Atlantic coast: south of NE is bordered by broad, low plains washed by gentle waves;  these coastal planes break up into a series of complex bays that are commonly fronted by barrier islands;

Pacific coast: narrow, rugged coast; sculpted cliffs, pocket beaches, and crashing waves
 

constant action of the waves suggest a direct relationship between the sea and the shape of the coasts.

expanded view:  global-scale organization of the Earth's surface?    plate tectonics
- explains the underlying forces that  have created the mountain ranges, ocean trenches, and other major geological features of the Earth's surface.
 

Continental Drift Hypothesis

Alfred Wegener - 1912 - continental drift 200 M years ago
- with the advent of more accurate map making
- Sir Frances Bacon - first noticed in 1620; series of catastrophies?
- Wegener believe that the continents had slowly drifted apart from a super continent, Pangea; single ocean, Panthalassa

- using geologic and paleontologic evidence

- mountain belt
- fossils
- glacials scars
- matched coast lines

- could not provide the mechanism of continental drift; suggested the gravitational pull of the moon was the force

Sea Floor Spreading Hypothesis

research was hampered because only 29% of Earth's surface was accessible for study

naval operations paved the way for oceanographic studies and mapping of the ocean floor by using sonar readings
- discovered:
    - a continuos montain system circling the globe; ridge system with valleys along the peak and large fractures cutting across its length - called transform faults

- radiometric dating of rocks; older rocks furthest from center

- Paleomagnetic data:
    - pole reversal
 

Plate Tectonics Theory

Density of the oceanic crust:
- denser than continental crust
- becomes denser as the rock moves away from the oceanic ridge (spreading center)
    - ocean floor becomes gradually lower as it leaves the ridge

transform faults across the ridges are also evidence of sea floor spreading
- moving over spherical surface; strain on hard, flate plates moving over soft curved subsurface
 

Other pieces of evidence

Heat flow measurements show that the oceanic crust is warmest at the oceanic ridge

Plot of earthquake epicenters:
- highest concentrations occur at the trenches (subduction zones)

Hot spots
 

Plate Margins

Divergent
Convergent
Transform Fault
 

Lithosphere and Asthenosphere

lithosphere
- divided into 12 major plates
- in motion floating on the asthenosphere
- created at a spreading center and disappearing at subduction zones

Oceanic ridges: where the sea floor is stretched and weakened due to emergence of magma - pushed up to form a ridge
- new crust is being created at the oceanic ridge

Subduction zones: where the lithosphere (with oceanic crust) goes back into the subsurface
- plate with denser oceanic crust subducts under plate with less dense continental crust
- some of the rocks along its upper edge melt and rise up to the surface and cause volcanic activity: mountains and islands

Collision Zones: mountain building
- convergent margin where both lithospheric plates have continental crust

Convections Currents: release of the Earth's interior heat
- rock cycle
- rises at a oceanic ridge

Rate of movement: 1 - 10 cm per year

Continents ride on the back of tectonic plates
 
 

Tectonic Classification of Coasts

Interrelation between plate tectonics and coastal types;
- the location of the coast to a plate margin (boundary) and the type of margin has an impact on the type of coast.
- plate tectonics only explains the broadest features of a coast; there are different scales of time and size determining features of the coasts:
    - First-order: cover large geolgraphic distances - character of coasts are tied to tectonic conditions (1000's miles)
    - Second-order: smaller scale - e.g. erosion and sediment deposition (10-100's miless)
    - Third-order: local conditions prevail - wave action, tidal inlets, and individual barrier islands (miles)
 

Three major types of continental coasts:
- leading edge: located at a convergent zone
- trailing edge: located at mid-plate
- margin edge: located between the landmass and a volcanic island arc
 

Leading Edge Coasts

- develops where the oceanic edge of one plate converges with the continental edge of another; also know as collision or convergent coasts
- denser oceanic plate descends and the friction causes the lighter continental crust to fold and buckle
- rugged cliffed shorelines, large waves; e.g. Andes, Cascades
- because the angle of subduction is less steep under continental crust, volcanic range may be a distance from the subduction zone
- steep flowing slopes have rapidly flowing streams; transport large quantities of sediment; large waves erode any beaches that are formed by sediment deposition; no deltas are formed
 

Trailing Edge Coasts

- developed away from the edge of a plate; tectonically stable for at least 10s M years; types:
    - Neo: juvenile coast - located at the begining of a spreading center; e.g. Red Sea, Gulf of California
        - erosion is minimal because there are no large waves
    - Afro: inbetween juvenile Neo and mature Amero; forms on a continent that has coasts with only trailing edge coasts
        - e.g. Africa
        - developed pronouced continental shelves, but lack sedimentary features such as large deltas
    - Amero: mature coasts; temperate climate; development of large winding rivers with large deltas; large drainage areas
        - e.g. east coasts of N. and S. America

Marginal Sea Coasts

- near a collision of plates but kept apart from its influence by volcanic island arcs
- behave like a trailing edge
- e.g. China, Korea - protected from open ocean waves
 

Tectonic interpretations are large (first-order scale); at the regional and local scales, the second- and third-order features become more noticeable