--- title: "Lunar Mare | Earth Science" description: "Lunar mare are dark basaltic plains on the Moon formed by ancient lava flows, revealing volcanic history, surface age, and impact patterns in Earth Science." canonical: "https://fiveable.me/hs-earth-science/key-terms/lunar-mare" type: "key-term" subject: "Earth Science" unit: "Unit 1" --- # Lunar Mare | Earth Science ## Definition Lunar mare are the Moon's dark, smooth basalt plains created by ancient volcanic lava flows. In Earth Science, they are used to compare the Moon's younger volcanic regions with its older, cratered highlands. ## What It Is Lunar mare are the broad, dark plains on the Moon made of basaltic rock that filled in older low-lying impact basins. In Earth Science, you usually see them as the smoother, darker patches that stand out against the brighter, heavily cratered lunar highlands. The word mare means "sea" in Latin, but these areas are not water. Early observers thought the dark regions looked like oceans, so the name stuck. What actually covers them is solidified lava. After large impacts formed giant basins, molten material later rose and spread across the Moon's surface, then cooled into flat volcanic plains. That basalt matters because it tells you about the Moon's interior and volcanic past. Basalt forms from lava that is relatively rich in iron and magnesium, and it tends to look darker than the more reflective rocks in the highlands. That darker color is part of why the maria have a lower albedo, meaning they reflect less sunlight and appear dimmer to us. Mare surfaces are also less cratered than the highlands. That difference is a clue about relative age. The Moon does not have much atmosphere, wind, or liquid water to erase old features, so crater counts are a major way scientists compare surface ages. Fewer craters usually means the surface formed later and had less time to collect impacts. Most lunar mare formed between about 3 and 3.5 billion years ago, during a time when the Moon was still geologically active. Oceanus Procellarum, the largest mare region, stretches across a huge part of the near side of the Moon. A big pattern students notice is that the mare are concentrated on the side facing Earth, while the far side has far more highlands and much less mare coverage. That difference connects lunar geography with the Moon's history of impacts, cooling, and volcanic eruption. ## Why It Matters Lunar mare give you a way to read the Moon's surface like a timeline. In Earth Science, that means using color, texture, and crater density as evidence instead of just memorizing a map. If a surface is dark, smooth, and relatively uncratered, you can infer that lava resurfaced it after the oldest impacts formed. This term also ties together several big ideas from the Moon unit. It connects composition, because mare are basaltic, to surface appearance, because basalt has a darker albedo, and to geologic history, because the maria show that the Moon was once volcanically active. That is a lot of information from one landform. Mare are also useful for comparison. They contrast sharply with the lunar highlands, so they make it easier to explain why some parts of the Moon look older and more battered than others. On quizzes or short responses, you may be asked to identify which surface is newer or to explain why one region has fewer craters. Lunar mare are one of the best clues for that kind of reasoning. The term comes up again when you study how scientists use remote sensing, maps, and visible surface features to interpret other rocky worlds. Once you can explain why the Moon has dark basalt plains, you have a stronger model for reading planetary surfaces beyond Earth too. ## Connections ### Basalt Lunar mare are made of basalt, so the rock type explains both the dark color and the smooth, lava-flow look. If you know basalt forms from cooling lava, you can connect mare directly to volcanic resurfacing on the Moon. ### [Impact Craters](/hs-earth-science/key-terms/impact-craters) Crater counts help you compare mare with older lunar surfaces. The mare have fewer visible craters because they formed after major impact basins and then got resurfaced by lava, while older areas stayed cratered for much longer. ### [lunar highlands](/hs-earth-science/key-terms/lunar-highlands) The highlands are the bright, heavily cratered contrast to lunar mare. Comparing the two regions is one of the easiest ways to describe relative age, surface texture, and the Moon's volcanic history. ### [spectroscopy](/hs-earth-science/key-terms/spectroscopy) Spectroscopy lets scientists identify the composition of lunar surfaces from reflected light. It can confirm that mare are basalt-rich and help distinguish them from the rockier, more reflective highlands. ## On the AP Exam A quiz question might show two Moon photos and ask you to identify which region is a lunar mare, or ask why a surface patch is darker than the surrounding terrain. You use the clues: dark color, smooth plains, basaltic composition, and fewer craters. If the prompt asks for a short explanation, connect the mare to ancient lava flows and relative youth. In map or image questions, point out that mare are more common on the near side of the Moon and often sit inside large impact basins. ## lunar mare vs lunar highlands Lunar mare and lunar highlands are easy to mix up because both are on the Moon, but they look very different. Mare are darker, smoother basalt plains with fewer craters, while the highlands are brighter, rougher, and more heavily cratered. The contrast helps you infer which surfaces are older and which were resurfaced later by lava. ## Key Takeaways - Lunar mare are dark basaltic plains on the Moon, not bodies of water. - Their dark color comes from lower albedo, so they reflect less sunlight than the highlands. - Most mare formed after major impacts and later lava flows resurfaced the Moon's surface. - Fewer craters usually mean the surface is younger, so crater density helps scientists compare mare and highlands. - Mare are a window into the Moon's volcanic history and its geologic timeline. ## FAQs ### What is lunar mare in Earth Science? Lunar mare are the Moon's dark basalt plains, formed when ancient lava filled large impact basins and cooled. In Earth Science, they are a surface feature used to compare volcanic activity, surface age, and crater density across the Moon. ### Why are lunar mare darker than the highlands? They are darker because the basaltic rock in the mare has a lower albedo, so it reflects less sunlight. The highlands are made of brighter, more reflective material, which makes the contrast easy to see in Moon images. ### How do scientists tell lunar mare are younger than the highlands? They look at crater counts and surface texture. Mare have fewer impact craters and smoother surfaces because lava resurfaced them after the oldest bombardment events, while the highlands kept more of their ancient craters. ### Are lunar mare actually oceans? No, they only look like seas when viewed from Earth. The name comes from early astronomers who saw the dark patches and used the Latin word mare, but the regions are solid lava plains. ## Related Study Guides - [1.4 The Moon and Its Phases](/hs-earth-science/unit-1/moon-phases/study-guide/YhZHHUPXx59C9Uw2) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/hs-earth-science/key-terms/lunar-mare#resource","name":"Lunar Mare | Earth Science","url":"https://fiveable.me/hs-earth-science/key-terms/lunar-mare","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/hs-earth-science/key-terms/lunar-mare#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:52.120Z","isPartOf":{"@type":"Collection","name":"Earth Science Key Terms","url":"https://fiveable.me/hs-earth-science/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/hs-earth-science/key-terms/lunar-mare#term","name":"lunar mare","description":"Lunar mare are the Moon's dark, smooth basalt plains created by ancient volcanic lava flows. 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