gas and ash (up to 1000C)
-Fast-moving (up to 700km/h)
-Deadliest volcanic hazard
-Burn everything in their path
-Rock deposits from these are called ignimbrites

Question 12

What are the 3 Different ways in which Pyroclastic Density Currents Form?

Answer 12

-Dome collapse
-Directional blasts
-Column collapse

Question 13

How does Dome Collapse create a PDC?

Answer 13

-Structure at the top of the volcano collapses under gravity
-This forms an avalanche becoming a pyroclastic density current

Question 14

How does a Directional Blast create a PDC?

Answer 14

-Caused by a particular side of a volcano collapsing/ exploding

Question 15

How does Column Collapse create a PDC?

Answer 15

-Eruption column of gas keeps rising due to convection
-If this heat runs out, the column collapses
-Gas and ash roll down the valleys in the form of a PDC

Question 16

What are Tsunamis?

Answer 16

-Huge waves typically generated by earthquakes
-Volcanoes generate them by PDCs or landslides impacting water
Both displace huge volumes of water which forms a destructive wave
-Mega tsunamis are capable of crossing oceans and may be over 100m high (these are caused by the collapse of entire islands)