You’ve likely heard of Pangaea (not the one that sounds similar from Avatar), but you may not realize that it wasn’t the first supercontinent; several have been identified from the rock record. About a billion years ago, a supercontinent named Rodinia formed from the collision of a number of cratons which comprise parts of today’s continents. Evidence of the collisions that built Rodinia remains in a geological remnant called the Grenville mountain range.

Collisions of continents compress the crust between them, driving up a range of mountain peaks. We see a process like this going on today in the Himalayas, where the Indian plate is pushing northward into the Eurasian plate. With time, however, erosion will level out these mountains.

The Appalachian mountain range no longer reaches the impressive heights it once did because it has been eroding for over 400 million years. Deep in the roots of the Appalachians, though, we can see evidence of an even older mountain range that has long-since eroded from sight. The remnants of the Grenville range extend along the East Coast of the United States, but also continuing north into Canada as well as south through Texas and into Mexico.

Finding evidence of these mountain-building events along the edges of two continents is one way to piece together the collisions. Another is to find rock units of the same age and type that split apart as the supercontinent broke up. In addition, paleomagnetic information recorded in the rocks can be used to establish the past latitude of the continents and tie the pieces together. (The inclination of the Earth’s magnetic field relative to the surface changes with latitude.)

In the 1990s, rocks were found in North America and Antarctica that seemed to suggest attachment as part of Rodinia. The rocks were the same age, but it could not be determined whether they were the same unit. Now, a new effort to fingerprint those rocks has shown that the North American and Antarctic rocks are indeed related. This helps resolve the arrangement of the Earth’s continents at that time.

The North American rock in question is the Red Bluff granite suite in the Franklin Mountains of Texas, near El Paso. It’s related to the 1.1 billion year old Keweenawan (also known as the Midcontinent) rift event. Continents break up when volcanic rifts open to split them apart—the East African Rift Valley is a great modern-day example. Sometimes, though, a rift begins to form, but dies out before managing to divide the continent. The Keweenawan is the largest failed rift known.

Starting along the Keweenaw Peninsula of Upper Michigan (from which it gets its name), the rift is responsible for the Lake Superior basin. As it continues to the southwest, the evidence of the rift dives beneath the surface through Minnesota, Iowa, Nebraska, Kansas, Oklahoma, and into Texas. The Red Bluff Granite in Texas formed as part of the rifting.

Switching scenes to Antarctica, the relevant rock is a region of crust known as Coats Land, near the Weddell Sea and just east of the Transantarctic mountains. For an added degree of difficulty (since field work in Antarctica is widely regarded as "too easy"), there are only two accessible outcrops in Coats Land to work with. The rocks there are similar to the Red Bluff Granite in Texas, and formed at the same time. To determine the relationship between them, researchers used lead isotopes to fingerprint them. The ratios of these isotopes vary among different magma bodies, so a match indicates a shared magma source.

The rocks in Texas and Coats Land match perfectly. That simple result confirms a powerful story: Texas and Antarctica may not have much in common these days, but Coats Land is a chip off the North American block. When Rodinia finally did break up, this piece of crust was separated from North America and ended up in the core of what would become Antarctica. You certainly wouldn’t guess this from a map of the planet today, but that’s the gift that geology gives—unique pictures of the impossibly distant past.

Geology, 2011. DOI: 10.1130/G32029.1 (About DOIs).