Finish NW Earthquake Hazards (see previous weeks notes)
Review from Lab
Rocks are made of minerals
The amount of Silicon in a magma will determine which minerals form and therefore which rock type forms
The variation of composition will help determine…
The type of volcano that forms
The shape of the volcano
The hazards associated with the volcano
Felsic: high percentage of silica (SiO2) (>65%)
Intermediate: Silica from 55-65%
Mafic: silica from 40-55% and large amounts of magnesium (Mg) and iron (Fe)
Ultramafic: silica <40% and abundant iron and magnesium
Based on total silica in rock…all minerals together
Igneous rocks can be
Intrusive: form inside the earth
magma cool slowly so the minerals are large and can be seen easily
Granite, Diorite, Gabbro
Extrusive: form on the exterior or surface of the earth
Lava cools quickly so the minerals are very small and often can not be seen
Also known as volcanic rocks
Basalt, andesite, rhyolite, pumice, obsidian
Felsic:
Often pink and light in color
A small % of dark
minerals
intrusive: Granite
extrusive: Rhyolite
Intermediate
(Salt and Pepper)
intrusive: Dorite
extrusive:
Andesite (
Mafic
(Dark)
intrusive: Gabbro
extrusive: Basalt (Ocean floor)
Ultramafic
intrusive: Peridotite (Mantle)
Extrusive: Komatiites
Volcanic Rock types
Basalt—lava flow
Andesite—lava flow
Rhyolite—lava flow
Obsidian—lava flow (glass)
Pumice—pyroclastic
Scoria—pyroclastic
Ash Tuff—pyroclastic
Pyroclastic: any rock thrown out of a volcano during an eruption
Different magmas are created by melting different sources--> creates different rock types
Partially melting a rock will always produce a rock that is richer in silica
WHY?
How can we get a rock to melt?
Add Heat
Lower pressure
Add water
Where do volcanoes form and what processes is cause rock to melt and create magma?
Divergent Boundary
What processes is causing melt?
What is being melted?
What is being created?
Continental Rifts?
Subduction Zones:
Processes?
What is melting?
What is being created?
Hot spots
What process?
What is being melted?
What is being created?
How are Felsic Magmas/lavas created?
Primarily created by melting silica rich rocks.
Melt Andesites/Dioriteà Rhyolite/Granite compositon
Often associated with intra-continental hot spots and sometimes subduction zones
Are all basalts the same?
Are all andesites the same?
NO!!!
Composition of rocks will vary based on the starting material of the rocks
Columbia River Basalts vs Hawaiian basalt
By looking at the chemistry of a rock we can say something about it’s origin and the process that formed it
The composition of a magma will determine the shape, structure and size of the volcano as well as how violent the eruption will be
Physical properties of magma
How can the physical properties of magma vary?
Temperature
% silica
% of water
All of the above can affect the viscosity of a magma
Increasing viscosity
Lower temperature
Increase silica
Silica creates a framework structure in the magma
Lower the amount of water
water interrupts the silica structure and allows easier flow
Silica and Temperature: important for type of volcanic eruptions
Low silica = high temp = low viscosity
allows gasses dissolved in solution to easily escape
creates fluid lava flows or fire fountains
ex//
High silica = low temp =high viscosity
As magma rises to the surface pressure is lowered
Due to low pressure, any water or gas dissolved in magma comes out of the magma but can’t escape
pressure builds and explosion occurs
ex//
How does the viscosity of the lava affect the shape of the volcano that forms?
Types of volcanoes
Shield Volcanoes
created from mafic lava flow
high temperature and fluid flows
Cinder cones
mafic in composition
created by fountaining effect
Primarily scoria
Composite cone
Intermediate to silicic volcanism alternating layers of ash and lava
conical looking
Pumic cone
Silica volcanism
All pumice
Collapsed Caulderas
High silica content
Very explosive
Creates large crater area
10 x 20 miles
Scale was created based on explosiveness of a “typical” eruption
Hawaiian Activity
Mildest of all activities
Basaltic eruptions (high Tempà low viscosity)
Fire Fountainsà volatiles escapes easily
Lava flowsà long flows
Strombolian Activity:
Intermittent explosions lasting a few seconds
Basalt is higher in Silica (more viscous)
Lots of ejected fragments
Named for island between
Mt Erebus (Antarctic), Pacaya (
Vulcanian Activity
Small magnitude eruptions (<1 km3 of material)
Ash columns of 10-20 km
Explosive activity that can destroy part of ediface
Explosions last minutes to hours
Andesitic (intermediate) compositon
Pyroclastic flows evident
Vesuvian Activity
High Ash Column (~30 km)
Long sustained eruption
Composition varies from Rhyolite to Basalt
Vesuvius
Plinian Activity
Very powerful eruptions
Ash column up to 45 km
2-3 eruptions per century
Mt St Helens
Ultra Plinian Activity
Ash columns > 45 km
None in historic times
Research shows eruptions have occurred
Taupo New
Tambora
Phreatic explosions
Create when magma interacts with groundwater or lakes
Steam explosion
Creates tuff rings
Magnitude of the eruption
Amount of Mass ejected during eruption
Log10(masskg) -7
Why would it be difficult to calculate the magnitude of an eruption?
Ejected material can be widespread
Lava flows vary in depth
Density of products vary
To get the total of the mass ejected the density and distribution of all volcanic products must be known
Intensity of an eruption
Based on the mass eruption rate (kg/sec)
Log10(mass eruption rate) +3
Mass eruption rate controls
The height of the plume
Height of plume dictates how widespread debris is
How quickly lava flows
Combine magnitude and intensity to get VEI
Volcanic explosive index
Describes size of explosive eruptions
Scale goes from 1-8