Sunday, March 6, 2011

Geology of Baja California

Note - I will be adding some pics and maps to this post eventually, so please check back soon.

View from the volcanic cone (828ft) on Isla Coronados
Photo courtesy of Kurt on SV Raven

For those interested in some geological background, I found the following information in " A Field Guide To The Common and Interesting Plants of Baja California " by Jeanette Coyle and Norman C. Roberts. In attempting to condense three pages down to three paragraphs, I may have omitted significant facts, so don't sue me!

The jagged finger of the Baja California peninsula is relatively young in the history of the earth. About 150 million years ago, a chain of volcanic islands existed between the modern day Sierra Nevada of California, USA, to almost as far south as Puerto Vallarta, Mexico. To the west of the island chain was the deep trench of the Pacific Ocean and to the east, a shallow sea. Over millions of years, sediments eroded into the west trench and the volcanic rocks built up in the island arc. Limestone and shale to the east of the volcanic rock accumulated in the shallow sea. Heat and pressure has resulted in the complexity of these "metamorphic" rocks. Rocks that were formed in the deep sea are exposed in the mountain ranges of the west coast of Baja California. The shallow sea sediments are found all along the coast of the Sea of Cortez.

Volcanic activity stopped around the age of the dinosaurs and tropical rains eroded the mountains to expose granitic cores created earlier. Some of these mountains eroded completely and a plain extended hundreds of miles eastward. Rivers flowed down the current peninsula carrying away gravel and sand to the Pacific. About 25 million years ago the San Andreas Fault split California and northwestern Mexico apart and everything west of the fault began moving northward. Movement is still occurring at a rate of about one inch a year and has displaced Baja California by 450 miles since inception.

The movement was not uniform and stretching of the peninsula was accompanied by more volcanic activity. The Sierra de la Giganta is formed from the accumulation of the volcanic rock. Lava flows and ash layers has created the colourful black and pink layered landscape of these areas. The rivers that flowed from north Mexico were stopped by the separation of Baja California and the plain lifted up and tilted in the east. This surface can be seen today, rising from sea level of the Pacific to almost a mile high in the Sierra Juarez and Sierra San Pedro Martir. Scattered on the surface are river gravels, some gold bearing, evidence of old river systems.

The east side of this highland area is the " Gulf Escarpment", east of which is the San Felipe Desert, with some isolated mountain ranges. Between the desert ranges are valleys filled with sediment.

Baja California is still geologically active.

No comments: