Kealakekua Bay State Historical Park is a fragile protected environment. If you arrive by water you must stay on/in the water and if you arrive by land may swim in the bay and snorkel. . This park is a look only area as every habitat can be damaged by human contact, even the sands here have ecological communities that are damaged by being stepped on. If on the land, stay on the established paths and if in the water, stay floating at all times.
Those arriving by water must either have a permit or rent a vessel from a permitted vessel owner who has an current permit with the Division of State Parks. You will be able to see all the features from within the water and the permit clearly indicates that vessels may not land on the shoreline. As such, there is no need to leave the water to get to the exact coordinates. Due to exposure to rockfall as a result of the extreme instability of the cliffs and associated debris - do not swim or take a vessel into the area below the cliff that is marked with buoys.
The tall cliff on the landward side of Kealakekua Bay is a fault scarp that is likely the head scarp of a giant landslide. Before the landslide, the land surface extend out over the bay in a more gentle slope meeting the sea somewhere off to the west and then the slope continued on under the ocean surface.
For ocean slopes, the island of Hawaii rises out of the flat abyssal plain quite quickly, with slopes on the order of 3 to 9 percent. This is comparable to the 4 percent slopes of the continental shelf, some of the steeper slopes of under the ocean. These steep slopes occur because the erupting lava cools quickly in water and thus builds up on itself quickly.
Cooling of lava in air is slower, so the average slope of the above water portion of the island are not as steep. Lava flows out away from the source further before cooling thus making wider more gentle slopes. The lower slopes are between 3 and 6 percent while the slopes near the summit are near 11 percent. (Before you say it, these slopes are in relation to unweathered/unmodified slopes. Erosion by water have created steeper canyons)
The steepest average slopes occur at the air water interface. Lava that was cooling slowly in the air flows into the water and solidifies much more quickly. This results in a build up of rock near the ocean/air interface creating slopes that are between 13 to 17 degrees. These steep slopes are unstable and prone to slides and avalanches.
Some 200,000 to 100,000 years ago a debris avalanche broke off the western site of Hawaii and slid down the undersea slopes of the island. Geologists have named this event the Alika landslide. In that slide, an estimated 1,500 to 2,000 cubic kilometers of material separated from the island along the cliffs of this bay thus forming the Kealakekua Fault. Sea floor mapping has shown that material from this slide flowed 100 kilometers away.
The result is a bay that dives steeply down under the ocean after a short wave cut platform. This can be seen in the change in color of the ocean water as it progressed away from shore. Close to shore it is light colored and the coral reef can be seen through the clear waters. Very quickly the waters turn darker and the reef disappears and the floor of the bay drops away.
Such large movements of material under the sea are one of the sources of tsunamis. With an estimated 1,500 to 2,000 cubic kilometers of rock moving the tsunami resulting from this slide was likely very large and affected the entire Pacific. Lanai Island has tsunami deposits from around that time-frame that are 325 meters above sea level.
Following a common set of questions, there are two different sets of questions based on if you arrived by land via the Ka'awaloa trail, you may swim in the water or if you arrived via the water on a permitted vessel (and stayed in the water as landing your vessel is not allowed - unless you are with one of the 3 permitted tour guides that are authorized to do so). You only need to answer the questions related to your mode of travel.
Logging questions:
- The text "GC5DZ6P Alika Landslide" on the first line
- The number of people in your group (put in the log as well).
- What volume of material do you estimate was above water before it broke off the island?
- Use the formula for the area of a right triangle (½ base height) multiplied by the width of the bay.
- Assume the width of the bay is 1.6 kilometers (1 mile).
- Assume the base of the triangle (from the bottom of the cliff out to where the old shoreline was, the line between Keawekaheka Point and Palemano Point) is 1.8 kilometers (1.1 mile).
- You estimate the height of the cliff
- Compare your answer to #3 to the total estimated volume of the slide and estimate how much of the slide material was already underwater (or south of the bay and was covered over by new flows)
- Question based on how you got to the Bay
- Land Based Question: How far east or west of the underwater drop off are you?
- Water Based Question: What are the approximate coordinates to the point of the underwater drop off?
- For both: What feature did you use to find the underwater drop off?
The following sources were used to generate this cache:
- Slopes of the Hawaiian Ridge. Robert Mark and James Moore. Volcanism in Hawaii Chapter 3. in Volcanism in Hawaii. USGS Professional Paper 1350. 1987
- http://zsylvester.blogspot.com/2011/01/snorkeling-and-geology-in-kealakekua.html
- http://www.uhh.hawaii.edu/~kenhon/geol205/landslides/default.htm
- http://hvo.wr.usgs.gov/volcanowatch/archive/2008/08_10_16.html
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