Diverse species of flora and fauna inhabit the area, which is subject to frequent snowfall, occasional rain, and extreme temperature variation between seasons.

[20] Constructed towards the end of the Pleistocene epoch, these mountains are underlain by more ancient volcanoes that sank within parallel north–south trending faults in the surrounding region.

[21] The Three Sisters form the centerpiece of a region of closely grouped volcanic peaks, an exception to the typical 40-to-60-mile (64 to 97 km) spacing between volcanoes elsewhere in the Cascades.

[23] The 193 square miles (500 km2) area from the Three Sisters to Broken Top and Mount Bachelor features at least 50 eruptive vents for rhyolitic and rhyodacitic lava.

[23] East of Broken Top, the c, a 115.8 square miles (300 km2) area of andesitic and mafic scoria cones, features similarly rhyolitic and rhyodacitic lava deposits.

[6] The scoria, dike rocks, and lava that comprised this cone had a uniform composition made up of phenocrysts with plagioclase, olivine, two types of pyroxene, and magnetite.

[34] When Little Ice Age glaciers retreated during the 20th century, water filled in the spaces left behind, forming moraine-dammed lakes, which are more common in the Three Sisters Wilderness than anywhere else in the contiguous United States.

Analysis of the event by forest ranger David Rasmussen determined that it had originated from a lake on Broken Top, located at an elevation of 8,000 feet (2,400 m) with an area of 11 acres (0.045 km2).

[31] Cayuse Crater, an unrelated postglacial volcano which formed 11,000 years ago,[26] also sits on Broken Top's southern flank.

[39] It is made up of basaltic lava and scoria that erupted after Broken Top ceased activity,[25] suggesting that it formed due to an underlying magma chamber closer to South Sister.

[39] More recent Holocene activity on the volcano's flanks constructed basaltic lava flows, cones, and ash that have become interbedded with moraine and material from outwash plains from the Neoglacial period.

The remnants of a large scoria cone near Bend Glacier, for the most part destroyed by ice, include volcanic bombs that extend up to 8 feet (2.4 m) in length.

[28] During the late Pleistocene, six cinder cone and auxiliary vents between Broken Top and Tumalo Mountain erupted, yielding glomeroporphyritic basaltic andesite.

[27] When the first geological reconnaissance of the surrounding region was published in 1925, its author, Edwin T. Hodge, suggested that the Broken Top, the Three Sisters, and several other mountains in the area constituted the remains of an enormous collapsed volcano that had been active during the Miocene or early Pliocene epochs.

[42] In the 1940s, Howel Williams completed an analysis of the vicinity and concluded that Multnomah had never existed, instead demonstrating that each volcano in the area possessed its own individual eruptive history.

[44] Oregon State University geologist and volcanologist Edward Taylor's analysis in 1978[45] determined that the current Broken Top cone was constructed on basaltic andesite lavas that built a platform.

[29] As dikes and sills invaded the edifice over time, it formed a volcanic cone made of different lavas including basaltic andesite, dacite, and rhyodacite, as well as tephra and pyroclastic materials from more explosive eruptions.

A landslide at Broken Top, or an eruption from the nearby South Sister, could initiate a lahar (rapid flows of water, rock, and mud) that could cascade down river valleys in the surrounding area.

[47] An eruption from South Sister would pose a threat to nearby life, as the proximal danger zone extends 1.2 to 6.2 miles (2 to 10 km) from the volcano's summits.

[47] During an eruption, tephra could accumulate to 1 to 2 inches (25 to 51 mm) in the city of Bend, and mudflows and pyroclastic flows could run down the sides of the mountain, threatening any life in their paths.

[48] Over the course of the past century, five or more lahars have occurred at the Three Sisters and Broken Top, either as a result of glacier-outburst flooding or the failure of moraine-dammed lakes.

The hiking trail on Broken Top begins at the Green Lakes Basin at the trailhead east of Bend, running 12 miles (19 km) and gaining 3,450 feet (1,050 m) in elevation.