Geology 104 - Spring 2007
Lecture #26

Virgin Islands and Dry Tortugas NP

Dry Tortugas NP

Topography and historty

• "Dry" = no fresh water; "Tortugas" = turtles
• Discovered by Spanish in 1513
• NM 1935, NP 1992
• 7 keys + shoals (shallow areas of ocean)
  • Coral reefs
  • Sand
• Islands change in shape & size

Geology

1. Rocks

• No bedrock exposed. Probably underlain by:
  • Key Largo Limestone
  • Miami Oolite
• Most islands composed entirely of sand (oolites & broken coral)

2. Reefs

• Holocene & living corals
  • 15 m thick
  • Same species as in bedrock
  • These are "stony corals"
    • Colonial animals
    • Hexagonal
    • Calcite exoskeletons
  • "Soft corals" live on shoals
    • No hard skeleton
    • Octagonal
• Threats to reefs
  • Overfishing
  • Global warming
    • Too warm > "bleaching" > 95% die

West Indies Archipelago

• Consists of the Bahamas, the Greater Antilles, the Lesser Antilles, and the Netherland Antilles
• Each chain of islands includes volcanic rocks
• Only the Lesser Antilles are currently active

Virgin Islands NP

Topography and History

• Pre-columbian inhabitants, Danish plantations, bought by US
• NP 1956
• On island of St. John in the corner where the Greater Antilles meet the Lesser
• Accessible only by ferry

Geology

1. Stratigraphy

Rocks	Age
Coral reefs	Quaternary (Holocene)
Hans Lollik & other fms (igneous)	Tertiary
Outer Bass & Louisenhoi fms (lavas, limestones)	Cretaceous

2. Plate Tectonics

• Volcanic rocks were erupted along subduction zones
• Caribbean Plate has complex history
• All 3 types of boundaries can be seen in Caribbean today
• Continuing volcanic activity along eastern subduction zone
  • Mont Pelee on Martinique in 1902
  • Soufriére on Guadalupe in 1976
  • Soufriere Hills on Montserrat 1995-present

3. Marine features

• Terraces (= wave cut platforms) form at sea level by wave erosion.
  • When sea level rises/falls, terraces may form at different levels.
  • Sea level was affected by Pleistocene glaciers and by tectonic movements.
• Coral reefs
  • Patch reefs
  • Coral reefs are among the most diverse ecosystems on Earth.
• Beaches
  • No permanent streams on the island
  • Beaches form from material carried ashore by waves
    

Virgin Islands slides

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