Timeline of Martian Volcanism A crater-count study on Mars dates the calderas on 20 major volcanoes. High-resolution images from the Context Camera (CTX), onboard NASA’s Mars Reconnaissance Orbiter, allow a larger range of crater sizes than were ever used before. The timing of the last major summit eruptions correlates well with the crystallization ages (~165-170 million years) of the youngest basaltic Martian meteorites. Overview: A recent study of Martian volcanism presents a timeline of the last major eruptions from 20 large volcanoes, based on the relative ages of caldera surfaces determined by crater counting. Stuart Robbins, Gaetano Di Achille, and Brian Hynek (University of Colorado) counted craters on high-resolution images from the the Context Camera (CTX) on Mars Reconnaissance Orbiter to date individual calderas, or terraces within calderas, on the 20 major Martian volcanoes. Based on their timeline and mapping, rates and durations of eruptions and transitions from explosive to effusive activity varied from volcano to volcano. The work confirms previous findings by others that volcanism was continuous throughout Martian geologic history until about one to two hundred million years ago, the final volcanic events were not synchronous across the planet, and the latest large-scale caldera activity ended about 150 million years ago in the Tharsis province. This timing correlates well with the crystallization ages (~165-170 million years) determined for the youngest basaltic Martian meteorites. Image: MRO context Camera image of portion of the summit caldera area of Olympus Mons. Reference: Robbins, S. J., Di Achille, G., and Hynek, B. M. (2011) The Volcanic History of Mars: High-Resolution Crater-Based Studies of the Calderas of 20 Volcanoes, Icarus, v. 211, p. 1179-1203, doi: 10.1016/j.icarus.2010.11.012.

Timeline of Martian Volcanism – Olympus Mons Olympus Mons is shown on a basemap created with THEMIS Daytime IR images combined with MOLA altimetry-color-coded map. Olympus Mons, located in the Tharsis region at 18.5oN, 133.2oW, is mapped with six calderas numbered according to elevation. Lowest elevation is 1, next higher is 2, etc. The estimated age of each surface, determined by the crater size-frequency data, is shown next to the caldera number. (M is millions of years.) Five of the calderas have split ages where the younger age is at larger crater diameters and the older age is at smaller diameters. The crater size-frequency distribution, on the right, shows the data fit to isochrons. Surface ages are determined by the slopes of the isochron lines. The legend lists the ages of the six calderas on the summit of Olympus Mons determined by Robbins etal. (2011). Six calderas on Olympus Mons show ages ranging from about 140 - 420 million years.

Timeline of Martian Volcanism – Elysium Mons Elysium Mons is shown on a basemap created with THEMIS Daytime IR images combined with MOLA altimetry-color-coded map. Elysium Mons (25oN, 147oE) has a large, main caldera partly surrounded by a second caldera, and a third smaller caldera in the center. Ages are statistically identical. (G is billions of years.) The crater size-frequency distribution, on the right, shows the data fit to isochrons. Surface ages are determined by the slopes of the isochron lines. The legend lists the ages of the three calderas on the summit of Elysium Mons determined by Robbins etal. (2011). Three calderas on Elysium Mons show ages around 3 billion years.

Timeline of Martian Volcanism and Meteorites After Apollinaris Mons shut off, major volcanism ended throughout the highlands and Syrtis Major, then in the smaller Elysium volcanoes, and finally in the largest of the Tharsis volcanoes. Martian meteorites a few 100 million years old likely came from the younger flows of the Tharsis Montes. This timeline was produced by Robbins, Di Achille, and Hynek to illustrate caldera ages and last episodes of major, summit volcanism from 20 large volcanoes on Mars. Olympus Mons and Elysium Mons, the two volcanoes shown in the previous slides, are darkened. Geologic settings of the volcanoes are also noted along the x-axis. In addition to time given in billions of years on the y-axis, we also show color-coded stripes for the Martian epochs from youngest to oldest: Amazonian (blue), Hesperian (green), and Noachian (brown). The epochs are based on the times given by Hartmann and Neukum (2001). The colors overlap because the timing of the epochs is not known precisely due to different models of the rates of impact crater formation on Mars. The work confirms previous findings by others that volcanism was continuous throughout Martian geologic history until about one to two hundred million years ago, the final volcanic events were not synchronous across the planet, and the latest large-scale caldera activity ended about 150 million years ago in the Tharsis province. The shergottites are the most abundant type and the youngest of the Martian meteorites. They are basalts and basaltic cumulates with crystallization ages typically in the range of 165 to 170 million years. Crater counting tells us that young volcanic landscapes are widespread in the Tharsis region, leading Robbins and coauthors to agree with previous assessments that Martian meteorites a few 100 million years old likely came from the younger flows of the Tharsis Montes.