Lunar Volcanism
When Apollo astronauts landed on the moon, they stepped onto a surface that looked like a dry, dusty desert. They found vast, dark plains that contrasted sharply with the bright, rugged mountains surrounding them. These dark regions, which we call lunar maria, tell a violent story of ancient heat and molten rock. Just as a cracked sidewalk allows weeds to push through the surface, the moon’s crust once allowed deep magma to escape its interior. This process, known as lunar volcanism, shaped the face of the moon during its early, active life.
The Origin of Dark Plains
Unlike the volcanoes on Earth that build towering mountains, lunar volcanism was a quiet, spreading event. Molten rock flowed out from deep within the mantle through cracks caused by massive asteroid impacts. These floods of basaltic lava filled the low-lying basins created by those impacts millions of years ago. The lava cooled into smooth, dark rock that we see today as the maria. Because the moon lacks an atmosphere to weather these rocks, the maria remain preserved exactly as they were when the lava finally stopped flowing.
Key term: Lunar maria — the large, dark, basaltic plains on the Moon formed by ancient volcanic eruptions.
Comparing Highlands and Maria
To understand the moon, you must compare the two main types of terrain found there. The lunar highlands represent the oldest parts of the crust, which are heavily cratered and light in color. In contrast, the maria are younger regions that contain far fewer impact craters because lava covered them later. The following table highlights the key differences between these two distinct surface types:
| Feature | Lunar Highlands | Lunar Maria |
|---|---|---|
| Age | Very old (original crust) | Younger (volcanic plains) |
| Color | Bright and reflective | Dark and basaltic |
| Surface | Highly cratered terrain | Smooth and flat areas |
These differences reveal how the moon changed over time. The highlands formed when the moon was a cooling globe, while the maria represent a later period of internal heating. Think of the highlands as the original foundation of a house, and the maria as the new flooring installed during a major renovation. This renovation was driven by radioactive heat trapped inside the moon during its formation. As the heat built up, it melted the mantle rock, forcing it upward toward the surface.
The Mechanics of Lava Flow
This volcanic activity occurred because the moon had a thin crust in certain areas. When large asteroids struck the surface, they thinned the crust significantly, creating weak spots. Magma from the interior followed these paths of least resistance to reach the surface. Once the lava emerged, it spread across the basin floors like thick syrup on a cold plate. This process continued for hundreds of millions of years until the moon eventually cooled down. Today, the moon is geologically dead, meaning that no new lava flows occur to reshape the surface. We are left with a frozen record of a time when the moon was a hot, active world.
The dark lunar maria provide a visual history of how internal heat once escaped through impact-weakened crust to create smooth volcanic plains.
But this cooling process raises questions about how the moon’s internal state eventually helped stabilize the Earth's own rotation.