Field research target regions within two basaltic geologic provinces are described as Earth analogs to Mars. Regions within the eastern Snake River Plain of Idaho and the Big Island of Hawai'i, the United States, provinces that represent analogs of present-day and early Mars, respectively, were evaluated on the basis of geologic settings, rock lithology and geochemistry, rock alteration, and climate. Each of these factors provides rationale for the selection of specific targets for field research in five analog target regions: (1) Big Craters and (2) Highway lava flows at Craters of the Moon National Monument and Preserve, Idaho, and (3) Mauna Ulu low shield, (4) Kīlauea Iki lava lake, and (5) Kīlauea caldera in the Kīlauea Volcano summit region and the East Rift Zone of Hawai'i. Our evaluation of compositional and textural attributes, as well as the effects of syn- and posteruptive rock alteration, shows that basaltic terrains in Idaho and Hawai'i provide a way to characterize the geology and major geologic substrates that host biological activity of relevance to Mars exploration. This work provides the foundation to better understand the scientific questions related to the habitability of basaltic terrains, the rationale behind selecting analog field targets, and their applicability as analogs to Mars. These primary constituents are all nominally anhydrous. Following the emplacement of a fresh lava flow, a variety of secondary processes can act to form secondary minerals and textural modifications that will reflect the alteration process. The extent of alteration depends on the temperature, length of exposure time, and the compositions of the primary rock substrate and fluids/gases involved. Alteration conditions can be relatively hot and dry (syn-eruptive), hot and wet (hydrothermal or fumarolic), cool and wet (long-term meteoric water interaction), or even cool and dry (high desert conditions). 2E, G ) likely comprise secondary material introduced by fluids or directly deposited sublimates that are somewhat resistant to weathering processes. 2.1.4. Ambient temperature alteration This form of “low-temperature” alteration is essentially weathering related to the long-term effects of subaerial or subaqueous chemical change under ambient climate conditions (Dessert et al. , 2003 ; Adcock et al. , 2018 ). Each basaltic low shield was constructed by a series of low-viscosity, tube-fed basalt lava flows that initially erupted from fissures oriented perpendicular to the WSW-ENE trend of the ESRP. These monogenetic lava flow fields dominate the upper volcanic sedimentary regional ESRP sequence (Greeley, 1982 ; Leeman, 1982a ; Kuntz et al. , 1992 ; Hughes et al. , 2002a , 2002b ). By contrast, the polygenetic lava flows in the COTM field, which erupted intermittently since ∼15 k.y. ago along the Great Rift, have chemically evolved basalt-like compositions, such as hawaiite and latite ( Fig. 6 ). The Great Rift is the most predominant of several volcanic rift zones and numerous vent corridors, all of which are aligned perpendicular to ENE-WSW regional Basin-and-Range extension (Kuntz et al. , 1992 ; Hughes et al. , 2002b ). Most of the low shields on the ESRP were constructed along aligned eruptive vents active during different episodes. While tholeiitic basalts (low shields) dominate the terrain, the COTM system contains numerous steep cones and compositionally diverse lavas that make it unique in the ESRP. Extensive geologic mapping and petrologic analyses ( e.g ., Kuntz et al. Preliminary sampling and analysis further justified, with NPS permission to venture into otherwise restricted areas, our decision to engage in more detailed analysis. 3.2.2. Highway flow (RT 2) The Highway flow (RT 2) ( Fig. 4 ) is a thick (up to ∼15 m) chemically evolved latite (“HW” in Fig. 6 ) flow with a tall and steep flow front and an extremely rugged surface, ∼4–8 m relief, of jagged spires and steep-sided cracks. Generally it can be characterized as an ‘a'ā flow with block and slab-pāhoehoe components ( Fig. 3C, D ). The rugged high-relief morphology reflects its emplacement as multiple lobes of sluggish, viscous lava from an eruptive vent most likely north of the highway ( Fig. Lower volume magma with chemically evolved compositions erupted during the latter part of the shield stage results in steep-sided cinder cones and shorter lava flows. As eruptions diminish, growth stages are ultimately overtaken by loss of land mass due to erosion, landslides, and other geomorphic changes ( e.g ., Stearns, 1946 ). As the youngest subaerial volcano in Hawaii (and possibly the most active volcano on Earth), Kīlauea exemplifies the shield stage of Hawaiian eruptions with notable activity during the 19th and 20th centuries that has continued unabated into this century. Coupled with the absence of plate tectonics, this allows the construction of massive volcanic edifices such as the great martian shield volcanoes Olympus Mons, Alba Mons, Ascraeus Mons, Pavonis Mons, and Arsia Mons of the Tharsis region ( Fig. 10 ). These prominent landforms reflect a style of volcanism characterized by voluminous eruptions of basaltic lava that form large central shield volcanoes surrounded by many fissures, lava flows, and smaller (low) shields. Mission designs for crews can take on many forms and involve seemingly endless discussions as to target regions, duration, capabilities, and science objectives. Without the benefit of geologic reconnaissance missions on the surface, the analog locations can only be related to general target regions of Mars, with the notion that more specific targets to investigate will become known in time. While much of Mars' surface is basalt or basalt like, regional alteration of volcanic terrains is likely to have occurred early in the geologic history, whereas long-term alteration since then would be related to cold, dry conditions or the interaction of lavas with ice.