Our world's continents have been compared to both icebergs and onions. They are like icebergs in that they consist of a buoyant solid floating on a sea of deeper denser material, and also in that they are mostly unseen, with keels extending down to 250 km (150 mi) beneath the Earth's surface. They are like onions in that they consist of a central core surrounded by a sequence of younger layers that has grown around it. The process by which continents were formed remains a geologic mystery. Two of the keys to that mystery lie in the structure of the deepest part of the central core (or "craton", to use the geologic term) and of the boundaries between the layers ("sutures" or "terrain boundaries"). Earthscope's Transportable Array (TA) is a tremendous asset to seismologists studying Earth structure, because it provides extremely high-quality data for the entire land area of the US, a region that includes many important geological features, including plate boundaries, rift zones and hot spots. However, its impact on the "origin of the continents" question is limited, because the core of the continent is not beneath its footprint, but rather lies a little to the north, in Quebec. Our project, timed to take place when the TA is operating in eastern US, is synergistic and provides a companion data set specifically targeted at the cratonic core and surrounding terrain boundaries. We are deploying seismometers in a line stretching from coastal Maine (the edge of the continent) to northern Quebec (its core). The density of stations is variable, and is highest in the three places where terrain boundaries are crossed. We use earth-imaging techniques to test three significant hypotheses: 1) that the bottom of the continental keel is a distinct interface that deepens towards the cratonic core; 2) that the properties of the continent are regionalized (that is, each layer of the onion has its own recognizable properties), with terrain boundaries having a signature that extends right down to the bottom; and 3) that the part of the Earth below the bottom of the continents (the "asthenosphere") is flowing around the keel (in the same sense that ocean water flows around the keel of an iceberg). The project is supporting two graduate students, providing them with a venue to develop their professional skills as both researcher and teachers. Seismometer operations and data analysis involves undergraduate research assistants, in both summer internship and academic-semester employment modes. During the summer, educational features are incorporated into the fieldtrips to better introduce these undergraduates to the science underpinning the project and more generally to prepare them for a career in science. These include daily science discussions, stops to view important geological features and visits with scientists from collaborating institutions.
