5.1 Times and Tides

Overview of Ocean Systems and Coastal Dynamics

This series of notes covers the impact of time and tides on coastal systems, examining common misconceptions regarding oceans and coastlines, the dynamic nature of coastlines, and the specific mechanisms driving tidal phenomena.

Misconceptions About Oceans and Coastlines

Nature of Shorelines

  • Definition of Shorelines: Shorelines delineate the transition between marine and terrestrial ecosystems but do not represent the edges of continents. The continents extend to the edge of the continental shelf and down to the transition between continental crust and oceanic crust.

  • Shifting Shorelines: Shorelines are influenced by sea level changes, shifting back and forth over time.

Plate Tectonics and Coastlines

  • Plate Boundaries Misconception: Shorelines are often incorrectly assumed to be plate tectonic boundaries. For example, the eastern coast of North America has the nearest plate boundary located far offshore in the ocean basin, characterizing it as a passive continental margin. In contrast, the western coast parallels an active plate boundary further west.

  • Physical Features: The interaction of plate tectonics influences shoreline features:

    • East Coast (Passive Margin): Characterized by gentle slopes and extensive sandy beaches.

    • West Coast (Active Margin): Exhibits steeper slopes, and beaches are often narrower or nonexistent.

Sea Level Variability

  • Non-Uniform Sea Level: Sea level is not consistent; for instance, it's about 10 centimeters higher at Tampa Bay compared to Daytona Beach due to the influence of trade winds that push water into the Gulf of Mexico.

  • Geological Influence: Sea levels vary even more dramatically over submerged geological features:

    • Submerged Volcanic Mountains: Sea level can rise by up to 5 meters over submerged volcanoes and even 10 meters over mid-ocean ridges.

    • Deep-Sea Trenches: Coastal sea level can drop up to 30 meters over deep-sea trenches due to less gravitational pull from the surrounding rock.

    • Measurement Techniques: Satellites measure sea level variations, which helps map the seafloor's elevation.

Dynamic Nature of Shorelines

  • Tidal Changes: Coastlines are subject to frequent changes on various time scales, most notably daily and yearly due to tides.

    • Tidal Patterns: Tides can alter the beach landscape. For example, a beach may appear wide in summer (gentle waves moving sand onto the beach) but be significantly narrower in winter due to strong storm waves moving sand offshore.

Long-Term Changes

  • Historical Change Examples: Many bays along coastlines, resembling river valleys, were formed as sea levels rose at the end of the last ice age due to melting ice sheets.

  • Oceanic Processes and Erosion: Coastal erosion affects infrastructure, exemplified by the Cape Hatteras lighthouse, which was moved approximately 3,000 feet inland in 1999 due to rising seas and erosion.

Tidal Mechanisms

  • Tide Definition: Tides are the periodic rise and fall of sea levels caused by the gravitational pull of the Moon and the Sun, alongside the rotation of the Earth. Though the Sun is much larger, its distance diminishes its influence compared to the Moon.

  • Tidal Bulges: The Earth experiences two tidal bulges caused by the Moon's gravitational attraction, leading to high tides on one side of the Earth and a counteracting bulge on the opposite side due to centrifugal forces from rotation.

    • Anecdotal Analogy: Analogous to skating with a partner, when one partner is much larger, rotation occurs around a common center of mass closer to the larger partner.

  • Effects of the Sun: The Sun, while further away, still influences tidal heights. High tides occur when the Sun and Moon's gravitational effects align (spring tides), while lower tides occur when they are at right angles (neap tides).

Factors Influencing Tides

  • Geographic Considerations: The Earth's axial tilt, patterns of rotation, and elliptical orbits of both the Earth around the Sun and the Moon around the Earth all affect tides.

  • Local Topography: Tidal effects are amplified near coastlines with unique shapes and underwater features, creating more substantial tidal ranges in certain areas.

    • Bay of Fundy Example: Exemplifies a tidal range of up to 53 feet, far surpassing other locations due to distinct hydraulic conditions.

  • Tidal Friction: Interaction between land and water gradually slows down Earth's rotation. Historical records of coral growth suggest that the Earth’s rotation has slowed over time, proposing a future where tidal interactions may eventually bring the day to a standstill.

Conclusion

The intricate interplay between ocean dynamics, tidal forces, and geological formations provides essential insights into coastal systems. Ongoing changes driven by both natural processes and anthropogenic effects necessitate understanding these systems to mitigate potential impacts on human infrastructure and the environment. The next section will delve deeper into the behaviors of coastline systems.