Coastal Geology

1. Coastal Geology GLY 2010 – Summer 2013 Lecture 23 1 1

2. Coast • Entire land region bordering water - they extend inland until a different geologic setting is reached (plateau or mountain) • Dynamic zones, where change is constant 2 2

3. Interface • A boundary between different parts of a system • Transition zones between continent and ocean • Retreat of the coastline inward superimposes coastal features on pre-existing landscapes 3

4. Primary Coast • Coasts shaped by non-marine processes 4 4

5. Fjord Formation • Fjords drown coastal valleys scooped out by glacial erosion 5 5

6. Delta Formation • Deltas are formed by stream deposition, a non-marine process 6 6

7. Isostatic Rebound • Rebound following glaciation raised former beaches 7 7

8. Tectonic Uplift • Plate collisions may raise coast terraces 8 8

9. Carbonate Reefs • Growth of reefs can shape a coast 9 9

10. Secondary Coast • Coasts shaped by marine erosion or deposition 10 10

11. Wave Erosion • Breaking waves, especially when refracted, can shape a coast 11 11

12. Wave Animation 12

13. Coastal Features - Headland • Created by wave cut benches breaking the force of waves, thus cutting the energy before the cliff is reached • Slower erosion produces a headland 13 13

14. Coastal Features - Cove • In the absence of wave-cut benches, water quickly erodes the rock - may also be the result of less resistant rocks 14 14

15. Platforms and Terraces • Wave erosion creates wave-cut platforms • Tectonic uplift or falling sea-level changes them to marine terraces 15

16. Straightening Coastlines • Waves perform a double action, simultaneously wearing away the headlands and filling up the bays or coves • USGS animation with sound – click to play 16 16

17. Coastal Features - Sea Cave • Formed in cove areas where waves pass unimpeded to the rock, battering and tearing away less resistant rocks 17 17

18. Sea Arch Formation • Further erosion can create a sea-arch 18 18

19. Coastal Features - Sea Arch • Formed by caves on adjacent sides of a headland joining 19 19

20. Sea Stacks • Collapse of the arch forms sea-stacks 20 20

21. Coastal Features - Sea Stack • Results from further erosion and collapse of the arch 21 21

22. Tombolo Formation • An off-shore sea stack protects the beach from wave action, sometimes allowing sand deposits to reach out to the sea-stack 22

23. Tombolo Photograph • Mont St. Michel in France 23 23

24. Depositional Features • Longshore currents deposit sand across bays, creating spits, bars, and hooks 24 24

25. Baymouth Bar Photo • A baymouth bar on the Michigan side of Lake Michigan • Cut through the bar is man-made • Photo: Steven Dutch 25 25

26. Coastal Spit and Hook • Northern tip of Cape Cod shows a well-developed hook, caused be refraction of waves around the spit • Photo: Steven Dutch 26 26

27. Spit Formation Video • A sand-spit has formed off the end of this breakwater (with sound) 27 27

28. Organic Coasts • Coasts formed by erosion or deposition in conjunction with active biological processes 28 28

29. Shoreline • Boundary between a body of water (marine or fresh water) and adjacent dry land 29 29

30. Waves and Currents • We will examine several aspects of waves and currents: • Energy Transportation • Wave generation • Wave characteristics 30 30

31. Energy Transportation • Waves and currents transport energy • Energy sources • Solar radiation • Gravity (tides) 31 31

32. Wave Generation - Wind • Winds moving over the surface of water pile the water up into waves • The stronger the wind, and the longer it acts on the water, the higher the waves • Hurricanes forming off the African coast are usually the strongest 32 32

33. Wind Driven Waves • Wind driven waves breaking on rocky outcrop 33 33

34. Wave Generation - Seismic Disturbance • Earthquakes can trigger seismic sea-waves (tsunamis) • Sometimes this happens because the earthquake generates underwater landslides 34 34

35. Wave Generation - Tidal Forces • The sun and moon exert gravitational forces on the earth • Water tends to pile up on the line between the earth-moon and earth-sun centers - thus producing “tidal waves” 35 35

36. Tidal Change Video • Time lapse animation of a changing tide • Tidal waves can be substantial in some areas (Bay of Fundy - 20 meters) and Puget Sound (3-4 meters) 36 36

37. Wave Characteristics – Parts of a Wave • Waves have two parts • The crest is the top of the wave • The trough is the bottom of the wave

38. Wave Characteristics - Measurements • Waveheight – Vertical distance between crest and trough • Wavelength – Horizontal distance between adjacent crests • Velocity - Distance a wave travels per unit time (30 to 90 km/hr in mid-ocean) 38 38

39. Refraction of Waves • Bending of waves as they approach a shoreline - waves refract so they are more nearly parallel to the shoreline 39 39

40. Wave Refraction Video • Video shows waves approaching a beach and their refraction (with sound) 40 40

41. Wave Types - Tsunami • Seismic sea-waves • Large wavelength and small waveheight in mid-ocean • Very large waveheight and small wavelength in coastal regions 41 41

42. Chile Tsunami, 1960 • One of the more destructive Pacific-wide tsunamis of our time was the Chilean tsunami of 1960 42 42

43. Aerial View of Tsunami Damage • Aerial view of the coastal area of Isla Chiloe, Chile 43 43

44. Tsunami at Hilo, Hawaii 44 44

45. Wave Types - Tides • Waves produced by gravitation interaction, principally with the moon • High tides on opposite sides of the earth • Tides move as earth rotates • Tidal range is the difference between high and low tides 45 45

46. Tidal Cycle Animation 46

47. Tidal Inlets • Tidal inlets are periodically flooded, followed by withdrawal of water 47 47

48. Wave Types - Rip Currents • Water moving toward shore goes over a barrier, such as a sandbar or reef • The lower-energy return flow is blocked, and channeled so that a strong current flows directly away from shore • Great for surfers wanting a fast ride to the breaker zone • Impossible to swim against - instead swim sideways to get out of the narrow flow 48 48

49. Rip Current Diagram • After a set of waves break, a lot of water and energy is pushed towards the shoreline • Displaced water will move along the beach with the long-shore current until it finds its way back out to sea 49 49

50. Spotting Rip Currents • Narrow, river-like, currents that have been fed by the long-shore current and sets of waves • Between 50 feet and 50 yards wide and can flow up to hundred of yards past the surfline • Rip current produce foamy and choppy surface • The water in a rip current can be dirty brown (from the sand being turned up) and moves rapidly out to sea 50 50