---
title: "Wrinkle Ridges | Intro to Astronomy"
description: "Wrinkle ridges are long ridges from crustal compression on the Moon and Mercury, revealing cooling, contraction, and tectonic history in Intro to Astronomy."
canonical: "https://fiveable.me/intro-astronomy/key-terms/wrinkle-ridges"
type: "key-term"
subject: "Intro to Astronomy"
unit: "Unit 9"
---

# Wrinkle Ridges | Intro to Astronomy

## Definition

Wrinkle ridges are long, sinuous ridges on the Moon and Mercury formed when the crust compressed as the planet cooled and contracted. In Intro to Astronomy, they’re used to read a world’s geologic history.

## What It Is

Wrinkle ridges are long, low, winding ridges on the surfaces of the Moon and Mercury that formed when the crust was squeezed and buckled. In Intro to Astronomy, you usually see them as a surface feature that points to a planet or moon changing internally after its big early volcanic or impact-heavy phase.

The basic idea is thermal contraction. As a rocky body like the Moon or Mercury cools, its interior and outer layers shrink a little. The crust does not shrink smoothly like a soft rubber ball, so it responds by shortening and folding, which can push up narrow ridges at the surface.

On the Moon, wrinkle ridges are common in the lunar maria, the dark basalt plains made by ancient lava flows. Those lava plains later cooled, solidified, and were compressed, so the ridges often trace places where the maria were deformed after eruption. On Mercury, they show up a lot in the intercrater plains, the smoother regions between many impact craters.

The shape matters. Wrinkle ridges are usually long and sinuous, and they can curve, branch, or form intersecting patterns. They are not the same thing as a single crack or a volcanic mountain chain. They are a surface response to compression, so they tell you that the crust was under stress, not just that lava flowed there.

These features are useful because the Moon and Mercury do not have much water, weather, or active plate tectonics to erase old landforms. Once a wrinkle ridge forms, it can stay visible for a very long time. That makes it a surface clue for the thermal and tectonic history of a world that has otherwise been frozen in place for billions of years.

## Why It Matters

Wrinkle ridges matter in Intro to Astronomy because they are one of the cleanest clues that the Moon and Mercury did not just stop changing after their surfaces formed. They show that each body kept cooling, shrinking, and deforming long after the biggest impact and volcanic events.

That makes them a useful link between surface geology and interior structure. If you see wrinkle ridges in lunar maria or Mercury’s intercrater plains, you are not just identifying a landform. You are reading evidence that the crust was under compression and that the planet or moon was losing heat over time.

They also help you compare terrestrial worlds. Earth hides a lot of its old tectonic history because plate tectonics, erosion, and oceans constantly reshape the crust. The Moon and Mercury preserve these older features much better, so wrinkle ridges act like a record of the stresses that shaped small rocky bodies in the inner solar system.

In class, wrinkle ridges often come up when you are connecting surface appearance to geologic process. A map, image, or planet comparison question may ask you to explain why a smooth plain still shows ridges, or what those ridges suggest about the body’s cooling history.

## Connections

### Thermal Contraction

Thermal contraction is the process behind wrinkle ridges. As a rocky world cools, its volume decreases a little, and the crust has to adjust to that shrinkage. On the Moon and Mercury, the adjustment often shows up as compression features instead of a clean, even change in shape.

### [Lunar Maria](/intro-astronomy/key-terms/lunar-maria)

Lunar maria are the dark basalt plains where many wrinkle ridges are found on the Moon. The ridges formed after lava flooded low areas and then cooled and contracted. If you are looking at a Moon image, the maria are often the easiest place to spot them.

### [Lobate Scarps](/intro-astronomy/key-terms/lobate-scarps)

Lobate scarps are another compressional landform, especially important on Mercury. They are steeper, cliff-like features, while wrinkle ridges are lower and more gently rounded. Both point to crustal shortening, but they can look very different in photographs and elevation data.

### [Mercury’s surface](/intro-astronomy/key-terms/mercurys-surface)

Mercury’s surface preserves lots of old tectonic features because the planet has almost no atmosphere or active erosion. Wrinkle ridges fit into that broader pattern of a heavily cratered, ancient surface that still records internal cooling and crustal stress.

## On the AP Exam

A quiz image question may show a smooth lunar plain or Mercury map and ask you to identify the ridged, wavy lines as wrinkle ridges, then explain what formed them. The move is to connect the landform to compression from cooling and contraction, not to volcanism or impact fracturing alone.

If you get a short-answer prompt, use the ridge pattern as evidence. For example, you might say that wrinkle ridges in lunar maria suggest the basalt plains cooled and later shortened under stress. On Mercury, you could link ridges in the intercrater plains to a surface that contracted as the planet’s interior lost heat.

In a comparison question, use wrinkle ridges to show how rocky worlds preserve geologic history differently. The Moon and Mercury keep old compressional structures visible because they have limited erosion and little active resurfacing. That is the kind of observation instructors usually want when they ask you to interpret a planetary surface feature.

## Wrinkle Ridges vs Lobate Scarps

Both wrinkle ridges and lobate scarps form from compression, so they can be easy to mix up. The difference is mostly shape and relief: wrinkle ridges are lower, broader, and more sinuous, while lobate scarps are steeper, more cliff-like breaks in the surface, especially on Mercury.

## Key Takeaways

- Wrinkle ridges are long, winding ridges formed when a rocky crust is compressed as a world cools and contracts.
- On the Moon, they are common in the lunar maria, where ancient basalt plains were later squeezed into ridged patterns.
- On Mercury, wrinkle ridges often appear in the intercrater plains and point to a long history of crustal shortening.
- They are useful because they preserve evidence of internal cooling and tectonic stress on worlds with little erosion.
- If you see wrinkle ridges in an image or map, think compression, not just cracks or random mountain building.

## FAQs

### What are wrinkle ridges in Intro to Astronomy?

Wrinkle ridges are sinuous surface ridges on the Moon and Mercury created when the crust compressed as the body cooled and contracted. In Intro to Astronomy, they are a geologic clue that these worlds kept changing after their plains or maria formed.

### Where do wrinkle ridges form on the Moon?

They usually form in the lunar maria, the dark basalt plains made by ancient lava flows. Those plains later cooled and were squeezed, so the surface buckled into long ridges.

### Are wrinkle ridges the same as lobate scarps?

No. Both are compression features, but wrinkle ridges are generally lower and more rounded, while lobate scarps are steeper and more cliff-like. Mercury has famous examples of both, so it is easy to confuse them on a surface map.

### What do wrinkle ridges tell us about Mercury and the Moon?

They tell us that both bodies cooled, contracted, and experienced crustal stress over time. Because their surfaces are so old and not heavily erased, wrinkle ridges preserve a lot of their geologic history.

## Related Study Guides

- [9.1 General Properties of the Moon](/intro-astronomy/unit-9/1-general-properties-moon/study-guide/KSiYV5Dmq9zBkfqD)
- [9.5 Mercury](/intro-astronomy/unit-9/5-mercury/study-guide/LxhIn11HizbAf3V7)

## 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`)

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