Point Arena, CA USA


     On August of 2013, Becky (my wife) and I took a vacation from Utah to Northern California.  Our destination was  Point Arena, California. 

Northern California area map

     The City of Point Arena was a coastal community on the North Coast of California known for its beauty and small town charm.  Its Main Street Historic District hosted restaurants, delis, bars, and cafes.  However, the Point Arena Lands of the California Coastal National Monument provided kilometers of hiking along coastal bluffs.  Especially, the Point Arena Lighthouse provided a spectacular view of the coastal landscape.

I’m sitting on a bench viewing the Point Arena Lighthouse, California 2013

     Point Arena and adjoining lands are situated on a marine terrace underlain by an east-west geologic rock structure known as an anticline.  This anticlinal structure is millions of years old and was scraped flat by waves thousands of years ago, before tectonic uplift brought the rocks up from sea level and left the marine terrace deposits on top.  This creates an unconformity. The headland itself is carved on the sides by inlets of water and sea caves, all created by wave refraction processes.

Looking south from the Point Arena Headland,  at a group of sea inlets and caves

     Just to the north, and clearly evident while viewing from the headland (pictured below), a small sea stack is dramatic evidence of what this wave
refraction will ultimately do to the Point Arena headland. The rocks exposed along the edges of the cliffs are also weathering at different rates, giving Becky and I, a first-hand view of this differential  weathering. There were also rock falls and landslide features evident. All these are key
components of modern coastal erosion processes.

Looking north from the Point Arena headland

     A fantastic extra feature of the nearby sedimentary rocks further south is the dramatic concretions found at Bowling Ball Beach (pictured below). This location just a short drive from the Point Arena Headland and offers a fantastic place to bring home the ideas of sedimentary processes and differential weathering as the concretions wear down so much slower than the surrounding rocks that contained them.  Almost perfectly spherical about a 0.5 and 1.0m in diameter, the so-called bowling balls are actually a geological phenomenon known as “concretion,” (sedimentary rock formed by a natural process wherein mineral cements bind grains of sand or stone into larger formations).  These boulders are the result of millions of years of concretion and erosion, exposing the hard spheres as the mudstone of the cliffs receded around them.

Bowling-Ball Beach, 6 km south of Point Arena Headland, (looking south).  I’m standing on a one meter diameter concretion. 

     Just south of Bowling-Ball Beach, three tectonic plates (thin pieces of the Earth’s crust which float above the mantle) known as the North American, the Pacific, and the Gorda contact each other at the Mendocino triple junction.  Each of these plates slide against each other as they slowly move in opposing directions. Movement may be as much as two or three inches (5-6 cm) a year. Many times, this movement comes in the form of earthquakes, when built-up energy is released along fault lines that border the plates. Although the majority of these earthquakes are too small to be noticed, larger quakes are not uncommon.  Because the Gorda plate is subducting beneath the North American plate, there is the possibility of a “great earthquake” occurring in the future.

     Scientists believe that the two plates are partially locked together along a contact known as the Cascadia subduction zone. The frozen boundary between the two plates is called the megathrust. The megathrust is broken from time to time, but usually along small parts of the fault, resulting in small quakes. However, if a larger part is broken, a magnitude 8.0-plus quake is possible.

Water towers found near the Point Arena Lighthouse, (note how the extra reinforcement given at the bottom of the tower for the increasing pressures)

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