Volcanoes
Volcano – Mountain built of lava and volcanic ash which erupt
from a vent over rising magma.
Where do volcanoes occur? – *Most volcanoes occur along
______________________.
1. ______________ Boundaries – Where plates collide.
* Coastal Volcanic Mountains (Andes, Cascade Mts., Mt. St.
Helens – 1980) Usually Composite Cone Volcaones
__________________ – An area around the Pacific
Ocean of much volcanic & earthquake activity.
2. _______________ Boundaries – Plates spread apart.
*Most of the world’s volcanism takes place along the mid-ocean
ridge (Almost all is underwater).
3. _________________ – Areas in the middle of plates where
super hot magma burns through the crust making a volcano.
Example: Hawaiian Islands: The pacific plate moves over a hot
spot making the Hawaiian Islands.
3 Kinds of Volcanoes
1. Shield Cone – Made of layers of quiet lava flow.
*Commonly made with basaltic magma (ocean volcanoes) – ex.
Hawaiian Islands. (broad base & gentle slope)
2. Cinder Cone – Made of cinder and ash
*Commonly made with granitic magma (continental volcanoes)
– ex. Paricutan in Mexico.(steep sides)
3. C omposite Cone – Made of both layers of lava and cinder
and ash.
*Commonly found on Continents , Largest Volcanoes
*Ex. Mt. St. Helens, Mt. Fuji, Mt, Vesuvius
Factors that affect eruptions:
__________________________, _________________________, ________________________
Low Viscosity
High Viscosity
Low Gasses
High Gasses
Pyroclastics/Tephra: __________________________________________________
Magma and Igneous Rocks
How Magma Forms
•
•
•
Occurs when temperatures are high enough to melt the rocks involved (8001200 degrees C)
Pressure- because pressure increases with depth, it takes rocks longer to
melt—they need higher temperatures to melt.
Water- if a rock has water in it, it will melt at lower temperatures
Types of Magma
• ______________________high in silica-based minerals (quartz, feldspar,
mica) – Explosive eruptions (Mt. St. Helens)
• ______________________high in iron-based minerals (mainly dark—
pyroxene, olivine, amphibole) – Quiet eruptions (Hawaii)
• ______________________mixture of silica and Fe minerals
Igneous Rocks
Classifying Igneous Rocks
_____________: Determined by the cooling rate.
_____________: what they are made out of: Basaltic(Mafic) minerals
vs. Granitic(Felsic) minerals.
* ______________ igneous rocks—form from lava (on surface)—very small
minerals (aphanitic, vesicular, glassy)
* ______________ igneous rocks—form from magma (inside Earth)—large,
visible minerals (phaneritic).
Igneous Rock Textures
* If a rock has a:
* __________________ Texture, it formed extremely rapidly, allowing no
minerals to grow.

__________________ Texture, it cooled extremely rapidly. As it was
cooling, gas escaping from the rock formed bubbles

__________________ Texture, it cooled quickly, only allowing small
mineral growth

__________________Texture, it cooled slowly, allowing for large crystal
growth
__________________ Texture, had two-stage cooling—first slow
underground, then fast on the surface


_________________ Texture, it was created by the explosive release of
gases in a volcano welding lava fragments together.
Earthquakes:
The shaking or trembling caused by the sudden release of energy
Usually associated with faulting or breaking of rocks
Described by the Elastic Rebound Theory
Elastic Rebound Theory: ____________________________________________
_______________ – travel along the surface, cause the most damage
(Your teacher may divide these further into Love and Raleigh Waves)
(Slower than Body)
Body – travel through the Earth
• P waves- pushes and pulls the rock (twice as fast as S wave)
• S wave – slower, displaces rock at right angles, don’t travel through
liquids
Moho: The dept at which the P-wave velocity exceeds 8.1 Km/S is referred to
as the moho (after the seismologist Mohorovicic). The moho is both a seismic
and a compositional boundary, marking the transition between crust and
mantle materials.
Richter Scale:
Mercalli Scale: