How the Rocky Planets Got Their Shapes

The center of a complex crater often rebounds after impact, creating a central peak. In an impact basin, the central peak sometimes collapses and forms a central ring. Materials pushed out and thrown outside of a crater create a raised rim. Hokusai crater on Mercury, seen here, is a typical complex impact crater.

Simple craters have smooth, steep walls. Complex craters have shallower walls that often slump or collapse into terraces. Ejecta are materials thrown from the impact crater. Some are melted by the impact and called impact melt. 

Simple craters are bowl-shaped depressions surrounded by a raised rim and a blanket of ejecta. They are deeper relative to their diameter. They lack features like wall terraces and central peaks.  

This is a meteor crater in Arizona. Known as Barringer Crater, it is one of the best preserved craters on Earth. It formed about 50,000 years ago when an ironnickel meteorite about 98 feet (30 meters) in size slammed into the desert. The impact left a crater three-fourths of a mile (1.2 kilometers) across. Image courtesy of NASA. 

The size and shape of a crater are affected by many factors. The bigger and faster moving the object is, the larger the crater it will make. The more gravity a planetary body has, the faster an object will be moving when it hits. Gravity, the presence of an atmosphere, the angle of impact, and the nature of the surface material also affect crater shape. For instance, the atmosphere of Venus is so dense that some objects break apart and form clustered impact craters, like this one. Image courtesy of NASA.

Illustration of lobate scarps that were formed when the lunar crust was pushed together as the Moon contracted.  This causes the near-surface materials to break forming a thrust fault.  The thrust fault carries crustal materials up and sometimes over adjacent crustal materials.  

When forces pull a planetary surface apart, the most common landforms created are called a graben. These trough-like features form when a rock is pulled apart, breaks, and drops down between parallel faults. Image courtesy of Arizona State University.

These images show an identical area on Venus as imaged by the U.S. NASA Magellan spacecraft in 1991 (left) and the U.S.S.R. Venera 15/16 spacecraft in the early 1980's (right). In the area seen here, approximately 200 small volcanoes, ranging in diameter from 2 to 12 kilometers (1.2 to 7.4 miles) can be identified. Volcanoes form from the buildup of cooled lavas and ash on a planetary surface. Solids, liquids, and gases can erupt from a volcano. The amount and makeup of the erupting material affects the volcano’s size and shape. Venus displays the greatest diversity of volcanic features among the rocky worlds. Image courtesy of NASA.

Many of the rocky planets are covered in lavas and other types of volcanic deposits that have erupted from vents and fissures (long linear vents) that may or may not be associated with volcanoes. Many of these eruptions involve a hot mix of ash, gases, and lava, which scientists call pyroclastic. A pyroclastic flow is a mix of hot ash, gases, and lava fragments that moves rapidly downslope. Image courtesy of Wikimedia.

Molten rock does not always erupt onto the surface. Sometimes it stalls and cools just beneath the crust, forming what’s called an intrusion. Intrusions often push up and fracture the overlying surface, creating a system of cracks. Erosion sometimes wears away the overlying material, exposing the hardened intrusion. 

Shiprock in New Mexico is a preserved core of a volcanic vent exposed by erosion. The vent was active 30 million years ago.  Several sheet-like lava intrusions called dikes, in the foreground, radiate from the core on the left. Image courtesy of Wikimedia.

Channels and sinuous rilles (winding channels) are common landforms within volcanic terrains. They can form by either the buildup of lava along the side of a lava flow or when a lava flow carves into the surface. Our Moon has many sinuous rilles. Here, a small one lies within an older, larger one on the Aristarchus Plateau. Image courtesy of NASA.

Not all volcanic activity involves volcanoes. Many volcanic deposits on the rocky planets were produced by flood lavas, large-volume eruptions that covered vast areas. The lava usually poured out of vents now buried in low-lying areas, such as impact basins and craters.  

Over 80% of Venus is covered by relatively young volcanic plains, less than 500 million years old. The abundance of these plains suggests that Venus may have experienced catastrophic volcanic events that caused lava to flood large parts of its surface. Image courtesy of NASA.

Some lava flows erupt from long cracks, called fissures. This fissure on Mars extends more than 620 miles (1,000 kilometers). Features like this formed from the upward movement of molten rock. Image courtesy of NASA.