Introduction
This EarthCache requires you to circumnavigate Hawksbill Island, gathering information you observe underwater whilst swimming on the surface. You will answer four questions -- each based on the observable underwater geology of the eastern, northern, western, and southern sides. You will need a mask and (probably) a snorkel and fins. Depending on your swimming ability, this should take around 30-60 minutes. You can also paddle out with a friend and then swim. Please do not drop an anchor in the coral.
All answers are found underwater, and the information can easily be gathered by swimming on the surface and using a mask to look down. It is perfectly fine to swim out to Hawksbill Island or to paddle around it. People do this all the time and the nearby resort encourages it. The water generally doesn't get to more than 10-15 feet deep. This EarthCache should be accomplished by swimming a big circle around the island. It is about a 1.6km round trip swim from the parking area.
Warning: The water is generally very calm, but if you decide to swim to this EarthCache you should be a confident swimmer, able to swim in the ocean for a distance of 1 - 1.5km. Pay attention to the weather and do not attempt this if it looks stormy. There are plenty of places to stand up and rest, but do not attempt to swim this if you aren't comfortable swimming some distance in the ocean.
Other Warning: Even though GZ is centered on the island, please do not climb onto Hawksbill Island Rock. The island is home to nesting birds; climbing on the island disturbs them. Additionally, you would have to walk on budding coral to climb onto Hawksbill, which will destroy the coral. Or you would have to walk over sea urchins, which will be painful. Do not pet the sea urchins.
Other Other Warning: While you likely won't see any close to you, please do not pet any lionfish you might see. They tend to swim deeper down, and won't come up to bother you. Do not swim down to bother them. Stay away from them entirely. If you just can't resist, be aware that they have venomous spines, and their sting is widely regarded as one of the most painful wounds in the ocean. Even if it does not cause an anaphylactic reaction and kill you, the incredible burning pain from the sting of a lionfish will last for many hours. Lionfish are very pretty, and you might see bright blue and brown ones. They are best appreciated from a distance.
The Geology of the Lesser Antilles:
For the purposes of this EarthCache we are only concerned with plate tectonics. Very generally, the Lesser Antilles, of which Antigua is a part, sit on top of the North Atlantic Plate and the Caribbean Plate. The South American Plate is to the south of the Antilles, and also influences the geology. Moving from east-northeast to west-southwest, the Atlantic Plate is slowly diving under the Caribbean Plate, which pushes the Caribbean Plate upwards. The graphic above, taken from the Creative Commons article "20th-century strain accumulation on the Lesser Antilles megathrust based on coral microatolls," shows the direction of the North Atlantic Plate (labeled "NAM"), Antigua & Barbuda, and, in red, the location of two 19th-century earthquakes. If one were to zoom in, one would notice that Hawksbill Island sits on the western side of Antigua and so is subject to pressures from the east.
The Geology of Antigua:
The island of Antigua is somewhat unique in that its three regions represent a transition between geological periods and processes. Part of the island shows the remnants of volcanic action from 7-10 million years ago. Other parts show the subsequent deposition of carbonates. So, it's a combination of igneous (volcanic) rocks overlain by limestone (sedimentary) rocks.
Unlike some other Caribbean islands, Antigua has three distinct zones that reflect its geological history.
Basal Volcanic Suite (Volcanic Region)
This volcanic region takes up the southwest 40% of the island. It is the oldest unit (Eocene-Oligocene) comprising basalt (a fine-grained volcanic rock), andesite (a dark, fine-grained brown or grayish volcanic rock), with some outcrops of dacite (similar to andensite, but with bits of quartz) in the eastern portion and thick accumulations of pyroclastic debris (The Technical Advisory Committee 2005).
Central Plain Group
This unit overlies the Basal Volcanic Suite and comprises sedimentary rocks such as mudstones (a fine-grained sedimentary rock with silt- and clay-sized grains), tuffs (porous igneous rock formed by volcanic ash and dust), arenites (sand-grained sedimentary rock), conglomerates (looks like aggregate cement, with rounded bits bound together) and some limestone (very fine-grained sedimentary rock).
Limestone Region
They are geologically the most recent and are composed of mostly fossiliferous marine limestones (limestone with a noticable quantity of fossils) and marls with coral limestones (limestone with and a few lenses of interbedded, weathered and volcanic gravel.
The mix of geologic regions, and the previously-active geologic history, can sometimes lead to some interesting rock formations around Antigua. Upthrust from the subduction of the North and South American Plates beneath the Caribbean Plate can be seen in various places. There are also embedded concretions. A concretion is a compact mass of mineral matter, often spherical but sometimes disk-shaped, embedded in a host rock of a different composition. This can happen in a number of ways. In some instances, a rock that formed earlier in the region's geologic history can be carried by water or thrown by volcanic action. Rock, such as limestone or coral rock can form around the original rock. Concretions can range in size from microscopic to massive boulders.
The Formation of Coral
Coral reefs are formed by coral polyps known as "reef-builders" as they ingest minerals and "poop out" layers of calcium carbonate. This forms a hard skeleton to which other corals attach themselves. Over time the corals die and break into pieces. The rubble builds up, as do the shells of various mollusks, which decay to form a kind of cement which binds the reef together. The reefs you will see around Hawksbill have been building for thousands of years.
Hawksbill Island
Hawksbill Island, on the western coast of Antigua, is generally sheltered from the normal winds and tides, which come in from the Atlantic on the eastern side of the island. This spares Hawksbill from the worst effects of ocean weathering.
As you circumnavigate Hawksbill Island you will notice that the four Waypoint locations each show a different aspect of the geology of Antigua and the Lesser Antilles outlined above. They are:
- The subduction of the North and South American Plates beneath the Caribbean Plate, as evidenced by upthrust and overlaying slabs, as well as a deep fissure between the slabs, and a steeper dropoff beyond the slabs as one moves away from the island.
- The buildup of coral around a large round concretion approximately 1.5 meters in diameter and sitting at a depth of approximately 2 - 3 meters.
- Growth in coral at shallow, sheltered depths, extending several dozen meters out from the island.
- A long sandy runout, likely caused by ocean currents beating against the coral and breaking it down.
To Log This EarthCache Please Send Answers To The Following Questions From the Choices Directly Above:
1. Which aspect of the geology, listed above, of Antigua and the Lesser Antilles most closely matches the feature seen at Waypoint #1.
2. Which aspect of the geology, listed above, of Antigua and the Lesser Antilles most closely matches the feature seen at Waypoint #2.
3. Which aspect of the geology, listed above, of Antigua and the Lesser Antilles most closely matches the feature seen at Waypoint #3.
4. Which aspect of the geology, listed above, of Antigua and the Lesser Antilles most closely matches the feature seen at Waypoint #4.
5. Not required, but feel free to post a picture at, near, or under GZ.
The Fifth Waypoint Bonus -- Not A Required Question: What human-made object is lying on the bottom in the sand, roughly pointed towards shore, near Waypoint #5? It's in a bit of a hole, but there is no mistaking what it is. The coordinates are exceedingly approximate, as I didn't have my GPS with me when I spotted it. If you can provide better coordinates, you'll get your username up in lights.