Unit II. Volcanic Rocks, Weathering and Geologic Time

VOLCANIC ROCKS

Hundreds of active volcanos erupt worldwide each year. Material extruded (erupted) onto the earth's surface during a volcanic eruption can consist of:

1. Lava - molten rock, usually flows in thin sheets and exhibits a variety of textures (eg. aa, pahoehoe, pillows).

2. Gasses - most gaseous emissions consist of water vapor, carbon dioxide, and sulfur and nitrogen compounds.

3. Pyroclastics - hot, "fire-broken" materials such as ash, lapilli, and bombs.

Interesting volcanic rocks include pumice and obsidian.

Volcanic Structures ---------------

Volcanoes can exist in a variety of forms on the earth's surface. Several types of volcanoes are recognized by geologists:

1. Composite or stratovolcanoes
2. Shield volcanoes
3. Cinder cones
4. Lava domes

Each of these types of volcanoes exhibits a typical style of eruption, based largely on the type of magma involved (specifically its silica content).

Other interesting volcanic features include:

1. Calderas
2. Volcanic necks and pipes
3. Lava tubes
4. Lava plateaus and flood basalts

Distribution of volcanic activity worldwide ---------------------

1. Spreading center (divergent boundary) volcanism such as that which occurs along submarine mid-ocean ridges and in Iceland.

2. Subduction zone (convergent boundary) volcanism such as that responsible for volcanic activity in the Cascades.

3. Intraplate volcanism (hot spots) such as that in Yellowstone National Park and in Hawaii.

WEATHERING

Weathering is the physical disintegration and chemical decomposition of rock material at or near the surface of the earth. This is not to be confused with erosion, which is the transport of rock fragments by the wind, water or ice.

Mechanical weathering includes those physical processes that reduce a rock to smaller and smaller fragments. Examples:

Chemical weathering alters rock materials through chemical processes. Examples:

The rate at which weathering occurs is primarily controlled by the nature and composition of the parent rock and the intensity of the climate.

Soils ---------------------

Soils are a mix of weathered mineral matter, air, water and humus.

Soils can be described in terms of the horizons that are represented in a profile. A mature soil can have the following horizons:

The names of soils can also reflect their major characteristics and climatic settings in which they formed. Pedalfers are formed in humid climates, and Al and Fe accumulate in the B horizon. Pedocals are formed more typically in arid to sub-arid climates, and have calcium carbonate precipitates in their B horizons. Laterites are formed in tropical climates where weathering is most intense, resulting in thick and very red soils.

Other factors influencing soil development (in addition to climate) are the parent rock material, the degree of organic activity, topography and time. Soils can be either residual (formed in place) or transported (carried in by wind or water).

GEOLOGIC TIME

The earth is old. Really, really old. The best scientific evidence points to an age of about 4.6 billion years for the oldest rocks. Here's a nifty exhibit about geologic time at the University of California - Berkeley.

The geologic time scale is based on evidence gathered from the relative dating and radiometric dating of rock sequences.

Relative Dating ------------------

Relative dating involves the detemination of the relative ages of rocks (and geologic events) to one another without knowing their absolute ages. The fundamental principles used in relative dating techniques include:

1. The Principle of Superposition - In an undisturbed sedimentary sequence, the sedimentary layers grow younger in an upward direction.

2. The Principle of Original Horizontality - Sediment layers are deposited in mostly a horizontal position.

3. The Principle of Lateral Continuity - Sediment layers extend in all directions until they thin out or terminate against another rock unit.

4. The Principle of Cross-Cutting Relationships - A rock or structure (fault, dike) must be younger than the unit it intrudes or cuts.

5. The Principle of Inclusion - Fragments of rocks included in a layer of sediment must be older than the layer in which they are contained.

6. The Principle of Fossil Succession - Fossils succeed one another through geologic time in a regular and predictable order.

Breaks, or interuptions, in a depositional sequence where a long period of geologic time has elapsed are termed unconformities.

Radiometric Dating ------------------

Radiometric age dating techniques fall under the category of absolute dating because the actual ages of the rocks are being determined (unlike relative dating).

This is achieved by knowing the half-life of naturally occurring isotopes that are present in most rocks. These substances will undergo radioactive decay, producing "daughter" products. By measuring how much of the original isotope and daughter materials are present, one can arrive at an age for the rock.