The Olympus Mons volcano on Mars shares morphological similarities with many active volcanic islands on Earth, suggesting that it formed when a vast ocean once occupied the planet’s northern lowlands.
Nearly as large as the state of Arizona and three times the height of Mount Everest, Olympus Mons, located on the Tharsis rise in the northern hemisphere of Mars, is the largest mountain in the Solar System. This volcano with an estimated volume of over 300.000 cubic-kilometers was most likely formed when a stationary current of hot rock in the Martian mantle melted parts of the crust. As Mars is too small to possess active plate tectonics, the volcano sat for millions, maybe even billions of years, over this so-called hot-spot, providing a constant supply of lava and allowing the volcano to grow to its gigantic size. Reported ages for Olympus Mons volcanic activity range from 3.8 billion years to less than 10 million years (with some researchers believing it still is an active volcano).
A 6-kilometer-high escarpment surrounding the base of Olympus Mons has attracted attention since the Mariner 9 made the first images of the volcano in the early 1970s. Theories of formation included fossil glaciers, large landslides and wind erosion reshaping the mountain after volcanic activity ceased. But a new study concludes that the basal escarpment formed by lava flowing into liquid water and Mount Olympus was an active volcanic island.
The authors of the study compared MOLA topographic data and satellite images taken from orbit of Mons Olympus with field studies done on three active volcanic islands on Earth: the Portuguese Pico Island, the Canadian Fogo Island and the U.S. island of Hawaii.
The three-dimensional images show that the escarpment is formed by tilted layers and blocks, partially buried by younger lava flows. An analogous landscape can be found at the Ninole Hills on Mauna Loa, Hawai‘i. These hills are a prominent group of flat-topped ridges towering over the nearby Punaluu Beach Park. They are formed by a succession of basaltic lava flowing into the sea.
When lava transitions from air to water, the viscosity of the lava plunging from over 1.000 degrees to a few hundreds changes abruptly from low to high, quickly solidifying and forming steeply inclined rock faces. Eventually the solidified lava cracks and breaks and entire blocks topple over and become buried by subsequent lava flows. This likely also happened on Mars.
Similar features on the northern flank of the Alba Mons volcano, located more than 1,500 kilometers north-west from Olympus Mons, also support the idea of a vast Martian ocean.
Based on other geological evidence, like sediments deposited in Jezero crater, water was present on Mars up to 3.8 to 3.6 billion years ago. The sedimentary succession indicates that for much of its existence, an ancient lake feed by a gently flowing river was present in the crater. Then the climate became much more extreme, and mudflows triggered by sudden floods deposited large boulders onto the delta. Once the lake dried up, wind eroded the landscape, leaving Mars a dry, cold desert as it is today. The cause of this climate turnaround is yet unknown.
The study “A giant volcanic island in an early Martian Ocean?” was published in the journal Earth and Planetary Science Letters (2023).