Even though lots of people have written about this, I figure one more example that illustrates the methodology won’t hurt. Forty years ago, I worked on some very old rocks on Belcher Islands, Hudson Bay, that included volcanic deposits. Looking at the photos (35mm slides), I still marvel at the geology, the fact that something almost 2 billion years old is so well preserved, makes it look like the volcano just erupted.
Here are three ancient structures that were constructed by flowing basalt lava. Each can be compared with modern volcanic structures and processes that we can observe directly. We can interpret the ancient structures according to the similarities and differences between the modern analogues and the ancient versions. The examples are from strata known as the Flaherty Formation, a succession of volcanic rocks exposed on Belcher Islands, Hudson Bay.
Ropy lava: Slow-moving (pahoehoe) lava flows begin almost immediately to cool from the top down. As the crust develops, it forms curved, sinuous, and twisted ribs, initially near the front of the flow, gradually extending back up the flow. There is a resemblance to braided rope, hence the name. Crusty ribs become twisted and arcuate because flowing lava beneath the crust acts as a drag. This video link shows this process very nicely.
Pillow lavas: Lava extruded under water (seas, lakes) commonly forms bulbous masses called, fittingly, pillow lavas. Flowing lava cools very rapidly in water, forming a crust that expands as the molten rock continues to flow. Eventually the crust is breached, and the process begins anew. In this way flowing lava can form great piles of spherical, to tubular globules. Some pillows break off the pile and tumble down-slope, but many are interconnected. The process of pillow formation is nicely illustrated in this example from Hawaii.
Pillowed basaltic lavas are most common in oceanic crust, where they are produced at mid-ocean spreading ridges, and at volcanic hot-spots like Hawaii, Canary Islands, and other oceanic islands.
The close-up view (below) shows details that help add more clarity to our interpretation. Pillow margins, like rinds, have a different colour and, although not visible in these views, the minerals tend to be much smaller than the pillow interior; the margins are also glassy. These attributes, that we can witness in modern analogues, develop during rapid chilling of the original lava; they are referred to as chilled margins, or selvages.
Lava tubes: During an eruption, the most obvious exuded magma flows over the volcano surface. However, flow can also occur through conduits, or tubes beneath the surface; the tubes can be several metres wide, and 10s to 100s of metres long. Some of the recent (2017) lava flows on the coast of Hawaii Island have been fed by lava tubes (see image at top of page). When an eruption ceases, lava in the tube may solidify, or the lava may drain from the tube leaving an open conduit, or the tube may collapse.
Afterword: Although geologists have formalized the use of analogy (al la Archibald Geikie) as a tool to further our understanding of the world (universe), it has been an important part of the science armoury for centuries. Leonardo da Vinci used the analogy of modern seashells to explain the fossil equivalents recovered from mountain tops. Seriously famous icons like James Hutton, Charles Lyell, and Charles Darwin, all used analogy to help explain geological phenomena, long before Geikie’s pronouncement. Perhaps analogy is also an expression of common sense. Either way, we’d be lost without it.
About 100 images of modern and ancient volcanic-volcaniclastic rocks are included in the Atlas of sediments and sedimentary structures: Volcanoes and the products of volcanic eruptions