Geological Survey Bulletin 1309 The Geologic Story of Isle Royale National Park

VOLCANIC ROCKS

When we think of volcanic activity, we usually conjure up visions of a Vesuvius spewing forth billowing clouds of steam and ash or a Kilauea with fiery fountains of molten lava reaching skyward. This is partly because most people are familiar with the type of volcanism that builds imposing cones, such as Vesuvius, Kilauea Mount Rainier, Mount Fuji, and many others and partly because many of these volcanoes have erupted in historic times, sometimes with disastrous results. Individual eruptions of such volcanoes, while spectacular, generally involve relatively small volumes of molten lava. In other words, such volcanoes are constructed slowly by frequent eruptions of relatively small quantities of lava spewed out in all directions from a generally localized vent. The lava seldom travels far from the vent, and the tongue-shaped flows accumulate one upon another to form a conical pile (fig. 8).

VOLCANIC CONE—cross section. (Fig. 8)

However, in some of the world's most extensive volcanic areas, true volcanoes are rare or absent. In such areas tremendous volumes of lava, sometimes measured in tens or even hundreds of cubic miles, welled out through fissures not connected with volcanic cones to form extensive sheets of flood basalts or plateau basalts. These sheets of basaltic lava, piled up flow upon flow to thicknesses of thousands of feet, are the most extensive of all volcanic deposits, and such deposits have occurred at various places and times in the geologic past. Such eruptions are rare at the present time, however, the only known historic eruptions of this type occurred in Iceland. There, at Laki, in 1783, an estimated volume of 3 cubic miles of lava flowed from a fissure 20 miles long to cover an area of 215 square miles. But even this was small compared with similar eruptions during earlier geologic periods.

The volcanic rocks of Isle Royale are flood basalts, but before discussing their details we can look at a younger sequence of flood basalts that is still relatively undisturbed geologically, allowing us to deduce more about their origin. In the northwestern United States, between 18 and 10 million years ago, enormous amounts of basalt lava welled up through deep fissures in the earth's crust and flooded much of present-day Washington, Oregon, and Idaho to form the great Columbia River Plateau. These basalt flows are known collectively as the Columbia River basalt (fig. 9).

COLUMBlA RIVER BASALT—today. (Fig. 9)

Much of the Columbia River basalt has been eroded away, hut it is estimated that the lava flood once covered an area of 150,000 square miles, equivalent to the combined areas of Washington and Oregon. It was probably over 1 mile thick in places and had an average depth of 1,800 feet. The tremendous amount of basalt contained in even a single flow can be illustrated by the Roza lava flow in Washington (fig. 10). This flow which can be traced over an area of 8,800 square miles, averages 100 feet in thickness. Its volume of at least 160 cubic miles is roughly seven times the volume of Mount Rainier, one of the great volcanoes of the Cascade Range.

FLOOD BASALTS of the Columbia River Plateau form these cliffs along the Yakima River. The Roza lava flow lies near the top of the hill. Photograph by R. S. Fiske. (Fig. 10)

A schematic diagram of an eruption of flood basalt is shown in figure 11. The lava wells out of long fissures that cut through earlier, cooled and solidified flows, and it is erupted in such a great volume that it rapidly flows away from the source fissures when it reaches the earth's surface. Instead of building volcanic cones, the eruption forms a vast lava lake, and the resulting flow, when solidified, tends to have nearly level surfaces. A sequence of such eruptions results in the accumulation of a series of basalt sheets covering vast areas—much like a layer cake—having quite a different geometry from the outward-dipping concentric structure that results from the eruptions of the more familiar cone volcanoes.

FLOOD BASALT ERUPTlON—schematic diagram. (Fig. 11)

At first glance the volcanic rocks on Isle Royale seem to have a monotonous similarity—just nondescript dark-colored rocks. When studied more closely, however, numerous variations can be seen, some readily apparent and others more subtle. These variations are more significant than might be supposed. Some, for example, result in differences in rock hardness, which in turn affects resistance to erosion and thus the development of landforms. Other differences help to identify individual lava flows, thus enabling one to separate them from their neighbors and to trace them across the countryside. This recognition is critical to the preparation of a geologic map and to the unscrambling of the geologic history.

The compositional classification of volcanic rocks is largely based upon their silica (silicon dioxide) content, with a range from basalt, with a silica content of roughly 50 percent or less, to rhyolite, with a silica content of 70 per cent or more. Rhyolite and volcanic rocks of intermediate composition usually have an excess of silica, and so some of the silica occurs in the free or uncombined state as quartz, as well as in other more complex silicate minerals. In basalt, however, all silica generally is needed to form silicate minerals, and quartz is uncommon.

The volcanic rocks on Isle Royale are nearly all basalt, and their mineral constituents are plagioclase feldspar, pyroxene, and lesser amounts of olivine, magnetite, and other minerals. A few of the rocks are somewhat more silicic than basalt and would be classified as andesite. Volcanic rocks as high in silica as rhyolite are not exposed on the island. In fact, the basalts of Isle Royale are so much alike chemically that we cannot readily use chemical composition by itself to distinguish one volcanic rock on the island from another.

When lava cools and solidifies quickly, it forms glass. If it cools more slowly, it partly or completely crystallizes to mineral grains. The glass, in time, will also crystallize to mineral grains, but they generally will be finer than those formed during slow cooling. Differences in the cooling history, as well as in chemical composition, of individual lava flows result in differences in rock texture, a property that chiefly reflects the grain size, shape, and distribution of the minerals in the rock. The basalts of Isle Royale, and elsewhere in the Lake Superior region, do exhibit variations in texture; consequently, a rock classification based upon textures has developed through the years and is widely used in the Michigan copper district. Only an experienced geologist can estimate the chemical compositions of rocks in the field, but anyone can learn to recognize the different textures; remembering their names is the major hurdle. The textural classification is extremely useful because it can be applied directly in the field, at the outcrop, and thus is a powerful aid in identifying individual flows for geologic mapping. I must emphasize, however, that these textural terms are not always used with the same connotation outside the Lake Superior region, and some geologists have abandoned them altogether because of conflicting usage. But they will be found in any further readings on the geology of the volcanic rocks of the Lake Superior region. The rock textures are illustrated in figure 12.

VOLCANIC ROCK TEXTURES found on Isle Royale. (Fig. 12)

Ophite.—Rock with a mottled texture produced by crystals of pyroxene surrounded by a slightly darker matrix of finer grained minerals (fig. 12A). Pyroxene has good cleavage—the tendency to break along definite planes, producing smooth surfaces. Such surfaces reflect light better than the rock matrix, and on freshly broken specimens of ophite, the flashing of pyroxene cleavage surfaces in the sunlight is a distinctive feature. Cleavage reflections are dulled by chemical processes involved in weathering, but weathering actually accentuates the texture by increasing the color contrast between the pyroxene crystals and the rock matrix and by producing a knobby surface (fig. 13). The size of the pyroxene crystals varies from less than 1 millimetre to more than 2 centimetres, but crystal sizes are usually rather uniform in any individual specimen. Within a given lava flow, the pyroxene crystals are progressively larger toward the flow interior. Ophite is the most abundant volcanic rock type on Isle Royale.

COARSE-GRAINED OPHITE showing weatherede, knobby surface (Fig. 13)

Porphyrite.—Rock with a texture produced by well-defined plagioclase crystals scattered through a finer grained groundmass. The term is applied to two distinct varieties of such porphyritic rocks. One variety has small, blocky millimetre-sized plagioclase crystals rather uniformly distributed through the groundmass (fig. 12B); when the plagioclase crystals tend to clot together, the term glomeroporphyrite is used (fig. 12C). The other variety of porphyrite has larger, tabular-shaped crystals more sparsely distributed in the rock and commonly occurring in clots (fig. 12D); the large crystals are often as much as 2 centimetres long.

Pegmatite.—Rock with a texture in which all of the minerals, especially the plagioclase are larger when compared with those in most of the other rock types; the elongate plagioclase laths give the rock a matted appearance (fig. 12E).

Trap.—Fine-grained dark-colored massive rock showing none of the distinctive textures mentioned (fig. 12E). Trap commonly breaks with a curved fracture.

Felsite.—Fine-grained light-colored volcanic rock, generally reddish. Texturally it is most similar to trap, although commonly containing scattered crystals, but is applied to more siliceous volcanic rocks such as rhyolite. Felsite does not occur in lava flows exposed on Isle Royale but does occur as pebbles in most of the conglomerate on the island.

All the rock textures are more easily recognized on weathered than on fresh surfaces because color contrasts between many minerals are increased during weathering and most lava flows can readily be characterized as ophite, porphyrite, or trap. In some flows the texture is obscure, but in nearly all these flows the rock is ophite. Pegmatite is a unique rock found only in layers in the interior of some ophitic flows, generally the thicker ones; its formation involves late-stage crystallization of a liquid remaining in the central part of a flow after the bulk of the flow has already solidified.

Two other terms are useful in describing the volcanic rocks: vesicle and amygdule. As a lava flow cools, small circular cavities are formed by the expansion of bubbles of gas or steam during the solidification of the rock; these cavities are called vesicles. When the cavities are later filled with minerals deposited from solutions circulating through the rock, the vesicle fillings are called amygdules. A rock with abundant amygdules is called an amygdaloidal rock or simply an amygdaloid (fig. 14); Amygdaloid Island, for example, is named for a rock of this character. The rocks on Isle Royale are very old and have been saturated by mineral-bearing solutions long enough so that no vesicles remain—all have been filled and converted to amygdules. The uppermost part of most individual lava flows is conspicuously amygdaloidal, containing as much as 50 percent amygdules. The abundance of amygdules decreases downward toward the amygdule-poor massive basalt of the middle and lower part of the flow, with another usually much thinner amygdaloidal zone reappearing at the base of the flow (fig. 15). The amygdaloidal zones are less resistant and more easily eroded than the flow interiors and normally do not crop out as well. Because of this, the actual surface of a flow is only rarely observable (fig. 16).

AMYGDALOID with calcite amygdules and calcite vein at top. (Fig. 14)

LAVA FLOWS—cross section showing distribution of amygdules. (Fig. 15)

ANCIENT LAVA FLOW with ropy surface (above) on Isle Royale compared with a "modern" lava flow (below) at Craters of the Moon National Monument. (Fig. 16)

SEDIMENTARY ROCKS

The sedimentary rocks of Isle Royale, sandstone and conglomerate, are easily recognized as being the lithified or consolidated equivalents of sand and gravel. These rocks exhibit a wide range in coarseness from a very fine grained sandstone to conglomerate with boulders 2 feet in diameter. Most of the erosional debris that formed the original deposits of sand and gravel was derived from volcanic rocks with a wide range in composition. A striking rusty-red color is characteristic of all the sedimentary rocks—a color that reflects the presence of a significant quantity of hematite (iron oxide) in the fine groundmass in which the larger fragments are imbedded. The fact that many of the volcanic rock fragments themselves have a reddish tone enhances this coloration.

PYROCLASTIC ROCKS

In contrast to the relatively quiet fissure eruptions of flood basalts, at various times more violent eruptions of rhyolitic lava hurled vast quantities of ash and other volcanic material into the air. The resulting rain of volcanic debris covered extensive areas and formed deposits that because of their fragmental character, superficially resemble sedimentary deposits. Rocks resulting from the consolidation of such volcanic deposits are known as pyroclastic rocks; those formed from very fine material, such as ash, are called tuffs. Pyroclastic rocks are not abundant on Isle Royale but do occur sandwiched between some of the flood basalt flows.