field's proximity to an urban area, little was known about its composition until recent years. Weathering, fine grain size, and glassy content further limit argon–argon dating for the field. Recent argon–argon dating suggests that eruptive activity at the Boring Lava Field began between 2.6 and 2.4 million years ago, yielding far-reaching basalt lava flows, the Highland Butte shield volcano, a number of monogenetic vents, and one andesitic lava flow. These took place near the southern Portland Basin, and were followed by about 750,000 years of quiescence. About 1.6 million years ago, eruptive activity resumed to the north of the previously active area, with alkalic basalt lava flows generating the Mount Scott shield volcano. As eruptions shifted to the east over time, the Larch Mountain volcano was produced by eruptions in the foothills of the Cascade Range. Activity spread out over the area, extending to its current expansive state about one million years ago. In addition to spreading out geographically, the lava composition in the field's vents became more diverse. This period continued until about 500,000 years ago, with no activity until about 350,000 years ago, after which activity continued through roughly 60,000 to 50,000 years ago according to several sources, or about 120,000 years ago according to I. P. Madin (2009). R. Evarts and Fleck originally reported that lava flows at the Barnes Road deposit of the field represented the youngest eruptive products in the Boring area, with a radiometric dating age of 105,000 ± 6,000 years. These eruptions followed a relatively even age distribution over time; younger vents and associated deposits lie in the northern portion of the field, while older deposits are confined to the south. The products of the Boring Lava Field were erupted discontinuously over an erosion surface. Activity took place during the late Tertiary and early Quaternary, in what is now the Portland area as well as the surrounding area, with a particularly concentrated pocket of activity to the east. Nearly all of these eruptions were confined to single vents or small vent complexes, with the exception of a lava plain southeast of present-day Oregon City. Boring Lava generally consists of flowing lava; only one eruptive deposit contains tuff, ash, and tuff breccia, and one vent to the northeast of the Carver area displayed evidence of explosive eruptions that later became effusive. Recent activity and current threats Sometime less than 100,000 years ago, magma at