How an undercooling of moderate amounts, caused by chilling of the lava, can influence crystallization. Ta would be coarsely porphyritc Tb would be very fine grained porphyritic and Tc would be Trap.

Keweenaw rift rocks include a somewhat rare textural variety of basalt called ophite or ophitic basalt.  Ophitic texture is defined inconsistently,  but it is an important variety of basalt texture where pyroxene (or occasionally olivine) forms larger crystals and typically contains numerous crystals of plagioclase (right). Pyroxenes may vary from < 1 to 10 cm and may include as many as hundreds of plagioclases.  In the field the pyroxenes are often 1-2 cm in diameter and give the rock a distinctive aspect. There may be a brownish or orange region surrounding the pyroxenes which may represent a glassy remnant of magma melt. Overall the ophite is thought to represent a solidified remnant of a dendritic crystal mush. Crystal size and form in volcanic rocks is known to be influenced by the rates of cooling in the immediate vicinity of the growing crystal. Slow cooling in a pluton leads to large, equidimensional crystals, while very rapid cooling can lead to no crystals at all (glass or obsidian). Intermediate cooling rates can lead to unusual shapes of crystals (spherulites, “bow ties”, spinifex, and ophitic) as crystals nucleate or grow at accelerated rates as crystallization, which requires more time than allowed by the environmental cooling of the lava, cannot keep pace and exhibits disequilibrium (Lofgren, 1980). The rate of heat loss (undercooling or supercooling) during the solidification is thus thought to cause ophitic texture, where pyroxene is growing rapidly and plagioclase is forming many more nuclei. Because ophites may completely crystallize and can be coarse-grained, especially with respect to pyroxene, some are termed gabbro rather than basalt. At first geologists looking at ophitic lava flows in the Keweenaw wondered whether they were sills.

There is a tendency for ophitic textures to be found in large basaltic intrusive rock bodies such as sills, suggesting that overall they reflect relatively slow solidification.  Overall ophitic texture is ubiquitous and could be a hallmark of the Keweenaw Rift lavas.  Paces (1988; see right) found that the average thickness of ophitic Keweenaw flows was 33 m (range 11-140m), while subophitic ones were 12 m (range 4-45 m), and traps (melaphyres) about 5 m thick (range 2-60m).  We note that the overall average thickness of Keweenawan flows is about 10-11m, much greater than what we see at modern volcanoes like Kilauea (average flow about 0.5 m thick). The differences are likely the result of ponding within the rift valley, where volcanism filled the rift basin rather than running off a slope away from the vent, as happens at Kilauea. So ophitic texture is a hallmark of slow cooling that is apparently related to ponding of the lavas.

Ophite beach cobbles, Bete Grise

Typical Ophite Texture, Greenstone Flow.
Weathered surface of ophitic boulder from the Greenstone flow near Phoenix