Oceans have a continuous system of large ridges located some distance from continents, often midocean. Also, deep trenches occur at many places in the ocean floors, often around margins of ocean basins.
Seafloor Spreading —theory proposing that oceanic ridges are formed by currents of deep-seated magma rising up from the mantle (often during volcanic eruptions), creating new crust on the ridges (the newest crust formed on the planet).
Subduction — process proposed to explain trenches, making them the site where older crust descends into the interior of Earth, where it is presumably melted and recycled into the convective cycle that operates in Earth.
Theory of seafloor spreading supported by two sets of evidence:
Seafloor has a relative symmetrical pattern of magnetic orientation on both sides of ridges, indicating that it has spread laterally by addition of new crust (displaying how the magnetic field has reversed itself [more than 170 times]).
Ocean floor core sampling
Sediment age and thickness increase with increasing distance from the ridges, indicating that sediments farthest from ridges are oldest
Table comparing two eruptive styles and their characteristics
Basalt Andesite Common type of Lava Shield volcano Composite volcano Type of Volcano Subduction areas High silica content (felsic magma) More polymerization Thick magma More gas pressure build up High explosivity index Explosive Low silica content (mafic magma) Less polymerization Thin magma Less gas pressure build up Low explosivity index Magma chemistry Mid-ocean ridge and hot spots General locations Effusive (mild) Characteristics
Lava Domes —usually small, with irregular shape (e.g., Lassen Peak, California).
Cinder Cones —smallest of volcanic mountains (e.g., Sunset Crater in Arizona).
Calderas —uncommon, but large, steep-sided, roughly circular depression resulting from the explosion and subsidence of a large volcano (e.g., Oregon’s Crater Lake).
Volcanic Necks —rare but prominent sharp spire that rises abruptly above the surrounding land. It represents the pipe or throat of an old volcano, filled with solidified lava after its final eruption. The less resistant material that makes up the cone is eroded, leaving the harder, lava-choked neck as a remnant (e.g., Shiprock in New Mexico).
Volcanic gases include carbon dioxide, sulfur dioxide, hydrogen sulfide, and fluorine. Affects can range from acid rain that destroys local vegetation to droplets affecting solar insolation and lowering global temperatures (e.g., Mount Pinatubo in 1991 lowered temperatures for more than a year).
Lava flows cause more property damage than loss of life.
Eruption Column and Clouds
Violent ejection of pyroclastic material and gases; can reach elevations of 16 kilometers (10 miles) or more.
Pyroclastic flows —terrifying high-speed avalanche of searing hot gases, ash, and rock fragments (e.g., one on Martinique in the Caribbean killed nearly 28,000 inhabitants of one town in a matter of moments in 1902).
Volcanic Mudflows ( Lahars )—fast moving, and sometimes hot, slurry of mud and boulders; one of most common volcanic hazards. (e.g., Nevada del Ruiz volcano in Columbia produced a mudflow that killed more than 20,000 people in a town nearly 50 kilometers [30 miles] away).
Fig. 14-40 Eruption of Mount St. Augustine,
Alaska. A pyroclastic flow is moving down the slope to the left.
Laccoliths —an igneous intrusion produced when slow-moving viscous magma is forced between horizontal layers of preexisting rock. The magma resists flowing and builds up into a mushroom-shaped mass that domes the overlying strata. If near enough to Earth’s surface, a rounded hill will rise above the surrounding area (e.g., South Dakota’s Black Hills).
Dikes —a vertical or nearly vertical sheet of magma that is thrust upward into preexisting rock; probably most widespread.
Sills —a long, thin intrusive body that is formed when magma is forced between parallel layers of preexisting rock to solidify eventually in a sheet.
Veins —small igneous intrusions, usually with vertical orientation.
Normal fault —the result of tension producing a steeply inclined fault plain, with the block of land on one side being pushed up, or upthrown, in relation to the block on the other side, which is downthrown (displacement is mostly vertical).
Reverse fault —a fault produced from compression, with the upthrown block rising steeply above the downthrown block, so that the fault scarp would be severely oversteepened if erosion did not act to smooth the slope (displacement is mostly vertical).
Thrust fault —a fault created by compression forcing the upthrown block to override the downthrown block at a relatively low angle; complicated in structure.
Strike-slip fault —a fault produced by shearing, with adjacent blocks being displaced laterally with respect to one another (displacement is entirely horizontal).
A mountain formed under certain conditions of crustal stress, whereby a surface block may be severely faulted and upthrown on one side without any faulting or uplift on the other side. The block is tilted asymmetrically, producing a steep slope along the fault scarp and a relatively gentle slope on the other side of the block.
Primary or P waves are the fastest moving waves, moving through Earth like sound waves, alternately compressing and relaxing the material they pass through.
Secondary or S waves are slower moving waves, also passing through body of Earth, producing both strong side-to-side and up-and-down “shearing” motion.
A third type of waves, surface, do not travel through Earth like P and S waves do, but only travel across surface, immediately after S waves, and produce strong side-to-side and up-and-down “rolling” motion.