1. BHS Earth and Space Science
Volcanoes
Mt St Helens before and after 1980

2. Density Density is the key to understanding volcanism.
Remember mantle convection from two weeks ago.
Hotter, less dense rock within the mantle rises and sometimes finds its way through the lithosphere to the surface. Rut roh.
To do this, it must either melt or break through the crust, which is anywhere from 6-50km thick.
Density

3. Most volcanoes burst onto Earth’s surface along the boundaries of the planet’s shifting tectonic plates.
Unique types of volcano pop up on different spots on the plates.
Locations

4. Locations Volcanoes can be found:
Where two oceanic plates converge (lots of examples in Indonesia)
Where two oceanic plates diverge (Iceland)
Where a hot blob of magma within the mantle rises to the surface (Hawaiian islands)
Where an oceanic plate subducts under a continental plate (Mt. St. Helens, WA)
Locations

5. Shapes Volcanoes can have several shapes:
Shield
Cinder cone
Composite (AKA stratovolcano)
Lava dome
Caldera
The shapes are determined by the magma, as we will see in this week’s lab. Talk about anticipation!
Shapes

6. So you live next to a volcano. How silly do you feel
So you live next to a volcano. How silly do you feel? Depends on the type of magma within.
Volcanoes either erupt quietly or explosively.
For a million extra credit points, which one sounds worse?*
*Extra credit will not be awarded.
Meh or Arrgh?

7. Shield volcanoes Shield volcanoes usually erupt quietly.
Named because they are shaped like a warrior’s shield (really).
Form from continual flows of highly fluid (low-viscosity) basaltic lava that cool in thin sheets.
Can be 5-6km in diameter and m high.
Shield volcanoes

8. Both Mauna Loa
Shield volcanoes

9. Cinder cone volcanoes Simplest type of volcano. Erupt explosively.
Built from particles and blobs of congealed lava ejected from a single vent.
Fragments solidify and fall as cinders around the vent to form a circular or oval cone.
No more than 300m high.
Cinder cone volcanoes

10. Cinder cone volcanoes

11. Composite volcanoes Alternate between explosive and gentle eruptions.
Gentle lava flows build up the volcano into an impressive mountain.
Near the peak, pyroclastic material can form a steep-sided cone.
Can be over 2km high!
Composite volcanoes

12. Mt Fuji – 16 eruptions since 781.
Composite volcanoes

13. Lava dome Form around vents that erupt lava high in silica.
This viscous (sticky) lava doesn’t flow far, so forms a steep-sided, rounded dome.
The dome’s vents may be blocked up by hardened lava.
Pressure of the underlying magma may build, causing an explosion.
Lava dome

14. Lava dome

15. Lava dome

16. An explosive eruption can empty a magma chamber beneath a volcano.
This may cause the roof of the chamber to collapse under its own weight. Stupid gravity.
Leaves a huge crater that may fill in and become a lake, but may erupt explosively again.
Caldera

17. Caldera

18. Caldera

19. Lil YouTube clip
Mt. St. Helens