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!