Pillow Basalts and Radiolarian Cherts

November 01, 2018

Radiolarian cherts_sm (1)

Photographer: James Van Gundy 
Summary Author: James Van Gundy 

This photo taken near the mouth of Kachemak Bay, not far from Homer, Alaska, shows rocks of the Cretaceous McHugh Complex. The black rock with white speckles on the left is a pillow basalt lava that was extruded by submarine volcanoes, probably into relatively shallow water. The white speckles are mineral-filled cavities that originally formed as gas bubbles in the lava or as spaces between adjacent extruded basalt pillows. Such voids are subsequently filled-in by light-colored minerals like zeolites, quartz, and calcite. Gas bubbles only develop in submarine lavas that were extruded into shallow water since the pressure exerted by deeper waters prevents bubbles from forming.

The red and green rocks on the right are banded radiolarian cherts that were originally deposited on top of the pillow basalts on the seafloor. Radiolaria are single-celled planktonic creatures that produce a protective shell of silica (SiO2). When they die, their bodies decompose and their insoluble silica shells sink to the bottom of the sea, where they accumulate as sediments. If other sediments, such as clay, are excluded radiolarians may form extensive and thick deposits of relatively pure silica that are eventually converted into the sedimentary rock called chert. The red and green colors in these cherts are due to iron impurities.

Assemblages of pillow basalts and associated radiolarian cherts are often indicative of subduction zone processes where shallow marine rocks are deformed, metamorphosed, and plastered onto a continental margin to form a coastal mountain range. A similar suite of rocks can be seen along Route US 101, just to the north of the Golden Gate Bridge in Marin County, California. Photo taken on August 21, 2018.

Photo Details: Nikon Coolpix P600 camera; F3.8: 1/60 second exposure. 

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