Basic course of volcanism - heating from the interior produces melts that are less dense than surrounding country rock; these rise to the surface (or intrude higher rocks) and produce erupting magmas.
A. Common volcanic rock types:
Compositionally, commonly classified by silica content:
1. felsic, light-colored, lots of quartz (SiO2) and feldspar [(K, Na, or Al)Si3O8] 70% Si by weight. Example: Granite
2. intermediate
3. mafic - example: basalt
4. ultramafic - low (<40%) silica content, lots of olivine[(Mg, Fe)2 SiO4] and pyroxene [(Mg, Fe) SiO3]
B. Crystal size usually indicates depth of eruption, with larger crystals indicating longer cooling time and hence deeper depths.
1. Granite, Grabbo - intrusive rocks.
2. Basalt, Rhyolite - cooled at the surface.
3. Pumice, Obsidan, tufts - cooled while traveling in air, very extrusive.
C. Various volcanic flows:
1. Aa - Black, jagged, slow moving flow.
2. Pahoehoe - More fluid (a few km/hr), has ropy texture.
3. Pyroclastic deposits - ash, dust, glass bomb, etc. Usually rough with lots of vesicles, breccias (angular clasts in a matrix).
4. Nuée ardente - fluid avalanche of hot ash and dust.
II. Principle locations of active volcanism on Earth;
A. Spreading centers - where two tectonic plates pull apart, basalt erupts and fills in the gap. Example: Mid-Atlantic Ridge.
B. Convergent margins - the subduction process takes H2O to great depths where it serves to lower the melting point of the rocks. Result is volcanism in the overriding plate, often with felsic composition. Granite volcanism is probably rare on other planets. Example: The Andes.
C. Intraplate - Volcanoes whose heat source is deep in the mantle; the volcano source appears fixed relative to the moving plate, often resulting in a chain of basaltic volcanoes where only the one on the end is currently active. Example: Hawaiian Islands.
III. Primary, Secondary, and Tertiary Magmas.
A. Primary magma - Forms at the end of planetary accretion, a magma ocean extending several hundred km in depth. The lunar highlands are a remnant of this primary magma, but no remnant exists of Earth's magma ocean.
B. Secondary magma - Forms when later radioactive heating partially melts rock in the planet's interior. The lunar maria, and Earth's spreading centers and hot spots are examples of secondary magmas.
C. Tertiary magmas - form from remelting of crust produced by secondary magmas. Granitic arc volcanism is an example.
IV. Different types and shapes of volcanic structures are particularly dependent on composition.
A. Basalt flows fairly freely and is often emplaced as sheets (flood basalts) or broad, low shields.
B. Mare granitic compositions have more volatiles that are harder
to lose because the magma is more viscous. The result is often
explosive ash flows and volcanoes with conial tops.
Tectonics. The faulting and folding that results from stress in the lithosphere.
I. The basics of stress and strain
Strain (deformation) results from applied stresses (measured in force per unit area). The stress field at any point can be characterized as resulting from three perpendicular "principal" stresses. At the earth's surface one principal stress is always vertical. At the Earth's surface 3 types of faults can arise:
A. Normal - vertical stress is most compressive principal stress
B. Reverse or thrust fault - vertical stress is least compressive
C. Strike slip - vertical stress is intermediate
II. The strength envelope - a description of the resistance to inelastic deformation with depth.
A. In the upper several kilometers of the Earth deformation is in the "brittle regime".The crust is assumed to be highly fractured so that resistance to faulting is controlled by the frictional resistance to sliding along a fault, an assumption known as Byerlee's law. If the difference between the largest and smallest principal stress exceeds this strength than faulting occurs. Strength in the brittle regime increases with depth because of increasing hydrostatic pressure on the sliding plane.
B. The resistance of rocks to ductile deformation (flow) decreases with depth, in part because viscosity of rock decreases exporentially with increasing temperature. At some point with increasing depth it becomes easier to deform the rocks ductilely than brittlely. This is the "ductile regime".
C. In a compositionally layered lithosphere (e.g., crust overlaying mantle) it is possible to go from brittle regime to ductile regime to brittle regime to ductile regime (and so on) if the lower layers are stronger than the upper layers.
III. Some typical examples of tectonic deformation on the planets:
A. Earth's deformation is dominated by faulting and folding caused by plate tectonics.
B. Venus has some highly deformed areas called tesserae, but no obvious current system of plate tectonics.
C. On the moon tectonic deformation often takes the form of ridges and graben resulting from the cooling and subsidence of mare basins.
D. Planetary contraction (Mercury) and expansion (icy satellites) as a planet cools can produce large-scale faulting.
Principal agents - wind and water
Erosion dominates the appearance of Earth's surface
Erosion makes most of the land surface flat.
Stream and glacier cut valleys obscure the faults that form mountains, etc.
Erosion-produced soil covers tectonical volcanic features.
Mars is the only other planet with significant erosional features.
Sanddunes
Large outflow channels
Dendritc stream valleys