Dana Point Harbor is one of the largest man-made marinas in Southern California. To protect boats from large waves and the nearby infrastructure from erosion, large breakwaters were constructed made from local rock quarried from Vista, California and also Catalina Island. The above coordinates are for one of the main breakwaters at the harbor's entrance. Breakwaters are meant to break the waves crashing on shore. It has some negative, but also some positive side effects. So how are breakwaters made and how do they disrupt the geology?
For starters, waves here typically come from the north to the south. It's why the water here is cold as cold water from the arctic regions of northern Canada and Alaska move down to the equator. When the water moves, it carries sediment with it which moves sand down the coast along with silt and clay which eventually gets deposited a few miles offshore. The sand on the beaches here were from rock that was eroded way somewhere up the coast.
By building a breakwater, this natural course of events where sand is moved down the coast is broken. When a breakwater Is constructed, it slows down waves which in turn, makes sand be deposited. Know that sand is suspended in the water when the water is choppy and moving. When the water slows down at a breakwater, the sand drops out of the water and falls to the seafloor. Over a few years, this sand will build up and eventually small beaches will form out of break waters. They usually form at corners where the receding water isn't as strong to pull the sand away and where waves can come in at two different angles, depositing sand twice as fast. Every few years, dredgers will help take this sand and move it off shore and the process will begin all over again.
By having the sand pile up here, beaches down the coast are being eroded faster than the sand can be replaced. This is a problem with roads, bridges, homes, and business that are at the water. Unfortunately, it is something we now have to live with. Breakwaters are formed in a few different ways, but in this earthcache I'll only be discussing the way they are created here. Large boulders are piled up on top of each other and overtime they each the surface. The large holes in between the rocks are good to move water in and out of the harbor, to recycle the water per say. They also allow small animals to use them as their habitats such as clams or muscles. Small fish can swim through these holes all together. When originally placed, many of the boulders were quite sharp. However, given a few years with continuos water erosion from the ocean the rocks that make up the breakwater become more and more smooth.
There are three main types of rocks, of which all can make up a breakwater depending on the conditions of its environment. Igneous rock is a volcanic rock. The most common is basalt which is black and is what makes up most of the big island of Hawaii. The next type is metamorphic rock. It forms when preexisting rock is under a lot of heat and pressure. This changes some of chemical properties of the rock. An example of this is marble, which forms from limestone, a sedimentary rock. The last rock type is sedimentary rock, which forms from sediments like sand into sandstone. See the chart below to get a better idea of the three main rock groups.
The rock type that makes up the breakwaters in the Dana Point harbor is an ingenious rock called Granite. This specific rock that is within the breakwater was quarried in two different places. Rock from Vista, California (in northern San Diego County) was used as the foundation and the support for the much larger granite from Catalina Island. About 25 million years ago, Southern California didn't have the infamous San Andreas Fault system it has today. Instead, it was much more similar to the current geology off the coast of Japan. An oceanic trench about 100 miles offshore created a convergent plate boundary, where one plate slides under the other. As the plate bends under, it eventually reaches the mantel, about 50-150 miles away from where it entered the earth. Here, the plate begins to melt and become part of the mantel. Eventually this makes the molten rock rise to the surface an erupt as a volcano. Within the magma chambers of these volcanoes is where granite was constantly forming and reforming. Today, in Vista there is remnants of a volcanic caldera at GC51CJ5 and in Orange County, you can see igneous rock such as basalt at GC3W57A. This is how the granite before you was created.
Works Cited
https://www.researchgate.net/figure/254318368_Typical-cross-section-of-rock-mound-breakwater
https://en.wikipedia.org/wiki/Breakwater_(structure)
http://www.surf-forecast.com/weather_maps/California
http://articles.latimes.com/1987-03-15/travel/tr-10688_1_rocks-dana-point-breakwater
TO LOG A FIND ON THIS CACHE YOU MUST GIVE ME THE CORECT ANSWERS. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER. ANY INCORRECT ANSWERS WILL RESULT IN A DELETED LOG
1. "Dana Point Harbor Breakwater" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching. However, this is not required.
2. Describe the (a) sizes, (b) shapes, (c) colors, (d) and textures of the granite boulders that make up the breakwater.
3. Besides granite, are there other rocks that make up this breakwater? How can you tell?
4. What side of the breakwater has the most wave energy AND see if this matched any high/low tides.
5. Look for any build up of sediments (sand) along the breakwater. Which side, if any, has sediment AND explain why this might be.
6. Using the last diagram on this cache page of mineral compositions, what is the (a) most and (b) least prominent minerals in these granite boulders?
7. Based off of your answer above, do you think these granite boulders were quarried at the same place on Catalina Island? Justify your answer.
8. Why can't you see the granite rocks that were quarried from Vista? According to the cross section of a typical breakwater diagram on this cache page where should you be able to find them?