Transcript The Coastal Environment

Ocean Basins and
Coastal Landforms
Five Ocean Basins and “Seven Seas”
A Closer Look: Continental Shelf and Continental Slope
 Ocean basins cover 71% of Earth
 2 to 11 km in depth (approx. 5 km average)
 1.4 billion km3 water
 No basins, water would be 2 km deep everywhere
 Where did water come from?
 Native volatiles – water and carbon dioxide - from the molten early Earth were
gases from initial coalescence of planets and the solar system
 Cometary water hypothesis – solar wind and the moon-forming impact would have
caused net loss of volatiles – Earth’s early surface had neither water nor an
atmosphere but the early solar system was full of comets formed by escaped
volatiles from the inner planets – comets later crashed onto Earth’s surface
returning 90% of the volatile compounds that eventually formed Earth’s
atmosphere and Oceans
 Why do volcanoes still spew water vapor? Volatiles recycled with tectonic activity
Sea-level has risen and fallen in the past
 Sea-level is rising presently approx. 3mm/yr
 Warming temperatures raise sea level by a) Thermal expansion
of water and b) Melting and release of glacial water
The Coastal Environment
 The World Ocean covers 71% of Earth’s surface, and the world’s
shoreline is of enormous length
 Earth’s coastlines have tremendous resource, and they are
biologically and geomorphically diverse
 Their spectacular scenery bring in more tourists than any other
natural environment, and continue to attract new residents
 But they are also subject to an array of natural hazards, and
human-induced environmental problems; ironically, some of
our most polluted waters are found in coastal locations
 Global warming and resulting sea level rise will have profound
consequences for coastal zones
The Coastal Zone: Definitions
Coastal Zone: General region of interaction between the land
and the ocean
Shoreline or Coastline: Constantly changing contact
between the ocean surface and the dry land
Sea Level: Average position of the ocean shoreline from
which all elevations are measured
Swash: The thin sheet of water rushing towards the shoreline
Backwash: The return flow of water to the ocean
Nearshore Zone: Comprises Breaker Zone, Surf Zone and
Swash Zone (See Fig. 17.1, p. 407 in Text)
Offshore Zone: Undefined zone seaward, beyond the
breaker zone
Waves and Currents
Waves : are travelling, repeating forms of alternating highs and lows
called wave crests and wave troughs – induced by Wind
Currents : horizontal movement of ocean water, usually in response to
atmospheric circulation  also, rip currents, long shore currents
Wave Refraction – waves change directional trend
as they approach shore.
Raglan, New Zealand
Basic Concepts
I.
Repeated sea level changes in the past
• Estimated 422 ft. rise since 18,000 years ago
• specific landscapes submergent (Passive) or
emergent (Active) depending on tectonic change
II. Waves and resultant currents erode, transport, and
deposit sediment load.
• rocky headlands
• beaches
III. Tides affect all coastal life but have little
topographic effect.
• tidal bore/tidal wave/storm surge/tsunami
Submergent vs. Emergent Coastlines
• During the ice ages (or glacial periods) sea level sinks as
more water is tied up in glaciers
• During interglacial periods sea level rises, flooding many
river valleys, creating bays and estuaries
• Submergent Coastlines  Rias: drowned river valleys;
Fjords: drowned glacial valleys
• Sea levels rise also causes many features of the former
shore to lie underwater
• Also known as Passive-margin Coasts – e.g., Atlantic
Coast – because tectonic activity occurs in the middle
of the ocean, and the coast is tectonically passive.
Submergent
Coastlines
Sea level rise
inundated many
coastal valleys
creating bays and
estuaries. These
coasts are often
referred to as Ria
coastlines.
Estuary – An arm
of the sea that
extends inland i.e.,
an inlet) to meet
the mouth of a river.
Emergent Coastlines
where water level has fallen, or land has risen due to tectonic activity
In areas with
“rapid” tectonic
uplift,
Emergent
Coastlines and
cliffs are
common, and
bays are rare
Also called Activemargin Coast – e.g.,
Pacific Coast
Tectonic forces lift coastlines faster than sea-level rises. Dramatic sea
cliffs and marine terraces tower above the sea. Also, sea stacks.
Most activity occurs along active subduction zones and transform plate boundaries.
COASTAL PROCESSES AND LANDFORMS
Coastal Erosion – dependent on wave size, angle, and
frequency. Focused where waves contact coastline.
• Headlands, sea cliffs, bluffs, sea stacks, natural bridges
• Beach Erosion
Coastal Transportation – Large waves move beach sand
offshore. Small waves push it back onshore.
•
Wave action creates strong currents parallel to shore –
Longshore currents, and longshore drifting.
 Also Littoral drifting and beach drifting.
Coastal Deposition – where wave action is reduced,
beaches and dunes form.
• Beaches, dunes, sand spits
Coastal Erosion
Hydraulic action: Sheer physical force, the
pounding of waves against coastal rock
material.
Abrasion: The process of wearing down or
rubbing away by means of friction.
Isla Vista Beach, October 2001
Isla Vista Beach, December 2002
Beach Erosion
Formation
of
Marine
Terrace
Marine Terraces
Point Reyes National Seashore
California
Sea Stacks
Port Campbell National Park, Victoria, Australia
Natural Bridges and Arches
Port Campbell National Park, Victoria, Australia
Erosional Landforms
1. Sea Cliffs: When Waves pound directly against steep
land
2. Sea Caves: Erosion, corrosion and hydraulic action
along lines of weakness
3. Sea Arches: 2 caves meet from each side of hydraulic
action of a headland
4. Sea Stacks: A resistant pillar is left standing – the
remnant is called a sea stack
5. Marine Terraces: Formed by tectonic activity uplift out
of reaches of wave action
6. Headlands: Prortruded land, flanked by receding bays
Depositional Landforms
1. Beach: Landform of coastal deposition, continuous with
the mainland  Sandy beaches, Pebble and
boulder beaches, White coral reef beaches, Black
sandy beaches – in volcanic Big Island, Hawaii
2. Spits: Coastal depositional landforms connected to the
mainland at just one end
3. Barrier beaches: long depositional feature constructed
parallel to mainland, protection from direct wave
attack
4. Lagoon: Body of water partially or fully enclosed by
baymouth bar
5. Tombolo: Strip of sediment connecting mainland to an
island
Tombolo
Tombolo
Sand Spit
Sand Spit
Barrier Islands
Barrier Islands
Point Reyes
National
Seashore,
California
Point Reyes Lighthouse
Islands
Types of Islands
1. Continental Islands – separated from
mainland due to shore submergence
2. Oceanic Islands – volcanic islands
2. Atoll – coral formation
Tropical Coasts
and Coral Growth
Factors Correlated with Healthy
Coral Reef Growth
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•
•
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Water temperature range: 18 – 29C
Normal seawater salinity: 32 – 35 ‰
Clear, transparent water
Little or no sedimentation
Vigorous water motion
Tropical Mangroves
Mangroves create tidal forests. These rich ecosystems provide
habitat for countless creatures and help to stabilize and, by
catching sediment, even create coastline.
Tides
Low Tide, New Brunswick
Tides
Tide Cycle
Tides rise and fall with the rising moon, an interval of
24 hours and 50 minutes. There is a bulge on both sides
of the earth. Thus two high tides each cycle.
Flood tide (low to high) takes 6:13 and then ebb tide
(back to low) is another 6:13.
Tides
Tide Range
Tide range dependent on
relative position of earth,
moon, and sun.
Spring Tides - highest
tidal range
Neap Tides - lowest tidal
range
Occurs every 2 weeks
Tidal Patterns in the U.S.
Tides
Not significant as a geomorphic agent, but can be very consequential
when coinciding with storm surges or tsunamis