Question

Isle Royale, the islands of Cape Hatteras National Seashore, Nantucket, California's Channel Islands, and Elliott Key in Biscayne National Park are all islands, but they all formed in different ways. Describe the different geological processes that created these different islands.

Answer 1

Ans:

1)    Isle Royale : Isle Royale is an Island in Lake superior . It is a long ridge which extends along the lenght of the island. It has been formed by eons of geological activity and glacial activity. The island was formed as glacial ice withdrew during the last Pleistocene ice age 10,000 years ago. The island is a high ridge rose above the great basin that later became Lake Superior.

2) Islands of Cape Hatteras National seashore : The Island of Cape Hatteras are barrier islands. Most of them are lond and narrow and separated from the mainland by wide shallow water. The geologic framework of the islands was created by the cyclic rise and fall of relative sea level. The framework of the islands control the sediments available to the modern barrier island . Some 5.3 million years ago the sea level was much higher and marine sediments were deposited to what is now a coastal plain.During the glacial period beginning some 2.6 years ago , the level of the sea rose and fell many times and rivers cut the previously deposited marine sediments, leaving a kind of paleotopography that controls the estuarine geometry and location of the barrier islands. These ancient river channels were backfilled and buried by holocene sediments during the last 10000 years as the level of the sea rose to its present level.The sediments underlying the the barrier islands were a complex structure of marine, esuarine , riverine and coastal plain deposits.

3) Nantuket : It was formed by the Laurentide Ice sheet during the recent Wisconsin Glaciation and shaped by the rise of sea level.The northern section of the island has a low ridge system which was deposited as glacial moraine during a period of glacial standstill which is a period during which till continued to arrive and deposited as the glacier melted at a stationary front. On the southern section of the island is an outwash plain which slope away from the arc of glacial moraine and shaped at its margin by the actions of longshore drift. Nantunket became an island when the rise in sea level covered the connection with the mainland.

4) California"s Channel Islands : The formation of these islands goes back to 100 million year ago. It was formed by activity of tectonic plates. Upto about some 30 million years ago, there were convergence of two large tectonic plates along the western edge of North America.As the oceanic plate approached the the continental North American plate from the west , it subducted to form a deep trench and was destroyed by melting in the mantle. On the landward side of the trench , a marine basin was formed and for million of years the sediments that were washed off the land were deposited into this basin . The deposited sediments solidified to become the basement rocks of the island and thus formed the island we know today.

5) Elliot Key in Biscayne National Park : Elliot key is one of the 42 islands in Biscayne National Park . Elliot Key is the park's largest island. The island is a remnant of ancient coral reefs and sand bars which were present during a period of higher sea levels some 125000 years ago during the pleistocene epoch. During the last ice age some 100,000 years ago, the sea level dropped and exposed the ancient coral reefs and sand bars . These ancient coral reefs fossillized over time to form the rocks of the island that we see today. Two rock formation in the Key are Key Largo limestone and Miami Oolite.During the time of the lower sea level, the Florida Bay was forested . The glaciers and polar ice caps started melting 15,000 years ago, The combination of flooding of land ( due to of melting glacier) and tidal influence changed the topography of the keys and the surrounding areas