Oahu Geology

Yesterday my friend, Barb, and I headed out of the Marriott vacation resort to go over to the Aloha Bowl Flea Market.  We ended up in the wrong traffic lane in the crazy traffic that seems to be a constant around here and were forced onto the highway H2.  The next thing we knew the sign said that the next exit was not for 10 miles.  I am really glad that traffic forced us out of our way.  At first there was a rain forest on both sides of the road.  The vegetation was amazingly dense. 

Then we drove through a tunnel only to emerge on the other side to one of the most beautiful scenes I've ever seen.

 

 

The dramatic cliffs just blew me away so I had to research exactly what caused these nearly vertical cliffs to form.  Oahu is the third largest Hawaiian island comprising 597 square miles with 112 miles of coastline  (180 km).  The volcanoes that formed this island first erupted on the ocean floor more than 2.5 million years ago. It eventually reached sea level and continued to grow in elevation until about 1.7 million years ago, when the volcano became dormant. The volcano remained dormant for hundreds of thousands of years, during which time the original smooth sides eroded away. The highest elevation originally exceeded 9,800 feet (3,000 meters), but over time the peaks have eroded down to around 3,100 feet.

The island has an unusual irregular shape that was caused when two large landslides from two shield volcanoes removed approximately one-third of the northeastern portion of the island and approximately half of the western side. What remains along the western coast of Oahu is the Koolau range  (2.7 million years in age).  The volcano was destroyed in prehistoric times when the entire eastern half, including most of the summit caldera, slid into the pacific Ocean.   The dramatic remnant of the shield volcano has been transformed over thousands of years of constant gales that have carved beautiful rippling rock faces, near-vertical sides, and precipitous cliffs. The other mountains make up the older Waianae Range (3.9 my in age) that extends along the west side of the island.  Remains of these dramatic landslides lie as massive fragments strewn nearly 100 miles (160 km) over the ocean floor to the northeast of Oahu.

 

 

 

  

 

Last night I gave my agate talk to the Honolulu mineral club.  Thanks to Barb and Tom for driving as well as attending and helping me.  The club had a few specimens on display.  Below is a sulpher specimen collected on the Big Island.

 

 

 Below is a specimen of goethite... 

 

 

Dog tooth calcite....

 

 

Calcite....

 

 

Another sulphur....