Observing Prince William Sound's Rocks

excerts of

Chapter 3
Geology of Prince William Sound


(Photographs and illustrations omitted)


Fortunately, there are not a great variety of rock types in Prince William Sound and the novice with a little practice can become fairly adept at recognizing most of the common varieties. Geologists classify rocks into three general categories depending upon their mode of origin. Igneous rock is rock which has been crystallized from a molten magma. If the rock has been extruded beneath the ocean or beneath the atmosphere to cool quickly into a fine crystalline form, it is known as "volcanic" or a "lava." If it intrudes the crust and cools slowly into a coarse crystalline form, it is known as an "intrusive" or as "plutonic" rock. Metamorphic rock is rock that has been recrystallized in a solid state under the influence of heat and/or pressure. Any of the three major rock types may undergo metamorphism. Sedimentary rock is rock formed usually in an aqueous environment from the fragments of other rocks (clasts). The fragments can vary from extremely find (siltstones) to extremely coarse (conglomerates) and are often cemented together by finer fragments known as the matrix. The cementing process occurs under pressure when the sediments are compacted and is known as lithification.

Over long periods of time, each rock group can be transformed by geologic forces into each of the other groups. This process occurring over millions upon millions of years is known as the rock cycle.



Volcanic rocks in Prince William Sound consist of basalts that were extruded onto the ocean floor either at a mid-ocean ridge or through a leaky transform fault. These lavas differ from land-based flood basalts as contact of these heated rocks and magma with the minerals in seawater have chemically altered them. Notably they have become enriched in sodium and somewhat depleted in calcium, magnesium, and silica. The released calcium and magnesium ions can combine with dissolved carbon dioxide in seawater to form into magnesium enriched limestones similar to those found at Ellamar while the released silica is often lithified into chert. Small deposits of limestone and chert and sometimes associated with the basalt flows of the Sound. Secondly, the minerals epidote and chlorite form as a by-product of this reaction giving the lavas of the sound a slightly greenish tint. Regional metamorphism of basalt also results in the creation of epidote and chlorite and may have likewise contributed to the greenish color. For this reason the basalts of the sound are called "greenstones." Greenstones are heavy, dark, mafic rocks usually exhibiting fine crystalline structure caused by rapid cooling of lava on contact with cold water. In prince William Sound greenstones assume three distinct forms: pillow basalts, sheeted dikes, and massive flows.



Pillow basalts form when magma derived from the earth's mantle spills out onto an inclined surface of the sea floor. Upon contact with the cold water, a glassy surface layer forms around a molten core which still continues to flow. The confining pressure of the surrounding water molds this plastic mass into a roughly spherical shape as the lava oozes forth like toothpaste. Sometimes the molten core of an elongate pillow will burst the surface forming subsidiary multi-lobed pillows.

Because of its molten core, the basalt is still somewhat plastic when it comes to rest; the bottoms of the pillows readily conform to the underlying surface. Pillows flowing over older, chilled pillow often overlap two or more of these. When this happens, gravity tugs at overlying plastic mass causing it to fill the roughly triangular spaces between pillows. This "tail" (or "keel") points to the bottom of the bed. When the lava flows out onto a relatively flat seafloor covered with sediments, the bottom of the pillows are flattened indicating the bottom of the bed. Pillows are often elongated in the direction of flow. Sometimes, the contact between hot magma and cold water is so violent that the surface of the pillow explodes producing a burst of glassy shards which forms a glassy breccia filling the interfaces between the pillows. Under some circumstances, one finds whole flows where entire pillows have been shattered by this violent interaction. Geologists refer to these shattered pillows as "pillow breccia."

One can often find pillows cracked open so that their interiors are exposed. Here a glassy surface gives way to a fine, crystalline interior. Usually, radial cracks resulting from contraction on cooling are visible. Sometimes, one can observe rings of tiny pockets filled with a white or greenish mineral; these represent the former sites of tiny gas bubbles called "vesicles."



Sheeted dikes represent the feeder vents that supplied the extruded pillow basalts. These most likely had as a source a shallow magma chamber near an oceanic spreading center. Repeated intrusion of older dikes and pillows by progressively younger dikes creates sheets of vertical intrusions resembling a deck of cards standing on end. On central Knight Island and on Glacier Island the cliffs of sheeted dikes exhibit distinctive, almost columnar, vertical structures. Seawater seeping down through cracks in the seafloor have altered these basalts to greenstones similar to those of the overlying pillows. The alteration to greenstones may also be due to regional metamorphism during uplift.



Relatively featureless masses of basalt which lack pillow or vertical dike forms are referred to as "massive flows." These may represent large, lakelike extrusions of basalt onto a flat ocean floor where there was insufficient flow for pillows to form, or they may represent intruded sills.

The greenstone lavas of Prince William Sound are found overlying slate and graywacke beds (Galena Bay), lying beneath conglomerate beds (Galena Bay), and intruded into slate and graywacke beds (Evans Island, Knight Island). The interlayering of greenstone lavas and turbidites imply that these lavas erupted repeatedly and were repeatedly covered by turbidity flows or that they intruded preexisting trench-fill sediments.

This association of greenstone flows with trench-fill turbidites in Prince William Sound suggests that these rocks were formed by a spreading center interacting with a subduction trench.

(Chapter 3. pp. 86-91).



Sedimentary rocks are formed by the consolidation of sediments which have in most cases settled from an aqueous medium. Geologists classify sedimentary rocks according to their grain size and mineralogy. Fine-grained rocks with grain sizes of less than .0025 inches are called "mudstones." Rocks with a grain size of between .0025 and .08 inches are referred to as "sandstones" while rocks which contain grains ('clasts") greater than .08 inches are called "conglomerates." Conglomerate clasts may vary from granule to pebble size and even to boulder size.

The minerals composing mudstones, sandstones and conglomerates of Prince William Sound indicate that they were derived from the erosion of a volcanic arc eroded to its plutonic roots. Some geologists believe that the island arc terrane of the Alaska Peninsula may be the source of these sediments (Dumoulin, 1987). In the case of the orca rocks, some geologists indicate that the Valdez rocks could be a likely source (Gergen and Plafker, 1988).



Most sedimentary rocks in Prince William Sound are "turbidites" (sometimes referred to as "flysch"). Indications are that the original sediments were deposited by turbidity currents in a subduction trench (probably the Border Ranges Trench) roughly between 80 and 50 million years ago. Turbidity currents, often referred to as 'density currents" or "gravity flows," occur when great quantities of water-logged sediments perched on a steep submarine slope begin to slump and slide. In a seismically active trench environment, earthquakes are often thought to initiate the flow. As the liquefied sediments gain momentum, turbulence develops violently mixing grains of mud, sand and gravel. The swirling vortices in these currents are often energetic enough to suspend small boulders in the flow. The density of this flow then is much greater than that of the surrounding seawater, hence currents may develop enough momentum to carry the suspended sediments hundreds of miles offshore. The swirling, suspended sediments give these bottom-hugging currents great erosive power. It may be that the submarine canyons which dissect the inner trench walls have been scoured and enlarged by turbidity flows. As these fast-moving currents spread out onto the deep trench floor, they begin to lose some of their momentum and turbulence and the coarser boulders, gravels and sands begin to drop out, followed by less coarse sands, and finally be fine muddy deposits. Under these conditions, a graded bed often results with the coarser conglomerates lying at the bottom, grading upward to finer and finer sandstones and finally into a slate or shale mudstone layer.

In a normally graded bed, such as that depicted above, one can often distinguish the bottom of the bed from the top even though the bed has been severely tilted. First, the conglomerate tends to identify the bottom of the bed. Secondly, of one closely examines the contact between the two mudstone layers and the sandstone layers (assuming the conglomerate is absent as it often is), he will sometimes note a sharp contact in one instance and a graded contact in the other. The sharp contact represents the bottom of the bed where the heavier sand settled suddenly out of the current on top of the upper muddy layer of the previous turbidity flow. Because the finer sand, silt and clay settled out of suspension (perhaps over a period of a week or so) the upper mudstone layers are finely graded. The sandstones of these beds are of a dirty, poorly sorted type called a "graywacke." The mixing in the turbulent vortices of the turbidity current probably accounts for the presence of the fine-grained muddy, volcanic particles mixed in with the sand. . . .

(Chapter 3. pp. 102-103).