Magma: liquid, molten rock underground
Lava: magma that reaches the surface
The composition of lava depends on
dissolved gases and silica content
Dissolved Gases
Example: water vapor, carbon dioxide (CO2), sulfur (S)
*More gas = more explosive eruption
Silica Content More silica (SiO2) = thick magma
Viscosity- ability to resist flow.
High viscosity = thick flow
Low viscosity = thin flow
Two classifications of lava:
1) Felsic: much silica, light-colored, slow moving (continental crust)
2) Mafic: low silica and gas, dark-colored, fast moving (oceanic crust)
2)
Pahoehoe
= ropy
3 types
of mafic
lava:
1) Aa
Lava=
choppy
3) Pillow Lava= underwater
Hardened Lava
In Hawaii
Volcano: place where magma reaches the surface
Volcanism: when a fissure or crack develops in the crust allowing magma to reach the surface
Pahoehoe
HOT SPOTS: areas of volcanic activity in the middle of lithospheric plates
Hawaiian Islands
•Volcano Anatomy
1. Vent: opening from which lava flows
2. Crater: funnel-shaped pit or depression at top of volcano
3. Caldera: depression where the walls have collapsed
Volcanic vents turn into plugs when the lava cools
Diamond Head Crater in Hawaii
Pyroclastic Material (tephra):solid particles produced in eruptions
1. Ash (<2mm)
2. Lapilli (<64mm)
3. Bombs
4. Blocks
ash
lapilli
bombs
blocks
Types of volcanoes…
•composed of calm lava flows
•form gently sloping, dome- shaped mountain
•basaltic (mafic) magma
Example: largest = Mauna Loa in Hawaii
•Made mostly of tephra and other rock particles
•Formed from explosive eruptions
•small, narrow base & steep sides
•Grantic (felsic) magma
•Symmetrical build up of alternating layers of rock and lava
• highly explosive eruptions with high silica and high gas content
•forms large, cone-shaped mountains
Mt. St. Helens- before
Composite example 1
Mt. St. Helens- after
Ex. 2 -Mount Vesuvius - Pompeii
20 feet of
volcanic ash
smothered
The city!
Mount Vesuvius – Pompeii= composite
20 feet of
volcanic ash
smothered
The city!