How do I recognize geologic change in my environment? What is the tectonic journey of this rock?

Radiolarian Ribbon Chert
Classification:	sedimentary
Environment of formation:	deep ocean floor

Folded layers of chert in the Marin Headlands

This rock contains billions of microscopic fossils. The fossils are skeletons of single-celled plankton, called radiolaria. These plankton are most common in nutrient-rich ocean waters. Like quartz, radiolarian skeletons are made of silica.

From a distance, radiolarian chert appears red-brown. A closer look reveals many colors. The colors are evidence of different amounts of minerals and oxygen in the sea water when the rock formed.

Radiolarian chert is very hard, and often feels smooth. Indigenous people made spear points from this glassy rock. The Army crushed chert to make roads. Pieces of chert often have a blocky shape.

When radiolaria die, their tiny skeletons fall to the ocean floor, forming an ooze. This ooze eventually hardens into rock. The chert forms horizontal layers about 5 –15 cm thick. It took about 1,000 years for a 1 mm thickness of chert to form! When the flat layers of rock are carried on a tectonic plate, they may collide with the edge of a continent.  This slow collision will bend and fold the layers.

Chert in Crissy Field riprap	Close-up of red and green chert	Magnified radiolaria skeletons

How do I recognize geologic change in my environment? What is the tectonic journey of this rock?

Pillow Basalt
Classification:	igneous (extrusive)  Environment of formation:	underwater volcano

Basalt is the most common igneous rock on the planet. When hot basalt erupts from an underwater volcano, cold seawater hardens lava into “pillows,” or jelly bean-shaped blobs, each about one meter long. As the volcano continues to erupt, stacks of pillow lavas form, with the newest pillow on top of the stack.
The pillows start out black, gray or green. If they are exposed to rain, the iron in the basalt will oxidize.

Pillow basalt stack at Point Bonita Lighthouse

Weathered pillow basalt looks rusty and crumbly.

Geologists sometimes call Franciscan pillow basalt “greenstone.” The green color seen in the basalt comes from an interaction between the basalt and hot, mineral-rich seawater.

Pieces of basalt you find on the beach may look like they have “polka dots.”  The white dots on the rock are minerals that filled in the gas bubbles in the lava. Pillow basalt with polka dots erupted at a shallower depth. In very deep water, such as at a seafloor spreading center, the weight of the water squeezes out the gas in the lava, and the basalt that forms has no vesicles (gas bubble holes). Most of the basalt in the park has no holes or polka dots, but at Point Bonita, and in some Crissy Field riprap rocks, you will see pieces of basalt with white dots. What does this tell you about its environment of formation?

 	 	

Basalt rock at Crissy Field riprap

Pillow lava forming underwater near Hawaii

Weathered pillow basalt at Point Bonita

Close-up of serpentinite, showing white asbestos veins