Question

Volcanoes, such as the Hawaiian chain, that form over mantle plumes are some of the largest shield volcanoes on Earth. However, several shield volcanoes on Mars are gigantic compared to those on Earth. What does this difference tell us about the role of plate motion in shaping the Martian surface?

Short answer

The lack of plate motion on Mars allows plume-fed volcanoes to grow larger due to continuous magma supply in one location.

Step-by-step solution

Understanding Plate Motion and Volcanism

To determine why Martian volcanoes are larger than Earth's, we need to understand plate tectonics. On Earth, the lithospheric plates move over mantle plumes, causing the formation of smaller volcanoes as the plates change position, creating a series of aligned volcanic islands or mountains.

Examining Martian Geology

Mars lacks active plate tectonics. The absence of plate motion means that mantle plumes remain stationary underneath the same crustal area for extended periods. This allows volcanoes to grow larger as the plume continuously supplies magma to the same location.

Comparing Volcano Sizes

The gigantic size of Martian volcanoes, such as Olympus Mons, compared to smaller Earth shield volcanoes, indicates that the lack of plate motion on Mars allows for the accumulation of massive amounts of volcanic material over time.

Key concepts

Plate Tectonics

Plate tectonics is the scientific theory that describes the movement of Earth's lithospheric plates. These plates are massive slabs of solid rock, floating atop the semi-fluid asthenosphere beneath. Their motions give rise to various geological features, including mountains, earthquakes, and volcanoes.
On Earth, mantle plumes—columns of hot, upwelling mantle material—often remain stationary while plates drift over them. This creates a series of volcanoes in a chain-like formation. Think of the Hawaiian Islands; each island represents a point in time where magma from a mantle plume broke through the shifting crust to create volcanic landforms.

The constant motion of plates ensures that any one spot over a mantle plume remains active for a limited time, leading to smaller and less long-lived volcanoes compared to planets without such tectonic activity.

Mantle Plumes

Mantle plumes are responsible for creating many of Earth’s large volcanic islands and mountain chains. These hot, buoyant upwellings originate deep within the mantle, potentially as far as the core-mantle boundary. They rise because they are hotter and less dense than the surrounding rock, bringing heat and material to the lithosphere.
When a mantle plume reaches a planet's surface, it results in volcanism. On Earth, these plumes are often obliterated by plate movement, but on Mars, where plate tectonics is absent, mantle plumes can persist in the same location for eons, leading to enormous volcanic structures.

• A steady supply of heat and magma from plumes allows for long-term lava accumulation.
• The lack of lateral tectonic movement on Mars means that plumes continuously supply the same volcanic site.

This underscores the role mantle plumes have in building some of the largest volcanic features in our solar system.

Shield Volcanoes

Shield volcanoes are large, broad volcanic structures primarily composed of fluid, low-viscosity lava flows. They are characterized by their gentle, shield-like shape, which results from the way lava spreads over wide areas due to its low viscosity.

On Earth, shield volcanoes are commonly found over hot spots where mantle plumes reach the surface. The Hawaiian Islands are classic examples of shield volcanoes that were created as the Pacific Plate drifted over a fixed mantle plume.
In contrast, Martian shield volcanoes—due to the lack of plate tectonics—can grow much larger and thicker because they are not restricted by moving plates. This results in extensive lava plains rather than linear volcanic chains.

• Martian shield volcanoes build up gradually over millennia with successive lava flows piling up.
• They provide insight into the planet’s geological history and volcanic activity.

The distinct size and formation processes of these volcanoes make them key subjects in comparative planetary geology.

Olympus Mons

Olympus Mons is the largest volcano in the solar system. Standing at about 13.6 miles (22 kilometers) high, it dwarfs any volcano on Earth, both in height and volume.
Located on Mars, this massive shield volcano provides critical insights into the planet's geological past. Unlike Earth, Mars does not have shifting tectonic plates that break the formation and growth of volcanic features. Therefore, Olympus Mons has been able to continuously grow over millions of years as the stable Martian crust sat stationary above a hot spot.

• Its immense size indicates a long, uninterrupted period of volcanic activity.
• Lava flows are believed to be hundreds of kilometers long, contributing to its wide base.

The existence and scale of Olympus Mons highlight the profound differences in volcanic processes between Earth and Mars, serving as a living testament to how planetary geology shapes celestial landscapes.