Lecture 9 - Diversity of Volcanic Activity and IgneousRocks

Outline

I. Diversity of Volcanic Activity

II. Classification of Volcanoes and Volcanic Products

III. Why so Much Variety?

IV. Minerals- The Building Blocks of Rocks

II. Classification of Volcanoes and Volcanic Products

A. Volcanoes

  1. Shield Volcano-broad, gradually sloping sides produced from accumulation of successive lava (called magma until it reaches the surface of the Earth) flows, HAWAII
  2. Strato (or Composite) Volcanoes-steep, symmetric sides composed of alternating layers of lava flows and airborne volcanic material, CASCADES
  3. Rift Zones- produce flat thick plateau of cooled lava, Craters of the Moon National Monument, Idaho; portions of Hawaii
  4. Cinder Cone- small steep sided cones composed of airborne volcanic material that cooled in the air and did not weld together
  5. Volcanic Dome- Lava flow that is too thick to flow very far and builds up a Dome

B. Volcanic Products

  1. Lava- molten or liquid rock, pahoehoe (smooth) and aa (blocky)
  2. Pyroclastic Deposit or Tephra-Volcanic material ejected in the air, classification based on particle size such as ash is a very fine grained pyroclastic deposit (don't need to know all the specific names)
  3. Lahar- hot mudflow, common on composite volcanoes where eruptions melt snow and cause landslides.

III. Why so much Volcanic Diversity?

The diversity of volcanoes and volcanic products is due to differences in the composition of the magma (molten or liquid rock beneath the surface of the Earth) which controls its VISCOSITY, and the gas content. To understand this lets look at why volcanoes erupt.

Liquid rock is less dense than solid rock so magma migrates towards the surface. Gas becomes less soluble (dissolved) in magma as it approaches the surface (pressure decreases) and causes gas to come out of the magma, often violently just like opening up a soda bottle causes gas to escape due to a decrease in pressure. The greater the gas content of a magma, the more explosive the eruption.

What about VISCOSITY? Viscosity is the resistance of a material to flow, oil is more viscous than water and honey is more viscous than oil. The more viscous the magma, the harder it is for the gas to escape during its trip to the surface and the more explosive the eruption.

How does composition affect viscosity? Magmas rich in silica (Si is the most common rock forming element next to oxygen) are much more viscous than those poorer in silica. This is true because silicon bonds to 4 oxygens making up the silicon tetrahedron (siO4) in such away as to create a very strong unit. This unit attaches with otherSi04 units to form a very strong network that is difficult to break, therefore more viscous and more like a solid. We give volcanic rocks of different compositions different names that depend on the amount of silica (measured as silicon oxide, SiO2) present:

SiO2 Content:

45-53%

53-70%

>70%

Volcanic Name:

Basalt

Andesite

Rhyolite

Plutonic Name

Gabbro

Diorite

Granite

Description:

mafic

intermediate

felsic

Viscosity:

low

intermediate

high

General Color

dark

intermediate

light

IV. Minerals- The Building Blocks of Rocks

Mineral- naturally occurring crystalline solid with a specific composition
crystalline-atoms making up the mineral are arranged in an orderly predictable manner

The chemical composition and arrangement of the atoms give minerals their specific properties and make them unique. Although thousands of minerals exist, only about 30 are common minerals that make up most rocks. The most common rock forming minerals are made up of the two most abundant elements in nature, oxygen and silicon and are called silicate minerals. Calcite (CaCO3) is a common mineral found in limestones and is not a silicate (does not contain silicon).

Silicate minerals are classified according to how the silicon tetrahedra are linked together.

  1. Isolated Tetrahedra- Olivine- very common mineral in oceanic crust (basalt) and mantle (peridotite), usually green (due to iron) gem quality called peridot
  2. Single Chains of  Tetrahedra- Pyroxene- also common mineral in oceanic crust and mantle
  3. Double Chains of Tetrahedra- Amphibole- common mineral in continental crust (diorite or andesite)
  4. Sheets of Tetrahedra-Mica- common mineral in granites or rhyolites
  5. Framework of Tetrahedra- Feldspar (common mineral in granite and rhyolite, in continental crust (diorite or andesite) and even in oceanic crust (basalt) Quartz -common mineral in granites and rhyolites.

Extrusive

Rhyolite

Andesite

Basalt

 

Intrusive

Granite

Diorite

Gabbro

Peridotite

Occurrence

Up. Cont crust

Ave. Cont. crust

Ocean crust

Mantle

Minerals

Feldspar, quartz

Feldspar, amph.

Feldspar

Olivine, py.

 

mica

 

py., ol.

 

In addition to Si and O, these minerals contain other elements like iron (Fe), magnesium (Mg), calcium (Ca), aluminum (Al), sodium(Na) and potassium (K). These elements fit between the silicon tetrahedra.

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This page was last reviewed on 1/26/03.