Transcript Oceanography Chapter 13

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Two Classification Systems
 Without coastal dynamics, the processes that create and shape the oceans’
coastlines, history would be significantly different.
 This first classification system is based on geology
and processes that take place over very long
periods of time.
Chapter 13 Pages 13-3 to 3-5
Coastal Classification
 Active Coasts: These are close to plate collisions
that result in volcanic activity and earthquakes
(the Ring of Fire).
 Passive Coasts: These lie far away from active
plate boundaries with little volcanic activity and
few earthquakes.
 Oceanographers believe that many other coastal
processes occur. This second classification system
is based on short-term coastal dynamics.
 Primary Coasts: Formed by geologic processes not
directly related to the ocean over an extended period of time.
 Secondary Coasts: Formed by marine action, process takes much less time.
 A combination coast is one that can be both primary and secondary.
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Primary Coasts
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Chapter 13 Pages 13-5 to 13-8
Coastal Classification
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Scientists attribute primary coast development to nonmarine forces. These include
land-based erosion (from running water, wind or land ice), sedimentation, volcanic activity
and tectonic activity.
Land-based Erosion Coasts – include fjord coasts
and drowned river valleys. Scientists believe these
occurred from erosion cutting into the land during
periods of low sea level, then the sea level rises,
flooding the eroded area. Both fjord coasts and
drowned river valleys can form estuaries –
salt wedge estuaries, vertically mixed estuaries,
slightly stratified estuaries and highly
stratified estuaries.
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1. Fjord Coasts – formed by glaciers moving toward the
coastline. The glaciers cut large, deep grooves in the land,
which then flooded when the sea level rose (Southern Alaska).
2. Drowned River Valleys – formed by rivers cutting a
valley through, which then flooded when the sea level
rose (Chesapeake Bay).
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Chapter 13 Pages 13-8 & 13-9
Coastal Classification
Primary Coasts (continued)
 Sedimentation Coasts – form when
materials carried by rivers flow into the
ocean, deposit and accumulate. Often
sediments accumulated form a wide
triangular shape – a delta (Nile delta
in Egypt).
 Volcanic Coasts – formed by volcanic
activity (Hawaiian Islands).
 Tectonic Activity Coasts – formed by
tectonic activity. Primarily includes fault
coasts which form as plates collide. Results
when the collision uplifts the seafloor above
the water surface, allowing the sea to flood
the new area (Tomales Bay, California).
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Chapter 13 Pages 13-9 & 13-10
Coastal Classification
Secondary Coasts
 Secondary coasts result from marine processes that
include wave erosion, material deposited by seawater motion,
and marine life.
 Wave-erosion Coasts – constant pounding by waves erodes
and changes a coastline, wearing away land protrusions. Can
result in spectacular formations like sea caves, arches and
sea stacks.
 Marine-deposition Coasts – form when sea action causes
ocean sediment to accumulate in one place. Involves ocean
sediments moved by water motion in the sea. Barrier islands,
beaches, salt marshes, and mud flats are all types of
deposition coasts.
 Marine Organism Coasts – marine organisms build coasts by
providing a structure that reduces the effects of waves and
currents. Best known of these is the Great Barrier Reef of
Australia built by coral.
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Longshore Drift
 One of the most significant forces shaping the coast is longshore drift.
Chapter 13 Pages 13-12 & 13-13
Coastal Dynamics
 The tendency for materials to move along the coastline due to a longshore current.
 Waves arrive on shore at somewhat of an angle, but the water recedes at nearly a
90º angle. Sand and sediment flow back with the water at this angle. The net
motion of this backwash combines with the net motion imparted by the waves to
cause a longshore current. Longshore drift occurs as the current moves material
down the coast.
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Beach Dynamics
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Sand comes from erosion. It may be from
the effect of waves pounding the shoreline
or inland erosion.
In tropical regions sand also comes
from biological sources, like the erosion
of coral reefs.
A beach is composed of three sections:
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Chapter 13 Pages 13-13 to 13-15
Coastal Dynamics
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1. Foreshore – region from the high- to low-tide mark.
2. Backshore – region rarely touched by seawater.
Includes dunes and grasses.
3. Offshore – beyond the low-tide terrace.
Factors that shape a beach: They are grain
size of beach sediments, wave energy and
the degree of beach slope.
Coastal Cell: a local region of material
transport mechanisms that, when combined,
form an area with no net sand gain or loss.
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Large Scale Sand Features
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Chapter 13 Page 13-16
Coastal Dynamics
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Spits form when a longshore current turns a
beach corner into the relatively calm water of a
bay. The current slows and can’t carry as
much sediment. Sand settles out of the water,
forming the spit.
Tombolos are spits that extend between two
islands or from an island to the mainland.
Tombolos form when the longshore current
slows. In this case, current slows around two
sides of land, accumulating sand on both
sides until the two spits grow together.
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Large Scale Sand Features (continued)
 Barrier Islands: there are currently two theories on how they form.
 1. Sediment accumulates offshore, eventually building up into an island.
 2. They were giant sand dunes that became islands with the last major sea-level rise.
 Typical barrier islands share five features:
Chapter 13 Pages 13-17 to 13-19
Coastal Dynamics
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1. Ocean beach
2. Ocean dune
3. Barrier flat
4. Salt marsh
5. Lagoon
 Deltas: there are three types.
 1. River-dominated deltas have strong rivers and mild wave and tidal action
(Mississippi River delta).
 2. Tide-dominated deltas occur in areas with strong tidal changes (Essex River
in Massachusetts).
 3. Wave-dominated deltas have significant wave energy that redistributes river
sediments (Senegal delta in west Africa).
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Coral Reefs
 Individual polyps create a calcium carbonate external
skeleton as they grow. It is this part of its structure
that creates coral reefs.
Chapter 13 Pages 13-21 & 13-22
Biological Processes and Human Activity
 Coral is perhaps the most significant of all biological
processes that affect the coast. Coral reefs can be
massive, but only the outside layer, the coral polyps,
is alive.
 Fringing reefs lie along an island or mainland coast.
They have a fore reef – the outer ocean side with most
biological activity. A reef crest is the top of the reef that
takes most of the wave energy. A back reef is on the
land side and has less biological activity.
 Barrier reefs have a similar structure as fringing reefs,
but lie further from shore. A barrier reef has a lagoon
between it and the main coast.
 Atolls are ring-shaped coral reefs that encircle a shallow lagoon.
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 In areas where the waves have generally low energy, plant communities can
dominate the coast. Seagrasses for example live entirely underwater, but most
marine plants live partly out of the water.
 Among the most important of the plant-dominated shorelines are the mangrove
swamps. Mangroves affect the coast directly by holding sediment in place
and absorbing wave energy.
Human Activities
Chapter 13 Pages 13-23 & 13-24
Biological Processes and Human Activity
Plant Communities
 There are two primary motivations for humans to modify the coastline:
 1. To create new coastal structures.
 2. To protect building and structures already on the coast from natural
coastal changes.
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Human Activities (continued)
Chapter 13 Pages 13-24 & 13-25
Biological Processes and Human Activity
 Human-built coastal structures include:
 Groins are artificial protrusions jutting out perpendicular to
the shore. These may be built to create an area relatively
protected from longshore current, or for recreation.
 Jetties are the same as a groin, except they are built to
reinforce a harbor entrance.
 Breakwaters run parallel to shore or start on shore and curve
into the sea. They are used to create an artificial lagoon for
use as a harbor or beach.
 Seawalls stand either at the water along the shore
or at the top of a beach. They act as a barrier to
block the waves from eroding the land.
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Human Activities (continued)
 Jetties and groins block longshore drift. This tends to cause sand to accumulate on the
upside drift and to become depleted on the downside drift. Spits may form at the top of
jetties.
 Seawalls effectively absorb energy but create problems at their ends. The unprotected
land next to the seawall tends to suffer from increased erosion.
 Beach renourishment is the process of bringing in sand or sediment from
somewhere else to replenish eroded sand caused by human projects.
Chapter 13 Pages 13-24 & 13-25
Biological Processes and Human Activity
 All of these structures can create problems:
 It is expensive and is only a temporary fix as the sand beach will again erode. Taking
sand from somewhere else effects that environment also and may damage biological
communities.
 The likely solution to problems created by human structures is a change in
coastal attitudes and management. In the long run it is more effective and less
costly to work with nature instead of against it.
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