Brookdale Comm Coll - Intro to Coastal Geology
RA Davis Text
Prologue and Ch 1 - set the tone, look at plate tectonics context
Why study coasts?
 Geology in action - see processes that have created our landscape
over millions of years
 Population pressure - beautiful place to be, lots of environmental
demands upon a limited resource - how do we best enjoy and
preserve?
Coasts defined:
 "zones of transition between ocean and land"
Extent of coastal zone: from "wave base" offshore to landward limit of high tide
(actual width variable: < 1 mile (steep Oregon coast) to > 60 miles ( gentle
Delaware Bay)
Coastal zone
High tide
Wave base
the "system viewpoint" - look at materials and processes….processes act upon
materials…materials change shapes, composition….in turn, materials influence
processes. Note the give-and-take of the system, always striving toward
equilibrium between materials and processes.
Processes of erosion and deposition always fighting to take material away or put
it somewhere

"erosional coast" usu. considered rugged, rocky…sediment eroded and
carried away - beaches small, slopes steep. Northern California, Oregon,
Washington

"depositional coast" - sand and gravel deposited, wide coastal zones,
perhaps lots sand beaches, sand dunes. Also see deltas, barrier islands.
Lec 1 p.2
Lots of factors to consider in coastal development:
 Wave energy
 Geologic setting (bedrock, structure, geometry)
 Sea level changes (both rise and fall)
- currently rising about 2mm/yr
Changes occur both short- and long-term Millions or thousands of years to days or hours.
Keep 3 time scales in mind when sorting through the processes 1. slow mt formation & erosion - millions of years (long term)
2. sea level change - thousands of years (intermediate term)
3. wave/tide/storm activity - years, days (short term)
man's study of coasts and coastal processes relatively recent - last 100 yrs or so
- coastal engineering projects to enhance shipping, reclaim land (e.g., Holland
dikes)
- military concerns - troop landings as on D-Day, etc.
- morphydynamics now in vogue - view origin and development by
observing processes acting upon materials
Chapter 1 - Plate Tectonics and the Coast
"marvelous variety of forms"
example - contrast east coast with west coast east low relief, sandy (Jersey shore)
west high relief, rocky (Oregon)
is there a Grand Scheme that controls the general nature of coasts?
Plate tectonics theory gives a framework for classifying types of coasts.
New way of looking at Earth - new way to ask and answer questions.
A. History of evolving thought
 Great ocean voyages of 16th & 17th century (1500s and 1600s) led to new
maps, poss fit of So. America to Africa became evident.
 Sir Francis Bacon - 1620 - wrote that continents would fit together
perfectly if the Atlantic Ocean weren't there….
Lec 1 - p.3

Alfred Wegener - 1912 - promoted first comprehensive explanation, but
theory not widely accepted at time - in fact, most did NOT accept theory
- Wegener showed several lines of evidence that suggested conts had been
connected (fig p.9) - glacial deposits, mountain belts, fossil locations
- But couldn't come up with acceptable driving mechanism to move the
continents - he suggested tidal forces, but they were shown to be not
strong enough
B. Evidence for continental drift
 Submarine features, not discovered til WWII and later, with technology
developed during the war, are very important
1. mid-oceanic ridge systems all over the globe mark location of new
ocean floor being generated fig p.14
2. paleomagnetic reversals, age of ocean floor show incr. age away from
spreading centers - mirror images suggest lavas were extruded at same
place, same time
ocean crust
Cont
Cont
+
older
younger
younger
+
older
cross section across a spreading center fig p.15 too
note how you can "close up" the ocean floor
3. sediments lying on top of the ocean crust mimic the age pattern of the
crust
All these data point toward same idea - new ocean floor develops when a
continent splits apart, then ocean grows and continents move, like a giant
conveyor belt fig p.16

heat flow is another line of evidence - hot where magma extruded, cold
elsewhere

earthquake distribution is another line of evidence - eqk activity in small
bands where plates touch each other fig p.17
Lec 1 p.4
So new technology has provided us with evidence from the ocean floor in form
of:
topography
magnetic patterns
age patterns
heat flow
earthquake patterns
that shows the ocean floor is not a boring, flat, featureless, place (so it's not like
the bottom of your bathtub….)
C. Plate Tectonic theory
Highlights / major points:
 12 major plates, some smaller ones too fig p.20
 all plates in motion relative to each other
 new lithosphere created at ridges, consumed at trenches (subduction zones)
fig p.19
 think of Earth as a system of huge converyor belts
 variable rates of spreading - 1 cm/year up to 10 cm/year
new
old
old
Consumed, remelted, re-introduced at ridge
D. Tectonic Classification of Coasts
Proximity to type of boundary very important to classifying coasts
Inman and Nordstrom developed scheme in 1971 - still used
Lec 1 p.5
Broad shape influenced by one of three tectonic settings fig p.22:
1. leading edge (usu at edge of plate)
2. trailing edge (usu mid-plate)
3. marginal sea (on continent, but with island arc offshore)
this classification sets stage for "broadest features of coast"
why is this classification good? Recognition of features at different size scales
a. first order: very major features ( +/- 1000 km or more) assoc with tectonics
(e.g, wide continental shelf vs narrow shelf along large portion of continent)
b. second order: large scale features (10s to 100s of km) such as rock types,
structural styles, bedding orientation; assoc with long-term processes at work
on a regional scale ( e.g., regional erosion or deposition patterns such as
those that produce barrier island systems such those off New Jersey or North
Carolina)
c. third order: small scale features (< 10 km) assoc. with short-term or local
processes, associated with a specific beach, or particular spit (e.g., sandy
Hook), or a single barrier island
1. leading edge coast fig p.24
 assoc with convergent boundaries (mt ranges, folds, etc.)
- rugged, cliffed shorelines
- large waves (move onshore from deep water quickly)
- narrow or non-existent continental shelf

sediments travel short distance, straight steep rivers, large quantities
delivered,
 sediments don't stay on the shelf, in the nearshore system, but instead get
moved offshore to deep ocean via submarine canyons
- interesting stat - NONE of the world's 25 largest deltas occur on a leading
edge coast (no nearshore or shallow area where seds can accumulate)
- one place where seds DO accumulate are in bays fig p.27
this occurs quite frequently on the southern California coast, which looks different
and sandier than N. California coast, where transform fault basins develop and fill
with sediment
Lec 1 p.6
2. trailing edge coasts
 these have the most diverse group of coastal settings
 a stable setting tectonically
broken into 3 sub-groups (fig p.22):
a. Neo-trailing
b. Afro-trailing
c. Amero-trailing
These mark the transitions in age from just after rifting (Neo-) to mature coast
long after rifting (Amero-).
a. Neo-trailing may resemble a leading edge coast: young (few million years),
steep, narrow ocean with little fetch distance results in minimal erosion.
Examples = Red Sea, Gulf of California
b. Afro-trailing coast
examples include Africa and Greenland
tectonically stable, but not as "mature" as an Amero-edge coast - contin shelves
not as wide, not many large deltas (well….what about the Nile?)
continent is of moderate relief, moderate-size river systems, moderate sediment
load delivered to coast. "abundant sediment for coastal development is not
available". Where large rivers DO exist, like Niger delta, sediment can build up
and make a broader continental margin than you'd see otherwise.
Digress here briefly - mention that climate plays a very important role, in addition
to plate tectonics, on the nature of first-order features.
- In high latitudes Northern North America, Greenland, northern Asia, little
water is available to generate rivers (precip usu in form of ice and snow)
- In other areas, desert conditions result in few rivers (portions of Asia,
Africa, SW U.S.
- So a key point here is, you need rivers to transport sediment to the coast
in order to get the right conditions for a "depositional" coast. Even in the
absence of large, erosive waves, if you don't get the sediment, you won't
get a depositional coast.
Lec 1 p.7
c.





Amero-trailing coast
Most mature (east sides of both North and South America
Tectonically stable
Lots of sed, has been delivered for a long time
Wide, shallow continental shelf causes large, long-wavelength waves to lose
energy as they come from deep water to shallower water; thus overall
depositional conditions are not disturbed (except during storms…)
Most sed delivered to US east coast currently being deposited in large
estuaries (essentially drowned river mouths that have been flooded by recent
sea level rise).
3. marginal sea coasts
- partially shielded from open ocean by volcanic arcs (e.g., Phillippines
protect South China Sea, Gulf of Mexico protected by central America and
Caribbean)
- behave like trailing edge coasts
- reduced fetch distance limits erosive wave size
- sizable sediment load (due to big rivers) plus low wave energy produces
big deltas, tidal flats, marshes, etc.
E. Application of the tectonic classification
To be used only as a start, on a very broad scale
Do NOT interpret tectonic classification based on local coastal features - could
easily be misled. Examples= Australia (looks leading but it's not) and Alaska
(looks trailing but it's not)
Second and third order features due to several diff processes: erosive power of
large waves, depos. tendency of small waves; sediment type and movement;
rock types and structure; climate, biology.