Earth's Moon

Key Questions

  • There are four types of rocks commonly found on the Moon: Basalt, Breccia, Highlands, and Regolith (or surface soil).

    The rocks on the Moon are the result of impact events, or meteoric collisions, throughout the Moon's history.


    Basalt: The Mare Rock

    Black volcanic basalts are found on 26% of the Moon's near side (and 2% of the Moon's far side). They formed when volcanic lava bubbled up into the Moon's cavernous basins through cracks formed by past meteoric impacts. Lunar basalts are similar to basalts found on Earth except for minor differences in chemical composition, such as fewer iron-like elements.

    Breccia: Shocked Rock

    Breccia are composite rocks formed from jagged and irregularly-shaped fragments that melted and then fused together during a meteoric collision. They are commonly found surrounding the Moon's craters. The pervasiveness of breccia suggests how often impact events occurred throughout the Moon's history.

    Highland Rock: Anorthosite

    Anorthosite is found across the Moon's lunar highlands and likely formed the primitive lunar crust. These rocks can be 4.6 billion years old at most, and their chemical composition suggests that the Moon's surface experienced frequent melting.

    Regolith Soil / Surface Layer

    Regolith is the loose, dusty soil that covers the Moon's surface up to a few meters on the Mare and sometimes triple that on the Highlands. It exists due to the Moon's constant bombardment with meteors - as the first line of defense, what might be larger rocks are ground into a powder upon these impacts.

    The composition of the Regolith mirrors that of the rocks underneath, which means the soil is mostly basalt on the the Mare and largely Highland rock on the Highlands. Glass spherules (small glass beads \ formed from small impacts on the Moon's surface) and orange dust from volcanic eruptions can also be found in the Regolith.

  • The Moon has many effects on planet Earth, but three major ones stand out. Today, the Moon:

    • Creates tides in Earth's oceans
    • Determines the length of Earth's days
    • Illuminates Earth's nights

    The Moon creates tides

    The moon has its own gravitational force that acts upon Earth as it orbits. As Earth spins on its own axis, gravity and centrifugal force keeps ocean waters at equal levels. However, the moon's gravitational force is strong enough to disrupt this balance, causing water to accelerate towards the moon and "bulge."

    This bulge moves as the moon orbits and Earth rotates, causing "high tide" in places experiencing the bulge. Without the moon, our oceans would still bulge (due to the gravitational pull of the sun), but bulges would be much smaller compared to those created by the moon.

    The Moon lengthens days

    We know that due to tidal friction caused by the moon, Earth’s rotation slows down (very slightly) over time. As it slows, our days get longer (because Earth is spinning more slowly, we experience longer periods of light from the sun).

    When we extrapolate this backwards, we see that our 24-hour day today is a product of a slowing rotation over time (it used to spin much faster!). Without the Moon to slow the rotation, the day would be only 6 - 8 hours long!

    The Moon illuminates nights

    The Sun's brightness causes our daylight, and at only 1/400,000th of the Sun's brightness, the Moon's brightness doesn't really compare. However, the next brightest object in the sky, Venus, is only 1/14,000th as bright as the full Moon!

    This suggests that without it, our experience at nighttime would be a very different one (much, much darker) and our night vision would never have evolved to what it is today.

    One final effect the Moon has on Earth: the Moon has long been a source of inspiration for poetry, literature, and other works of art. You can read one of my favorite poems that references the moon here.