INSTRUCTIONS: A. Which of the 7 coastal areas above (Ireland, New Zealand, Baja

Question

INSTRUCTIONS:

A. Which of the 7 coastal areas above (Ireland, New Zealand, Baja West Coast, Baja East Coast, Yucatan, Cape Cod, BC Canada) could be classified by the following terms? Be sure to explain each of your choices!

Passive plate margin:

At this type of margin, plate motions are far from the coastline, so the sediments accumulate both vertically and horizontally, creating a broader shelf with deep sediment layer.

Which 4 coastal area(s) can be classified as being far from (more than 100 km) to an Active Plate Margin, and therefore considered to have a Passive Plate Margin? Explain each.

a.

b.

c.

d.

Explanation / Answer

a) Ireland is a passive plate margin. 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).Palte motion are far from the coastline.

b) Baja West Coast and Baja East Coast (Baja Peninsula) are passive plate margin. 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.

(c) Yucatan Peninsula is a passive margin. Around 100 million years ago, much of the Yucatan was submerged under warm shallow seas and over 2300 meters of limestone strata, created by coral reefs and seawater-precipitated limestone, was deposited there. Over 150 cm of rainfall per year falls here, to which atmospheric carbon dioxide lends a slight acidity. 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 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.

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