(as edited by Elaine Kennedy, Geoscience Research Institute)
Geoscience Reports 28:1-3 (Fall 1999).
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Introduction
Flood basalts are lava flows that arise from fissures and spread laterally over a very large region. The Columbia River Basalts are one such example. At one time the Columbia River Basalt Group1 extended across southern Washington, northern Oregon and west-central Idaho, blanketing a total of 163,700±5,000 sq. km., with an estimated volume of 174,300±31,000 cu. km (see Fig. 1).2 Some of the lava flows extend from the vents in Idaho to the Pacific Ocean.3 These flows contain structures that give geologists clues about the environment in which they were deposited.
Figure 1. Extent of the Columbia River Basalt Group. The circle marked "F" identifies the region of the fissures supplying the source material for the flood basalts. (Modified from Reidel and Tolan 1992)
This article will briefly explain the variety of conditions that influence the structures seen in flood basalt flows: those that cool subaerially (exposed to air), and those that cool subaqueously (exposed to water). Three of the latter scenarios are described: 1) cooling structures that form when a lava flow blocks a river, 2) interaction of a lava flow with ocean tides, and 3) flows covered for an extended period of time by large amounts of water.
Subaerial Structures in Basalt
Examination of a cross-sectional block from a lava flow reveals a complex set of features. Figure 2 illustrates this point. Lava flows cool from both the top and the bottom. As they cool, unique, laterally extensive, structural features form that are indicative of the cooling process. The five units (beds) in this diagram represent three basic kinds of structures that occur in a single subaerial basalt or lava flow. Unit A (Fig. 2) contains gas bubbles, some of which are elongated and suggest the direction in which the lava was flowing. Unit B has vertical columns. As the molten lava cools, the columns form along parallel fractures.4 Unit C, representing the interior of the flow, has an irregular, blocky appearance. This region represents the mixed cooling rates that have extended from the top and the bottom inward. Cooling that has extended up from the base produces small gas bubbles that are seen in Unit E. Overlying Unit E is the columnar Unit D. In Figure 2 the flow has been interpreted as cooling in a subaerial environment; i.e., exposed to the air while it cooled. There was no water to affect the cooling rate. Researchers studying cooling times on recent lava flows in Hawaii report that any lava flow 40 m (120') thick would require approximately 14 years to cool subaerially.5 Underwater the flow could solidify in as little as five years.
Figure 2. Cross-section of basalt flow enlarged on right. See text for details.
Subaqueous Cooling: River
Figure 3 illustrates the cooling of a single flow that blocked a river or stream. Unit A contains gas bubbles. Unit B has slightly irregular columns rather than the sharp vertical columns of Figure 2. Unit C, called the entablature, has the appearance of distorted (e.g., splayed or jumbled) columns. The entablature unit cools as water invades the area through the overlying columnar fracture system. Unit D is the columnar bed that has cooled from the base upward. Unit E consists of pillow basalt formed as the flow came into direct contact with the water (see Fig. 4).
Figure 3. Variable cooling due to the presence of water. See text for details.
Figure 4. Pillow basalt (A) overlain by heavily eroded columns (B). (Photo courtesy of Jim Gibson)
Even though there appear to be separate layers in the
outcrops in Figure 4, the contacts in this case are continuous, suggesting that
these multiple layers (i.e., the entire outcrop) represent a single flow.
Such complex structures make it difficult to distinguish
separate flows from separate beds or units within a single flow when the contact
is covered because the crucial criteria for determining the number of flows at a
site are found at the contact between the units. In addition, some contacts may
be difficult to access as the result of the shear face along the basalt cliffs.
At this locality (Fig. 4), windblown sand from the nearby Columbia River has
been trapped on the lee side of a large outcrop from the same flow. It is
possible to examine the contacts at this locality and determine their continuous
nature, verifying that the units constitute a single flow. Without access to the
contacts any speculations in regard to the number of flows is virtually
meaningless. Figure 5 illustrates the issue of complexity more clearly.
Figure 5. Repetitive columns and entablature structures in a single flow.
Subaqueous Cooling: Tidal
The intermittent flow of tides can produce repetitive beds within a single flow. In Figure 5 Units A and G contain gas bubbles which apparently help define the upper and lower contacts; however, under certain conditions these small vesicles can also occur in the middle of a flow.6 It is important to note that the gas vesicles cannot be used as definitive criteria to mark the upper and lower boundaries of any flow. Units B, D and F contain columns that have formed as the cooling process or "solidification front" passed through the flow from the upper and lower boundaries. Units C and E represent contact with water that has moved into the flow through the fracture system that has been forming as the tide rose, fell and rose again. Figure 6 shows lateral breaks in the units that probably represent contacts between solidification fronts during the cooling process.
Figure 6. Lateral fractures (at arrows) related to cooling process. (Photo courtesy of Jim Gibson)
Many people look at the lateral breaks across the basalt in Figure 6 and think that each level represents a new basalt flow. However, closer examination of the contacts show that these fractures are associated with the cooling process in one very large flow. As the flow cools from the upper and lower boundaries, temperatures may be quite variable. This variability is primarily due to the amount of water passing through the developing fractures resulting in the formation of blocky and irregular structures within the basalt. Pseudo-columns may form also as intermittent, subaerial exposure occurs. As the body of molten lava solidifies, cooling temperatures may stabilize for a period of time, and it is along this "line" that the horizontal fractures occur with associated gas vesicles. Lateral disruption may also occur when an upper solidification front encounters a solidification front rising from below. Gas bubbles often occur above and below these "breaks" or regions of temperature change and variability.
Subaqueous Cooling: Large Influx of Water
Some flows consist entirely of entablature structure. In Figure 7 Units A and C contain gas vesicles. Entablature structure (Unit B) dominates the basalt flow when the lava is inundated by a large body of water. A continuous supply of a large amount of water filling the fracture systems speeds the cooling process. The cooling is so rapid that the entire flow is cooled from the top down such that the columnar structure does not form near the base.
Figure 7. Cooling in a large body of water.
Comments
Structures within the flows of the Columbia River Basalt Group provide clues to the cooling processes that solidified the lavas. Subaerial, as well as a variety of subaqueous conditions, can be described from the structures, textures and contacts within each flood basalt flow. Most of the flows in this region exhibit structures consistent with subaqueous cooling mechanisms. Research on the stratigraphic relationships of these characteristic flow features might provide information regarding flooding and flood stages during the eruption and cooling of the Columbia River Basalt Group.
ENDNOTES
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