Download Oceanography 101 Linda Khandro, MAT Homework 11: Dynamic

Oceanography 101
Linda Khandro, MAT
Homework 11: Dynamic Coastlines (20 points)
NAME:_________________________
Learning Outcomes:
By the end of this exercise students will be able to:
 Define and distinguish between active or passive continental margins.
 Define and distinguish between emergent or submergent coastlines
 Define and distinguish between primary or secondary coastlines
• Describe the effects of wave action on coastlines, producing erosion and deposition of
sediments.
Notes for Homework 11:
Earth’s continental coastlines are shaped and reshaped by many geological forces, all of which
occur over various time scales. In this Homework assignment you will study several relatively
small land masses to classify their coastlines on the basis of the forces that occur there. The
following is a short list of some of the forces at work on our coastlines:
• Effects of Earth’s crustal motions (plate tectonics). This produces either active or
passive continental margins.
• Effects of sea level. This produces either emergent or submergent coastlines.
• Influence of time on coastlines. This produces primary or secondary coastlines.
• Effects of wave action on coastlines. Waves are responsible for erosion and deposition
of sediments.
Resources that you will need:
I. Do your own Google or other type of web search for definitions of active and passive
continental margins, emergent and submergent coastlines, primary and secondary coastlines,
erosion and deposition of beach sediments.
2. Go here https://picasaweb.google.com/khandrolm
And scroll down through the Gallery to find web albums with pictures and illustrative websites
of the coastlines that are discussed below. They are all titled as “Coastlines Geo…”.
3. Read the following brief geological histories of:
Ireland, New Zealand, Mexico’s Baja and Yucatan Peninsulas, Cape Cod, and the coast of BC
and the Pacific North West.
IRELAND:
• Most of Ireland was formerly part of eastern US and Canada.
• It was moved to its present location as a result of the breakup of Pangaea (opening of
the mid-Atlantic).
• Massive limestone from an ancient ocean basin makes up much of the west side and
Atlantic coast, and produced the Cliffs of Moher and The Burren.
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The basalt columns of Giants Causeway in the northeast are the result of rifting about
65 million years ago; they are also found across the sea to the southwest of Scotland.
The climate is kept mild by the influence of the Gulf Stream ocean current, bringing
warmer water from the tropics.
NEW ZEALAND:
• New Zealand straddles a subduction zone between the Pacific and Australian plates;
where subduction is going in two opposite directions, stretching New Zealand with it.
• North Island rides atop the NW-moving Pacific Plate which is subducting beneath the
Australian Plate.
• South Island is mostly in the tear-apart (transform or strike slip) fault zone, but further
the south the SE-moving Australian plate subducts beneath the Pacific Plate.
• The climate is mild; warmer to the north and more seasonal to the south.
BAJA PENINSULA (Mexico):
• During the breakup of Pangaea (beginning about 200 million years ago), the North
American plate collided with the fast-moving Farallon plate to its west.
• A subduction zone formed, and with it, the subsequent spine of granitic mountains that
make up most of Baja’s structure and landscape.
• Even further west, the East Pacific Rise spreading ridge separated the Farallon Plate
from the Pacific plate; it ultimately subducted as well.
• Heat from the subducted East Pacific Rise weakened the continental crust and created
a rift zone that we now see as the floor of the Sea of Cortez, ultimately separating Baja
peninsula from the Mexican mainland like an opening zipper, from south to north.
• With the disappearance of the Farallon/Pacific spreading ridge, the boundary between
the North American plate to the east and the NW-heading Pacific plate to the west
became the transform (strike slip) boundary of the San Andreas Fault system
• The opening was complete and the sea rushed in to fill it roughly 5 million years ago,
and the Sea of Cortez was born.
• Baja California and the part of California west of the San Andreas fault, riding on the
NW-moving Pacific Plate, will tear off the continent entirely and become a very long
island, en route to the Aleutian Trench of SW Alaska in about 50 million years.
YUCATAN PENINSULA (Mexico):
 From about 100 million years ago to about 2 million years ago, much of the Yucatan
was submerged under warm shallow seas, at which time over 2300 meters of limestone
strata, created by coral reefs and seawater-precipitated limestone, was deposited.
 Over 150 cm of rainfall per year falls here, to which atmospheric carbon dioxide lends a
slight acidity.
 The limestone is very porous, and this acidic rainfall rapidly percolates towards a
shallow freshwater table. Steering a course to the Caribbean through bedding fractures,
this acidified water has dissolved conduits (caves) in the parent limestone since its initial
uplift from the ocean.
 Falling sea levels during active glaciation (2 million to about 10,000 years ago) resulted
in a considerable drop in the water table and thus, loss of water in the drainage
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channels. Unsupported cave ceilings near the surface collapsed creating caves,
caverns and surface sinkholes at odd intervals along the course of the conduits.
About 10,000 years ago sea level began to rise from 125 m below its present level,
raising the freshwater table. Deeper caverns were flooded by seawater and surface
caverns were flooded by freshwater.
The Mesoamerican Reef, 2nd largest reef in the world after the Great Barrier Reef, is the
southeastward extension of the Yucatan carbonate platform.
CAPE COD:
• The geologic history of Cape Cod mostly involves the advance and retreat of the last
continental ice sheet (named the Laurentide after the Laurentian region of Canada
where it first formed) and the rise in sea level that followed the retreat of the ice sheet.
• The sandy peninsula is the remnant of a huge terminal moraine, formed within the past
25,000 years.
• It remains an area of great geological interest because of its recent formation, and the
effect that rising sea level is having on the sandy margins.
BRITISH COLUMBIA, CANADA
• The geology of BC (and the rest of the Pacific Northwest) is too complex to discuss in
detail here!
• Suffice to say, most of the landscape from Alaska to California, and from the Pacific
coast to the western edge of the Rockies either originated somewhere else, or formed in
place as the “stuff from somewhere else” collided with ancestral North America.
• The BC coast of Canada is like that of Norway; a drowned coastline of former glacial
troughs and river valleys.
Procedure: (3 points per category=18 points)
A. Which of the areas above (Ireland, New Zealand, Baja, Yucatan, Cape Cod, BC Canada)
could be classified by the following terms? Note that most of the coastlines should fall into
more than one category below. Explain each of your choices.
1.
Active plate margin:
Plate tectonic motions are present nearby; for example, a subduction zone creates an active
plate margin with a relatively narrow continental shelf because oceanic and continental
sediments are deposited into the trench and become part of the subduction process. A nearby
rift zone can create an uplifted coastal region.
2.
Passive plate margin:
Plate motions are far from the coastline, so the sediments accumulate both vertically and
horizontally, creating a broader shelf with deep sediment layer.
3.
Emergent Coastline:
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Either the coastline is rising due to isostatic processes (rebounding after removal of glacial
weight) or tectonic processes (crustal compression and uplift), or sea level is dropping, or both.
This process exposes coastal seafloor, such as wave cut terraces and platforms.
4.
Submergent Coastline:
Either the coastline is subsiding due to tectonic processes (crustal extension or thinning), or
sea level is rising, or both. This is characterized by fjords (drowned glacial troughs), drowned
river valleys, and numerous islands and inlets.
5.
Primary-type coast:
“Primary” refers to the fact that processes such as river input, or coastal uplift, or subsidence,
dominate the coast, and that re-working of the shore by wave action is not yet well developed.
The types of sediments most likely found would represent the continental rock composition,
and the degree of their sorting and rounding would be less.
6.
Secondary-type coast:
“Secondary” refers to the fact that the dominant process now affecting the coastline is wave
action, which erodes headlands and deposits sediment and sand. Longshore current and drift
moves beach sand along the coast longitudinally, and storms bring up sediments from the
more distant continental shelf. Sands may be more rounded and well-sorted.
B. Do a Google or other web search for the continuous coastline of Washington, Oregon, and
northern California. Define this coastline in terms of the above categories. (2 points)
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