Radiolarian chert

If you’ve been to Marin Headlands and noticed the dark-red layered rocks, you’ve seen radiolarian chert.

Radiolarian chert is a type of very hard rock that forms from the tiny, glass-like skeletons of microscopic sea creatures called radiolarians. These creatures live in the ocean, and when they die, their skeletons sink to the ocean floor. Over millions of years, layers of their remains build up and get buried by more sediment. Eventually, the pressure and chemical changes turn this material into a solid rock made mostly of silica (the same substance found in glass). Radiolarian chert usually forms in the deep ocean, far below where shells made of calcium can survive, so it’s mostly made of these tiny siliceous remains.

To the eye, radiolarian chert often looks dark gray, red, or brown, and sometimes shows thin layers or bands from different periods of buildup. If you looked at it under a microscope, you’d see a beautiful, detailed pattern of tiny skeletons packed together in a background of fine quartz. Because it’s so hard and breaks with sharp edges, ancient people often used chert to make tools and weapons. Today, scientists study radiolarian chert to learn about Earth’s history, especially how the oceans and tectonic plates have changed over hundreds of millions of years.

Resources

• 3D image you can rotate (this doesn’t look as dark-red as chert appears in the amazing layered outcroppings, but it is chert from the Marin Headlands).
• More on Wikipedia

Glossary links