By studying the distribution and features of the post-impact ocean deposits and fragmented rock that filled the crater, Andrew J. Retzler of Idaho State University and colleagues present a new map characterizing the size and shape of the Alamo crater. Their results indicate that only about half of the Alamo impact crater and its related deposits are now exposed within the region, and they estimate its total diameter to be between 111 and 150 km.
This is more than double previous estimates and, if correct, places the Alamo crater as one of the largest marine impacts in the last 550 million years, conservatively larger than the well-studied Chesapeake Bay impact crater (about 35 million years old) on the eastern shore of North America.
To date, there are only 182 confirmed impact structures on Earth (see the Earth Impact Database). In contrast, most other planets consist of thousands to hundreds of thousands of impact structures. Regardless, Earth has undergone billions of years of bolide impacts, mainly taking place in its early history (about 3.8 to 4 billion years ago). Evidence of these early bolide impacts has been erased due to the dynamic nature of the Earth over millions of years. The thick atmosphere, deep oceans, and mobile crust unique to our planet has altered bolides and hidden their structures!
Only a handful of Earth's impact structures are relatively undeformed (for example: Manson, Mjølnir, Lockne, Popigai, and Ries craters), allowing geologists to better study bolide impact processes and their effects on Earth's systems. Most of these craters are exposed on the surface, which are more accessible than submarine impacts, such as the Mjølnir crater. Few submarine impact structures are known, and they are largely understudied due to the deep ocean depths in which they exist.
Lockne Crater in Sweden
The relatively young age of the oceanic crust also worsens the chance of submarine impact preservation. Oceanic crust is continually being created at ocean spreading ridges, where molten rock flows out and cools to form new oceanic crust. As the ocean floor spreads apart, older oceanic crust is pushed away and subducted underneath the continental plates, where it is melted and recycled beneath Earth's crust. Thus, any impacts that may have occurred on the ocean floor deep in Earth's history may have been subducted, re-melted, and recycled to create new oceanic crust.
The Alamo Impact Event represents a submarine impact that has since been exposed onto land. Such exposures allow geologists to better study and understand submarine impact processes, making the Alamo Impact an excellent case study. Other submarine impact structures that are now exposed on land include: the Chesapeake Bay (85-km diameter), the Montagnais (45-km diameter), the Mjølnir (40-km diameter), and the Ust Kara (25-km diameter) craters.
Contacts and sources:
Geological Society of America