Geological Basis of Geomorphology

1. Active role of the geological framework: endogenic processes
   • work performed on the lithosphere by internal heat
   • tectonic (structural) landforms
   • diastrophism
     • orogenesis: mountain building
     • epeirogenesis: continent building
   • vulcanism
2. Passive role of the geological framework: resistance to exogenic processes
   • lithology: physical and chemical properties of rocks and sediment
   • structure: orientation of rock units and discontinuities
     • determine the morphology of structurally-controlled landforms (erosional landforms in resistant materials)

Endogenic Processes

• source of energy: internal heat (geothermal energy) from decay of radioactive isotopes

• U²³⁸ and Th²³² have the longer half-lives, so they are presently the principal sources of radiogenic heat
• 2.4 x 10²⁰ cal/yr transmitted to the earth's surface (solar constant: 1.97 cal/sq. cm/min)
• K⁴⁰ is the most abundant radioactive isotope
• dominant mechanism of heat transfer is conduction, although convection occurs in the upper mantle and, while accounting for less heat transfer, the flow of molten rock is the cause of tectonism in the lithosphere

Plate Tectonics

• tekton (Gr): carpenter or builder
• outer 100 km of the earth (lithosphere) is composed of rigid plates
• convection in the partially liquid asthenosphere causes shearing of the molten rock and shearing stress applied to the lithosphere
• determines continental relief and volume of the ocean basins, and distribution of rock masses with varying lithology and structure
• three types of contact at plate margins

1. constructive: plates moving apart; molten rock injected adding new rock to the trailing edge of the plates and creating ridges, mostly on the floors of ocean basins
2. destructive: collision or, more commonly, subduction of denser oceanic plates below margins of less dense continental plates, creating oceanic trenches and mountain belts on the continental margins
3. conservative: transform faults where plates slide past one another (e.g. San Andreas fault)

Diastrophism (Gr: dia (across), strophe (turn))

• can be up to 10X more rapid then rates of erosion
• e.g. Alaskan earthquake, 1964: 8 m of uplift in a few minutes
  New Guinea: long-term uplift of 2000 m/ million years
  Postglacial uplift (glaciotectonism): 20,000 m/million years

1. Orogenesis (Gr. oros: mountain)
   • folding and faulting
   • occurs along destructive plate margins by crustal shortening and compression and thus in long narrow belts
   • associated with volcanoes and earthquakes (e.g. the circum-Pacific orogenic belt)
2. Epeirogenesis (Gr. epeiros: continent)
   • uniform, widespread crustal deformation with little or no faulting or folding
   • isostasy: the equilibrium maintained by crustal blocks because with increasing depth crustal rocks are denser and denser; therefore they subside with increasing mass (e.g. sedimentation or glaciation) and rebound with decreasing mass (e.g. deglaciation or erosion)
   • thus there is an absolute limit to the earth's relief since the crust is always moving towards an equilibrium elevation

Vulcanism

• more than 500 active volcanoes and 10s of thousands of extinct ones
• about 2 million km² of land underlained by 500-3000 m of mostly flat lava plains or plateaux, e.g. the Columbia Plateau of eastern Washington State
• 62% of all active volcanoes are in the Pacific Rim of Fire; also many on the mid-Atlantic ridge (sea floor spreading)
• like other structural landforms they form independently of climatically-controlled (exogenic) processes and thus have the same morphology everywhere

Types of Volcanic Activity

• defined by the products of vulcanism

1. exhalative: gas
   • vents that continuously or intermittently discharge hot water, steam and other gases, but rarely solid or molten rock
     e.g. hot springs, geysers, sinter mounds, fumeroles and mud volcanoes (where near surface hot water liquifies sediments and erupts as hot mud; typically 50-100 m high)
2. effusive: lava
   • silica poor lava has low viscosity and thus forms low angle landforms, most lava is silica poor, e.g. basalt is formed from silica poor lava
   • silica rich lava is more viscous and thus form steep angle landforms, e.g. volcanic plug
   • various kinds of lava: scoria is vesicular (bubbly) because alot of gas escapes as it cools; aa forms an angular blocky surface, pahoehoe is smooth and twisted (ropy) as it is hot fluid lava, pillow lava is pahoehoe lava that flow into or erupts into water, lava tubes flow out of a cooling hardened exterior
3. explosive: ash or tephra
   • solid fragments ejected from ash (silt texture) to lapilli (gravel sized) to blocks and bombs (boulders)
   • size-sorted during wind transport, also decreasing thickness of tephra deposit with distance from eruption
   • nuee ardente (Fr.): 'glowing cloud'
     • superheated steam and tephra erupted explosively
     • temperatures of 650-1060^o C
     • e.g. May, 1902 killed 30,000 people at St. Pierre, Martinique
   • tephrachronology: dating with the use of extensive ash (marker) beds
     • ashes distinguished by refractive index of glass shards and trace element chemistry
     • dated by correlation to tuffs (rocks composed of ash); K/Ar isotopic dating, fission track dating, or C14 dating of adjacent organic deposits (for late-Quaternary ashes)
   • significance for geomorphology: provide chronological control for determining age of landforms and deposits, rates of erosion or deposition, tectonic displacement, soil formation