Physical Geology - La Paz batholith

Click on photos for larger view.

 

This is a view looking across the bay that the resort town of Cabo San Lucas to the rocky tip of the baja peninsula, where a beach called Lovers Point occurs. The rock in the foreground is massive and grey and is composed of granite some 100-90 million years old. The tip of Baja is made of the same rock, and this give some small idea of the large extent of this granite body, which diagonally crosses the peninsula as an elongate blob on geologic maps. The batholith is a continuation of a string of Cretaceous batholiths that includes the Sierra Nevada of California. Note also the well developed fractures of common orientation - a joint set. Well developed joint sets are characteristic of granite intrusions.


This is a view of Lover's Beach on the Pacific side and the well jointed La Paz batholithic granite found there. One can see that several joint sets exist, and that the interaction of wave erosion undercutting the base of the cliffs and the fractures and other erosion processes produce a distinctive landscape. Note the rounded form of some of the granite blocks between the joints. One can often recognize when they are in a batholith by such distinctive features.


This is a fin of granite formed by erosion along parallel joints. Note the massive and consistent internal texture and coloration of the granite.


This outcrop of granite shows some orange iron staining along fractures due to weathering near the surface, a rounded dark xenolith to the left, and a coarse grained pegmatite vein diagonally in the lower center. The pegmatite likely represents a late stage crystallization product of the batholith.


This is a view of a relatively equigranular granite in the La Paz batholith that has been rounded by erosion along joints.


The rounded cobble of granite shows the typical mineralogy of a granite. The medium grey mineral is quartz and the light grey and whitish mineral are plagioclase and potassium feldspar respectively. The dark flecks are the accessory minerals biotite and hornblende. The sand beneath is a weathering product of the granite and one can see the grey quartz grains and white and pink feldspar grains. most of the biotite and hornblende breaks down and does not survive.


A stone church in the city of La Paz made from local quarry stone.


A block of rhyolitic pyroclastic tuff in the church walls. Note the angular fragments, a result of an explosive and violent eruption. This volcanic rock is of about the same age as the batholith and likely represents the volcanic arc material that foundered into the underlying magma chamber below that became the La Paz batholith when it eventually solidified.


A block of flow banded rhyolite from the church wall. This was a very silica rich lava that became banded as it flowed into place, and then was later solidified. Again, this part of the volcanic edifice above the La Paz batholith.


Harmon D. Maher Jr. reserves copyrights to the materials in this site. Material may be used for non-profit educational purposes as long as proper attribution is given. For permission for any other use please contact author. Thank you.