A mafic igneous rock from the top of Bowen's Reaction Series. Typically dark colored, although weathered specimens can appear quite light, or reddish. 

 

Olivine is sometime present as phenocrysts but is not essential. And although this specimen does not show it, vesicular (cellular) varieties are quite common, and these grade completely into scoria.

 

Minerals too small to identify (except phenocrysts). Composition will be from the top of Bowen's Reaction series (i.e. mafic) and we would expect ca plagioclase, pyroxene, and perhaps some olivine. Note that these minerals are present in this specimen, just too small to see.

 

basalt is one of the most common igneous rocks found. It is the major constituent of the upper layer of the ocean floors (usually as pillow lava), and hot spot volcanoes (such as the Hawaiian islands). basalt commonly forms on the continents too, usually the result of hot spot activity. Here it may exist as intrusive dikes and sills, or extrusive cinder volcanoes and lava flows. In the western U.S. such occurances are common and usually quite visible since the volcanoes are relatively young. But basalt is also common in the east, if you can get through the vegetation to see it. In the east basalt shows up mostly as dikes, or more spectacularly in the 1000 foot thick Palasides sill, across the Hudson river from New York City. 

 

Detail 

 

  

This closeup shows that minerals are too small to see, which is why it is an aphanitic rock.

 

 

 

A mafic igneous rock from the top of Bowen's Reaction Series. Typically dark colored, although weathered specimens can appear quite light, or reddish. 
     

This vesicular or cellular specimen is full of holes produced when gas expanded as the lava was extruded to the surface. Vesicular varieties vary widely, and grade completely into scoria.

 

basalt is one of the most common igneous rocks found. It is the major constituent of the upper layer of the ocean floors (usually as pillow lava), and hot spot volcanoes (such as the Hawaiian islands). Vesicular varieties such as this one are typically terrestrial, forming either on volcanic islands or on the continents. 

 

 

 

A mafic igneous rock from the top of Bowen's Reaction Series. Typically dark colored, as is the aphanitic portion of this rock, although the large plagioclase phenocrysts give it a lighter appearance. The phenocrysts indicate this rock had a two stage cooling.

 

The large phenocrysts in this specimen tell us it had a two stage cooling, having spent some time below the surface cooling slowly enough for the plagioclase crystals to form, and then being brought toward the surface for rapid cooling. But even the fine grained portion of this rock is more granular than the previous specimen, indicating it probably did not cool at the surface, but perhaps in a dike, sill or laccolith.