Earth Materials Lecture 16: Volcanoes and Products Flashcards
Summarise the structure of a volcano
Vents - the openings at the surface where volcanic eruptions occur
Conduits - the channel ways that lead from the magma reservoir to the vent
Magma Chamber - Reservoir of magma
Describe contents of volcanic products and their contents
SiO2 ,Viscosity and Volatiles increase together.
They are related to more pyroclastic explosions
Temperature and density increase together
They are related to Lava/effusive eruptions
Give the two types of Basaltic lava flows
Basaltic lava flows usually either have pahoehoe or ‘a’ā morphological forms.
The surfaces of pahoehoe lava flows are smooth, ropy, or billowy, and ‘a’ā flows are rough and clinkery.
A’a lava: rubbly, blocky flows. Common in basalts and andesites.
Describe Basaltic Lava flows
Basaltic Lava has a low silica content and a low viscosity. (less than about 52 weight percent silica)
Therefore, basaltic lava can flow quickly and easily move >20 km from a vent. (More runny than Andesitic)
Creates Aa or Pahoehoe
erupted at temperatures between 1100 to 1250 ° C
Describe Andesitic lava flows
Andesitic flows have higher silica content (Intermediate) and therefore are more viscous
Silica content 60%
thick lava flows, some reaching several km in length. Move a few kilometers per hour (couple feet per second),rarely extend over 8 km vents.
Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns.
Andesites erupt at temperatures between 900 and 1100 ° C
Overall is between in Basaltic and Andesitic in Viscosity and overall flow characteristics
Describe Rhyolitic lava flows
Viscous and tend to form thick blocky lava flows or steep-sided piles of lava called lava domes.
Rhyolite magmas tend to erupt explosively, commonly also producing abundant ash and pumice.
Erupted at 800 to 1,000 °C
High silica content (70% - higher than Andesitic)
Describe Explosive eruption products
Black scoria/Scoria
Extremely fine glassy froth
Accretionary lapilli
Large oxidised basaltic bomb
Describe scoria
A pyroclastic, highly vesicular, dark-colored volcanic rock formed by ejection from a volcano as a molten blob and cooled in the air to form discrete grains called clast
Highly vesicular looks like it has lots of holes.
Describe glassy froth/pumice
Pumice is a very light colored, frothy volcanic rock.
Pumice is formed from lava that is full of gas. The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes
Some pumice looks very similar to Scoria
Glassy froth looks kind of similar to metal sponge scrubber thing
Describe accretionary lapilli
Accretionary lapilli are pellets that form by the accretion of fine ash around condensing water droplets or solid particles
Describe the various types of volcanic bomb
Core bomb
Breadcrust bomb - Looks like cracked bread, or cracked mud
Cow pie bomb - Looks like cow poo
Explosion bomb - Looks hollow
Fusiform/spindle bomb - Looks like spindle or linear gastropod shell or a tear
Define Columnar jointing
Columnar jointing is a post emplacement feature containing sets of regularly spaced parallel fractures (joints) that intersect in a roughly prismatic pattern. Most columns are hexagonal, but they may have anywhere from three to seven sides.
Often found in Sills, Ignimbrites and Lava flows
e.g. Fiamme in welded ignimbrite (eutaxitic),
Mount Aso, Japan.
Columnar jointing in sill, USA
Columnar jointing in basalt lava flow, Isle of Staffa, Scotland
Columnar jointing in the Bishop’s Tuff, ignimbrite, USA
Define an Ogive/Pressure Ridge
These ridges can also be found on some lava domes known as coulées, sometimes found to as much as 30m high.
The ridges are formed as a result of compressional forces, parallel to flow of the coulée.
Look almost similar to a small mid ocean trench, however more tall and not as long.
Coulees - Short flow of viscous lava
Define a levee (in a lava flow)
Lava, from small overflows, cools and congeals along the banks of the lava channel to build lava levees.
Basically a stream of lava
