Question

If you look at the Moon, even without a telescope, you see broad areas where its surface appears relatively darker and smoother. These areas are individually called mare (plural: maria), from the Latin word for sea. The term is misleading, for they are not bodies of water but rather plains of igneous rock formed after huge meteors struck the Moon and formed very deep craters. These impacts occurred early in the history of the Moon. Propose a cause for the igneous activity, and suggest the type of igneous rock that fills the mare. (Hint: Think about how the presence of a deep crater affects pressure in the region below the crater, and think about the viscosity of a magma that could spread over such a broad area.)

Short answer

The cause of the igneous activity in the Moon's maria is the increased pressure generated by meteor impacts, producing magma. The magma has low viscosity, allowing it to spread over large areas, and the type of igneous rock filling the mare is basalt.

Step-by-step solution

Determine the cause of igneous activity in maria

When a huge meteor strikes the Moon, it creates a very deep crater. The impact of the meteor causes a sudden and significant increase in pressure in the region below the crater. This increased pressure generates enough heat to partially melt the lunar mantle, producing magma.

Consider the viscosity of the magma

In order for the magma to spread over a large area like the maria, it must have low viscosity. Low viscosity magma is typically less resistant to flow and can spread easily over the surface. This allows for the formation of the broad, relatively flat areas we observe in the maria.

Suggest the type of igneous rock that fills the mare

Considering the low viscosity magma that spreads easily over the lunar surface, the most likely type of igneous rock that fills the maria is basalt. Basaltic lava flows are generally more fluid and can cover large areas, forming the broad plains observed in the maria. This type of rock is also darker in color, which corresponds to the dark appearance of the maria compared to the rest of the Moon's surface. In conclusion, the cause for the igneous activity in the Moon's maria is the increased pressure generated by meteor impacts, which produces magma. The magma likely has low viscosity, allowing it to spread over large areas, and the type of igneous rock that fills the mare is basalt.

Key concepts

Igneous Rock Formation

Igneous rock formation is a fascinating process that occurs both on Earth and the Moon. When meteors crash into the Moon, they form deep craters, increasing pressure below the surface. This pressure creates immense heat, enough to partially melt the Moon's mantle, creating magma.
Once the mantle is molten, it begins its upward journey towards the Moon's surface. This rising magma cools and solidifies, forming igneous rock.

• Igneous rocks are primarily solidified magma or lava.
• They are classified by their mineral composition and texture.
• On the Moon, the primary igneous rock filling the maria is basalt.

This formation process is crucial in understanding lunar geology and the Moon's evolution.

Maria on the Moon

The term "maria" is derived from the Latin word for "seas," which is misleading since these are not bodies of water. Instead, maria are large, dark plains of igneous rock found on the Moon's surface. These regions formed billions of years ago from ancient volcanic activity triggered by massive meteoric impacts.

• The impacts created deep craters, changing the pressure dynamics beneath.
• The pressure increase melted parts of the mantle, forming magma.
• The magma erupted and flowed into the depressions, forming the maria.

Maria can be easily identified even from Earth due to their distinct dark appearance compared to the lighter lunar highlands.

Magma Viscosity

Magma viscosity is a critical factor in volcanic activity, dictating how magma flows. It is a measure of magma's resistance to flow:

• Low viscosity means the magma is more fluid and can travel further distances.
• High viscosity indicates thicker magma, which moves slower and tends to trap more gas.

On the Moon, the magma responsible for the maria had low viscosity, allowing it to spread over vast areas easily.
This low viscosity magma allowed for smoother, darker plains rather than jagged or mountainous landscapes. It is primarily composed of volcanic basalt, known for its fluid nature and ability to cover large areas quickly.

Basaltic Lava Flows

Basaltic lava flows are a hallmark of lunar geology and are significant in creating the mare regions on the Moon. Basalt is an igneous rock that forms from the cooling and solidification of magma.
This type of lava has low viscosity, enabling it to flow smoothly over the lunar surface, filling large craters formed by meteor impacts.

• Basaltic lava is darker, contributing to the maria's distinct appearance.
• These flows spread widely, creating the vast plains we observe today.
• The low viscosity allows basalt to level out over large areas.

Understanding basaltic lava flows helps explain the Moon's surface structure and the formation of its unique geological features.