GEOG101: Physical Geography

Lecture 15: Wednesday, March 18, 2009

Topics
1. Earth's Surface Relief Features
2. Crustal Formation Processes
3. Crustal Deformation Processes
4. Orogenesis (Mountain Building)
5. Earthquakes
6. Volcanism

Earth's Surface Relief Features
• Different orders of relief:
      1. Continental - Ocean (first order): the highest place: Mountain Everest, 8.8 km above sea level; The lowest place: Mariana Trench, 11 km below sea level.
      2. Within continents and basins (second order): for example, mountains, plains, and lowlands. The Alps, Rockies, and Tibetan are these order of relief within continents; mid-ocean ridges, oceanic trenches, submarine canyons, etc. belong to this order of relief within ocean basins
      3. Localized topographic relief (third order): for example, Las Vegas basin, central nevada belong to this category
• Hypsographic curve: display the distribution of Earth's surface by area and elevation in relation to sea level.
      The average elevation of Earth's solid surface is actually under water, -2070 m below mean sea level.
• According to the three orders of relief, Continents can be divided into six topographic regions (Figure 12.3, p. 362):
      Plains
      High tablelands
      Hills and low tablelands
      Mountains
      Widely spaced mountains
      Depressions

Crustal Formation Processes
• All continents have a nucleus of ancient crystalline rock on which the continents "grow" with the addition of crustal fragments and sediments
• Such nucleus is called Cratons
• Cratons are commonly stable, and are formed more than 2 billion years ago
• Continental Shields: are regions where cratons are exposed at the surface
• Continental Crust and Terranes: are added to the Cratons through tectonic cycles

Crustal Deformation Processes
• When the lithospheric plates or crustal blocks move relative to each other, they form different types of structures such as:
      Folding and Broad Warping (bending of the crusts)
      Faulting (Breaking, causes offset)
• Stress: forces caused by tectonic forces, gravities, and the weight of the overlying rocks. There are three types:
      tensional stress
      compressional stress
      shear stress
• Strain: The response of rocks to stress: folding (bending) or faulting (breaking)
• Three types of faults:
      Normal fault: Hanging wall move downwards relative to footwall--> formed by tensional stress
      Reverse fault (or thrust): Hanging wall move upwards relative to footwall-->formed by compressional stress
      Strike-slip fault: Blocks move along the fault plane horizontally-->formed by shear stress

Orogenesis (Mountain Building)
• Three Types of Orogenies:
      Oceanic-continental plate collision: Examples-- Andes, Rocky mountains
      Oceanic-oceanic plate collision: examples--Western pacific such as Japan and Indonesia, the Philippines
      Continental-continental collision: examples--Himalayas, Alps
      The Appalachian Mountains (the older cycles during the formation of Pangaea about 250-300 milion years ago)
• The other type of mountain building:
      uplifting of Fault-blocks by magmas or by isostatic rebound: Grand Tetons and the Sierra Nevada

Earthquakes
• Expected Quakes and Those of Deadly Surprise:
      Earthquakes are mostly along faults and plate boundaries
• Focus, Epicenter, Foreshock, and Aftershock
      Focus: The place where the motion of seismic waves is initiated-- normally in subsurface
      Epicenter: The area at the surface directly above the focus
      Foreshock: Smaller shocks before the main earthquake shock sharing the same Epicente-- Earthquake Forecast? 
      Aftershock: Smaller shocks after the main earthquake shock sharing the same Epicenter: Important for the rescuing arrangement
• Earthquake is recorded by an instrument called: seismograph

Volcanism
•Location and Types of Volcanic Activity: along active plate boundaries, e.g., subduction zones, rifting valleys, midocean ridges
• Effusive Eruptions: Gentle eruptions. Magma contains less gases and is rich in iron and magnesium, thus has low viscosity. Example-- Hawai'i;
• Explosive Eruptions: Violent eruptions. Magma contain more gases and silica/aluminum and has a high viscosity. They can block the magma conduits, causing high pressure buildup. Examples: along subduction zones. You are better not invited to have a close look at these volcanos.