A lava flow that erupts into an actively subsiding basin in which active sedimentation is on-going generally will be covered with younger sediments and become part of the sedimentary record when the eruption ends. Magma (molten rock which is only called lava when it erupts onto the surface), which forms tens of kilometers below the surface of the earth, must intrude into and through the overlying rock layers to reach the surface and erupt. The conduit through which magma travels is generally a fracture or fissure that necessarily cuts across the layers of the pre-existing rock. Upon cooling the magma remaining in the fracture solidifies to rock and forms a dike. In many places, the fracture breaks parallel to (concordant with) the rock layers. The magma that intrudes into such a concordant fracture forms a sill when it cools and solidifies to rock.
Early geological explorers of the Mesozoic Hartford Basin recognized layers of basalt (trap rock) that were, in most places, concordant with layers of sedimentary rocks (i.e. did not cut across the sedimentary rock layers). At several notable places, however, the contacts were discordant. Because the discordant bodies were obviously intrusive, there was some controversy about the nature of the concordant layers. In the New Haven area, the trap-rock ridges were intrusive, but what about the trap rock ridges in the rest of Connecticut? Were they also intrusive, that is sills, as some of the early geologists claimed, or were they lava flows1? This EarthCache explores one of the concordant layers, the Hampden Basalt, at outcrops near Trinity College in Hartford and makes observations to answer the sill vs. lava flow question.
Location: N.41.75094o, -072.69361o. Find your way to Zion Street in Hartford and park near the intersection of Glendale and Zion Street. Walk around the rock outcrops at the base of the ridge on the east side of Zion Street where our observations for this EarthCache will begin. Be careful because slopes are steep and footing is difficult. In places, broken glass and other hazards litter the ground surface. Poison ivy grows on some of the rocks. Eventually you will make your way to the corner of Vernon and Summit Streets (N. 41.75111o, -072.69301o), which is adjacent to Trinity College (public parking there is restricted and in short supply, so it may be better to walk rather than drive).
Background. How can a geologist determine whether a layer of igneous rock is a lava flow or a sill? Knowing the answer is critical to understanding the geologic history of the site2. In part, geologists rely on The Principle of Uniformitarianism, which simply states that physical processes we observe today operated in the geologic past as well and produced similar products at similar rates. Thus, we can use particular types of layering in modern day beach sand deposits to interpret that the same type of layering in an ancient sandstone formed on an ancient beach by the same physical processes. Likewise, we interpret that, particular types of cross-bedding found in ancient sandstones that are the same as cross-bedding found in modern river bars likely formed as ancient river deposits. Finally, we can conclude that concordant basalt layers that contain structures similar to modern day basalt lava flows likely were formed as lava-flows themselves. In short, the present is the key to the past. We will use this type of reasoning, along with inference, to distinguish between the lava-flow vs. sill interpretation for the layer of basalt at Trinity College. But first, let’s make some observations.
Observations and Interpretations. Begin your observation on a small grassy area at the bottom of the ridge at the inter-section of Zion and Glendale streets. The ridge is held up by the Hampden Basalt that in most places forms a 15-30 ft. (5-10 m) high cliff. Most of the ridge is covered by numerous trees and bushes (including poison ivy!) that hide the cliff and rock outcrops. About 10 yards north of the intersection there is a portion of the ridge that, instead of a cliff, is a steep soil covered slope on which abundant young trees and bushes have taken root. In times past a stairway at this location connected the sidewalks of Zion Street with those of Summit Street. Here the outcrops can be viewed. The Hampden Basalt forms the major part of the cliff. The East Berlin Formation forms the base of the cliff (Figure 1). The basalt weathers to a tea-brown patina, but on a freshly broken surface it is dark gray.
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Figure 1. Contact between Hampden Basalt and underlying East Berlin Formation (arrow). Note that the East Berlin Formation, which is normally reddish-gray to reddish-brown, is black. The black color gives way to reddish-brown about a foot below the contact. The heat provided by the molten basalt caused a change in the oxidation state of the iron in the East Berlin minerals which resulted in the black color. It’s a form of contact metamorphism. The hammer handle is 14” (~35 cm.) long for scale.
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It consists of small crystalline grains of plagioclase feldspar and pyroxene. It is fine-grained throughout most of the flow, but is very fine-grained near its contact with the underlying East Berlin Formation. This is the chilled base caused by more rapid crystallization adjacent to the relatively cold surface of the East Berlin. The middle of the Hampden cooled more slowly and hence is slightly coarser grained.
The underlying East Berlin Formation is typically reddish gray and reddish brown, layered siltstone, shale, and very fine grained sandstone. However, immediately adjacent to the contact with the overlying Hampden the rock is black and massive, with local pockets of calcite- filled steam-vesicles3 (hint: this suggests that the sediments were not lithified during the igneous activity). The underlying shaley sediments have been baked and metamorphosed adjacent to the hot (~1150-1200o C) basalt. They are now black hornfels.
Careful observation of the base of the basalt will reveal two different kinds of vesicles, both that are elongate in the vertical or near-vertical dimension. Pipe-stem vesicles (Figure 2a. and b.) are formed at the contact with underlying sediment. Each vesicle is a single tube that is about a cm in diameter but several cm long. Presumably they were formed by steam escaping from the underlying sediment. As such they should have bubbled straight upward through the basalt. These tubes, however, are tilted about 30o or more from vertical. Part of the tilt can be accounted for by the tilting (dip) of the sedimentary layers (10-15o) toward the east (see Figure 2c). The vesicles are interpreted to have formed while the magma or lava was still moving and hence would be dragged in the direction of flow (Gray, 1982; Ellefson and Rydel, 1985) thus producing the tilt. These are tilted toward the northeast, suggesting that the source of the magma/ lava lay to the southwest.
Cylindrical vesicles(Figure 2c. and d.) are about 5 cm in diameter and 1-2 m in length. Steam escaping from the underlying sediments likely formed these vesicle trains as they bubbled up through the congealing basalt. The cylindrical vesicles are not tilted, suggesting that they formed after the molten rock stopped moving.
PIPE-STEM VESICLES: a. Pipe-stem vesicles (arrows) near base of basalt. Contact with underlying sedimentary rock is just below the compass ( 4” long, ~10 cm). Sedimentary rock here was a mudstone, which has been metamorphosed to black hornfels. b. Pen (arrow) has been inserted into one of the pipe-stem vesicles to demonstrate the tilt, which here is ~30o from vertical. Cliff face is oriented approximately north (left)-south (right), so the vesicle tilts toward the west-northwest. Pen is 5.75” (15 cm) long. c. and d. VESICLE CYLINDERS: c. Longitudinal section through a vesicle cylinder that is about 2 m above the base of the flow. Index finger ~3” (9 cm) long. Note cylinder is tilted about 10o from vertical, which corresponds to the easterly dip (tilt) of the sedimentary layers. Right image is cross section through a vesicle cylinder. Scale bar ~2” (~5 cm).
Vesicles and amygdales (mineral filled vesicles) are more typically found near the tops of lava flows. They are formed by gas bubbles (commonly carbon-dioxide and/or steam) that float upward because of their buoyancy, while the lava is molten. Carefully make your way up the steep slope to the top of the cliff and cross the street. (Alternatively, you may drive around to Summit Street and try to find parking near the intersection of Summit and Vernon streets.) Vesicles may be seen in the outcrop on the corner of Summit and Vernon streets. They are more abundant in what looks to be a church yard (private property; actually it is a fraternity house) just to the north of the intersection (Figure 3).
Figure 3. Vesicles and amygdales in Hampden Basalt. a. Outcrop on fraternity property just north of Vernon Street. It appears vesicular even from the road. b. Detail of to of outcrop in yard of fraternity house. Spherical vesicles are 5-10 mm in diameter. Pen is 5.75” (15 cm) long. c. Vesicles from outcrop in Rock Ridge Park, south of Trinity College. Much of the vesicular rock has been removed by erosion, nonetheless it can be seen that vesicles are more abundant in places. The rock is locally scoriacious. (Scoria is a vighly vesicular basaltic rock.) Hammer head approximately 7” long (~18 cm). d. Amygdales in a core sample of Hampden Basalt about a mile north of Trinity College (scale in centimeters). Gas bubbles that form when lava is mostly molten will be roughly spherical. But gas bubbles that form after lava begins to solify will have irregular shapes.
The outcrop on the corner (Figure 4) stratigraphically overlies the vesicular basalt in the fraternity yard and is even closer to the top of the basalt; perhaps is the very top. This outcrop consists of a blocky conglomeration of basalt fragments with vesicular basalt between large massive fragments. Many of the vesicles contain calcite linings. The basalt fragments are angular and hence the rock is a breccia.
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Figure 4. Breccia at top of Hampden Basalt. a. Outcrop st the corner of Vernon and Summit Streets on the northwestern part of the Trinity College campus. Contrast the blocky nature of the basalt here with the massive nature of the basalt elsewhere (see Figure 1 for instance). Blocks or slabs as large as 2 feet (70 cm) may be seen. Hammer handle in a. and b. is 14” (~35 cm) for scale. b., c. Most of the blocks are massive basalt and are surrounded by a fine-grained groundmass that is vesicular. Block in c. is an exception in that it contains vesicles. Disc on key chain is 2” (~5 cm) for scale.
Why would the basalt layer look different at the top of the layer than it did at the bottom? If the basalt layer was a sill and intruded between layers of sandstone/siltstone, there would be no cause for it to be different at the top than at the basalt. The basalt would be massive throughout with chilled margins at both the upper lower contacts. In addition it would have baked the overlying sedimentary rock (definitive evidence for a sill) just as it baked the underlying sediments. The overlying rocks, however, are not exposed here. The surfaces of modern lava-flows are scoriaceous and characteristically rough; many consist of broken up slabs and fragments of cooled lava-rock. It is likely the outcrop we are observing is a flow-top breccia of some sort, but its exact mode for formation is conjectural. Gray (1982, p.186) suggested it could be a “local feature produced by lava squeezed out of the underlying sheet when parts of it collapsed, or …a separate younger flow.”
The breccia may have wider distribution than seen on outcrops. About a half mile north of Trinity College a series of boreholes were drilled to obtain samples for petrophysical testing prior to boring one of the Park River diversion tunnels. Two of those boreholes recovered cores through the top of the Hampden Basalt (Figure 5). One contains a flow top breccia similar to that seen here at Trinity College. The other borehole is located on the up-thrown side of a fault and the top of the basalt has been removed by erosion prior to deposition of the overlying sandstone (Hoffman and others, 1994). The sandstones overlying the Hampden Basalt show no evidence of contact metamorphism in either borehole.
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Figure 5. Illustrations are of slabs cut from samples of a cored borehole (PRFD-24T) drilled north of Trinity College. a. Flow-top breccia at top of Hampden Basalt. Note that the fragments contain amygdales. In both a. and b. the width of samples ~4” ~10 cm). b. Pebble and small cobble-sized fragments of basalt in overlying sandstone. Fragments are sub-rounded and contain amygdales. Dip of layers of Portland Formation is about 20o. Basalt presumably was eroded from up-thrown faulted block just to east of borehole location soon after eruption of basalt. A borehole just east of fault contains no flow-top breccia at the top of the Hampden Basalt (see Hoffman and others, 1994). Presumably it was eroded from that location prior to deposition of Portland Formation. Note that the overlying sandstone is not metamorphosed.
The above observations ad inferences are summarized in Figure 6 and Table 1 to help the EarthCacher interpret the outcrops and answer the accreditation question that follows.
Figure 6. A comparison of (a) basalt sill characteristics and (b) basalt flow characteristics. Orange represents cooled magma/lava.
TABLE 1. Comparative features of basalt sills and lava flows.
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Sill
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Basal contact
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Concordant; underlying rock“baked” by heat of igneous rock (contact metamorphism). Basalt at contact finer grained (chilled margin).
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Concordant or discordant; underlying rock “baked” by heat of igneous rock (contact metamorphism). Pipe-stem vesicles and vesicle cylinders at base of basalt formed by escaping steam generated in underlying sediment. Pillows may be present in basalt.
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Upper contact
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Concordant; overlying rock “baked” by heat of igneous rock (contact metamorphism). Basalt with a chilled margin. No eroded fragments of basalt in overlying sediment. No vesicles/amygdales in basalt. Possible small dikes of basalt intruded into overlying rock. Possible fragments of sediment included in basalt.
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Concordant; overlying rock unaffected (i.e. not “baked”). Overlying sediment may contain eroded fragments of basalt lava. Upper part of basalt may be brecciated and contain vesicles and/or amygdales. Upper part of basalt may have weathering horizon and/or soil.
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Footnotes
1. The interested cacher may consult McDonald, 1996, p. 17-20, for a concise history of the debate.
2. For instance, if the igneous layer is a lava flow it is essentially contemporaneous with sedimentation and we say it is synsedimentary. A radiometric date for the lava flow dates sedimentation and provides an age for the sedimentary rocks. If however, the layer is a sill, it post-dates the rocks it intruded and a radiometric date for the sill tells you only the age of the igneous activity, and, of course, that the rocks into which it intruded were around at that particular time. It tells you only that the age of the country rock is older.
3. A vesicle is a hole in the rock that formed when molten rock solidified around a gas bubble. Thus, strictly speaking, the term vesicle is an igneous rock term. The term is used here because muddy sediment (unlithified) has some rheological similarities to molten rock and because the steam that formed the vesicles was generated by an igneous event.
References cited.
Ellefson, K. J. and P. L. Rydel, 1985, Flow directions of the Hampden basalt in the Hartford
Basin, Connecticut and Massachusetts: Northeastern Geology, v. 7:33-36.
Gray, N.H., 1982, Mesozoic volcanism in north-central Connecticut,
in. R. Joesten and S.S.
Quarrier eds., Guidebook for Fieldtrips in Connecticut and South-central Massachusetts; New England Intercoll. Geol. Conf., 74th Ann. Mtg., Trip M-3, p.173-193.
Hoffman, D.L., Rondeau-Hodgson, C.R., Williams, C., and Steinen, R.P., 1994, Flow-top
erosion of the Hampden Basalt during syndepositional seismic activity on normal faults in the Mesozoic Basin of CT: Geol. Soc. America, Abstracts with Program, v.26, no. 3, p. 23.
McDonald, N.G., 1996, The Connecticut Valley in the Age of the Dinosaurs: A Guide to the
Geologic Literature, 1681-1995. Connecticut Geol. Nat’l. Hist. Surv. Bull. 116, 241p.
How to get credit for this EarthCache: Answer the following question.
The modern day interpretation of the basalt at Trinity College is that it formed as a lava flow. Cite at least three lines of evidence to support of that view. If you disagree with that interpretation, cite three lines of evidence to support your preferred interpretation.
Difficulty Rating: 1
Terrain rating: 3, if you walk up the steep slope. You could drive your automobile around to the corner of Vernon and Summit Street. If you do that the terrain rating is only 1.
This EarthCache is a product of the Connecticut Geological and Natural History Survey and the Connecticut Department of Energy and Environmental Protection written by Randolph Steinen, Margaret Thomas, and Lindsey Belliveau.