The Avon Volcanic District – Missouri’s Hidden Explosion Pipes
In the rolling hills of southeastern Missouri, a short drive south of St. Louis, more than eighty small explosion pipes and dikes cut through ancient sedimentary rocks. These are the Avon Volcanic District (also called the Avon Magmatic District), a little-known suite of Devonian-age ultramafic diatremes centered near the town of Avon in Ste. Genevieve County. Unlike the famous diamond-bearing lamproites of nearby Arkansas, these Missouri pipes have never produced diamonds. They remain, however, a rare window into the deep mantle beneath the mid-continent and one of the more unusual geological features in the Ozarks.
Discovery and Early Work
The unusual rocks were first examined in detail by geologist G.W. Rust, who published his preliminary notes on explosive volcanism in southeastern Missouri in the Journal of Geology in 1937. Rust recognized the pipes as diatremes—narrow, gas-driven explosion conduits filled with fragmented rock, lapilli, and magma. Later field work expanded the known number of outcrops to more than eighty within an area of roughly 75–200 square miles. Most exposures are small and poorly preserved, often revealed only in road cuts, stream banks, or farm fields.
What the Rocks Look Like
The Avon pipes are composed of dark, ultramafic rocks rich in olivine (frequently altered to serpentine), clinopyroxene, melilite, phlogopite, and perovskite. Many contain primary carbonatite—carbonate-rich magma that formed at depth. The rocks grade from olivine melilitite to alnöite to carbonatite, sometimes within a single outcrop. Pelletal lapilli and abundant xenoliths of the surrounding Cambrian sedimentary rocks (and occasional basement fragments) are common. The overall appearance is a heterogeneous, often weathered breccia rather than a clean volcanic flow.
How the Pipes Formed
These are classic diatremes. Gas-rich magma rose from the upper mantle and blasted upward through the overlying sedimentary layers, creating a narrow vertical pipe. The explosion fragmented both the magma and the country rock, leaving a filled conduit of lapilli, ash, and broken blocks. At the current erosion level we see the deeper parts of the pipes (the root zone and lower diatreme). K-Ar dating and fossil-bearing xenoliths place the activity in the Middle to Late Devonian, roughly 377–390 million years ago—long before the Cretaceous lamproites of Arkansas formed.
Location and Setting
The district sits near the southeastern end of the Farmington Anticline, between the Ste. Genevieve Fault Zone and the parallel Simms Mountain Fault Zone. The pipes intrude Cambrian sedimentary rocks (Lamotte Sandstone, Bonneterre Dolomite, and overlying units) that rest on Proterozoic basement. The area is part of a broader mid-continent alkaline province that includes other Devonian ultramafic occurrences farther north and east.
Why No Diamonds?
Despite the mantle origin and the presence of olivine and other deep-seated minerals, no diamonds have ever been confirmed from the Avon pipes. The magmas are melilitite–alnöite–carbonatite rather than the diamond-friendly kimberlite or lamproite compositions found in Arkansas. The Missouri Geological Survey and subsequent researchers have consistently reported that native diamonds are unknown from Missouri. Any historical “Missouri diamond” claims remain unconfirmed and are almost certainly unrelated to these pipes.
Why It Matters
The Avon Volcanic District is one of the better-preserved examples of Devonian ultramafic magmatism in the North American mid-continent. The pipes carry mantle xenoliths and provide information about the lithosphere beneath Missouri at a time when the continent was relatively stable. For rockhounds and students of geology, the district offers a chance to examine rare rock types (olivine melilitite, alnöite, carbonatite) that are seldom seen outside specialized collections. Most outcrops are on private land or poorly exposed, so access is limited, but the story of their formation is a clear and compelling example of explosive, gas-driven volcanism far from any plate boundary.
Further Reading
- Rust, G.W. (1937). Preliminary notes on explosive volcanism in southeastern Missouri. Journal of Geology, 45, 48–75.
- Modern petrographic and geochemical studies of the Avon Magmatic District (Bridges, Shavers, and others).
