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

Classification: igneous (intrusive)

Diabase

Environments of formation: in the vicinity of basalt eruptions and divergent plate boundaries

A sheet of diabase forms the cliff along the right side of the Point Bonita trail as you approach the tunnel

Diabase is an intrusive igneous rock. In Franciscan rocks, diabase formed from magma (molten rock) that didn’t erupt into seawater. Instead, the basaltic magma cooled slowly in fractures under the seafloor. When molten rock cools slowly, crystals can form. The slower the cooling, the larger the crystals. The same molten rock material can form both pillow basalt and diabase. If it erupts underwater, pillow lavas form. If it cools more slowly beneath the seafloor, it can form diabase.

Diabase in Franciscan rocks has light- colored crystals that are easy to see within its green-gray matrix.

 	

Don’t expect to find jelly bean shapes in diabase

Diabase appears speckled, since the crystals are easy to see

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

Serpentinite
Classification:	metamorphic

Environments of formation: upper mantle at subduction zones and other plate boundaries
Blue-green serpentinite at Baker Beach bluff, San Francisco
We have never been to Earth’s mantle, but the mantle comes to us in the form of serpentinite! This is the state rock of California. It is usually blue-green, flaky, slick and soapy.  It feels like a snake, thus its name.  The mixing of seawater with upper mantle rock produces serpentine minerals, such as chrysotile (asbestos) and talc, which are rich in magnesium and iron, but poor in potassium and calcium. This means that soils produced from serpentinite contain few nutrients that plants need to grow.  Some plants,
however, have adapted to serpentine soils. Several of these plants are unique to California, for, while serpentinite is abundant in the California Coast Ranges, it is rare in many other places. Look for serpentinite in areas where great tectonic movements occurred in the past, such as ancient, folded mountain ranges.

Sometimes pieces of talc-rich serpentinite are less flaky, and make good carving stone. Then you may hear it called soapstone.

The Chinese character for serpentinite contains the symbol for “dragon.” Can you guess why?

North Baker Beach serpentinite	Serpentinite, Crissy Field riprap

Close-up of serpentinite, showing white asbestos veins