Lesson 2 of 6 · Two terrains, several geological events
Two terrains, several geological events
10 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish ancient highland crust, impact basins, and later volcanic plains using material and contact evidence.
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The dark areas are volcanic plains
The Moon’s broad dark plains are called , the plural of mare, from historical names meaning seas. They are not present-day oceans. Samples and remote measurements show that many are made of , a volcanic rock formed when molten material cools. Basaltic lavas spread across low areas and produced surfaces different from the generally brighter, heavily cratered highlands around them.
Highland crust includes abundant , a rock dominated by feldspar. Basalt and anorthosite are rock names, while plagioclase and pyroxene are mineral names. Confusing those categories makes a chemical description less useful. A rock is an assemblage with a texture and history; a list of mineral ingredients helps identify it but still needs interpretation of how those ingredients formed and were assembled.
A basin and its filling have different ages
Many maria occupy large impact basins. Excavating a basin and filling part of it with lava are separate geological events, sometimes widely separated in time. The impact can fracture and melt target rock; later volcanism brings additional molten material from below. Calling every dark basin floor would erase this distinction and assign the wrong event to samples of volcanic basalt.
A mare surface commonly records a later episode than surrounding highland crust, but neither terrain is a single uniform timestamp. Multiple lava flows can overlap. Fresh impacts can excavate older material through a younger surface, and their can cover neighboring terrain. Ask whether an age belongs to a rock’s formation, a particular flow, an excavated basin, or the exposed surface being mapped.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Read a contact rather than a color alone
The Lunar Reconnaissance Orbiter view of Mare Moscoviense shows darker mare terrain at left and brighter highland terrain at right. The white curve is an added annotation marking a generalized . It is not a white ridge on the Moon. The source describes an image about 1.8 kilometers wide, so this is a local boundary view rather than a portrait of the entire basin.
Mare Moscoviense is on the far side. Most visible maria occur on the near side, but far-side maria do exist, and the far side receives sunlight. “Far” describes its usual orientation away from Earth, not permanent darkness. A local exception can help test why the two hemispheres evolved differently. The distribution itself does not establish a complete explanation for that asymmetry.
Lunar Reconnaissance Orbiter image of the Mare Moscoviense boundary, about 1.8 kilometers across. The white line is an added geological annotation separating mare terrain at left from highlands at right; it is not a bright feature on the surface.
Look closely
Find the contact, then identify what a rover would need to measure before assigning rock types to both sides.
See what to notice
Brightness and texture motivate the mapped boundary. Spectra and documented sampling could test composition, while local observations would help identify ejecta mixing and access limits. The source supplies the contact interpretation; the white line is not itself physical evidence.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Work the evidence: design a boundary traverse
Consider an invented 20-square-kilometer mapped region containing 6 square kilometers of dark plain and 14 of highland terrain. The plain covers 6/20, or 30 percent, of this region. That is an area fraction, not the percentage of the Moon that formed in one eruption. An oblique photograph would first need suitable geometric correction before its pixels could support this area calculation.
To test whether the mapped boundary separates basaltic and feldspar-rich materials, compare spectra and documented samples on both sides, with several sites away from obvious mixed ejecta. Record lighting and surface texture rather than assigning composition from brightness alone. A loose highland fragment found on the dark side might have arrived in an impact. The most informative route connects mapped units and their contact, while preserving enough context to recognize transported material.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: LPI: basalt, breccia, and highland samplesRock types, volcanic products, and material transported by impacts.
- Read: NASA: Mare Moscoviense contact imageAnnotated 1.8-kilometer-wide boundary on the lunar far side.
- Read: LPI: volcanic landformsBasalt flows and the difference between volcanic and impact processes.
- Read: NASA: lunar compositionHighland and mare differences in a broader geological setting.