Shield Volcanoes

I recently returned from a really awesome trip to Hawaii, where I participated in the 2008 NASA Planetary Volcanology workshop! The idea behind the workshop is that planetary scientists who are trying to interpret orbital data of other planets should have some experience doing the same on Earth. Now that I’m back, I have a lot to tell you about volcanoes. Today we will start by talking about shield volcanoes, starting with Hawaii.

First of all, what exactly is going in Hawaii? Why are there volcanoes there? After all, it’s smack in the middle of a tectonic plate, and most volcanoes form in fault zones at the edges of plates. It turns out that Hawaii is a textbook example of “hotspot volcanism”. The basic idea is that there is a “plume” of hot rock welling up from deep inside the earth right beneath Hawaii. (sort of the same as the hot “lava” welling up inside a lava lamp) When the plume hits the lower crust, the crust partially melts and the liquid rock works its way to the surface where it erupts as a volcano.

The cool thing is that the hotspot has remained pretty much stationary, but plate tectonics keeps moving the crust. That means that a volcano will form over the hotspot, but then the plate moves so that volcano goes dormant and a new one springs up nearby. In fact, if you look at a map of the pacific ocean floor, you can track the chain of islands and seamounts from the Big Island of Hawaii all the way up to the Aleutian trench, where the pacific plate gets subducted back into the mantle.

The Hawaiian volcanoes are known as “shield volcanoes”. Shield volcanoes are so named because they are very broad and convex, like a giant Greek warrior’s shield. They get this shape because the lava from a shield volcano is very fluid, and flows a long way before solidifying. As the lava oozes out over the surface, it continually builds up the “shield-shaped” profile. The Big Island of Hawaii is actually made up of several shield volcanoes. Here is a panorama that I took of the active Mauna Kea shield, as seen from the slopes of Mauna Loa.

Earth is not the only place in the solar system to have shield volcanoes. Venus and Jupiter’s moon Io have tons of them. But the best example of a shield volcano that we know of is Olympus Mons on Mars. Olympus Mons is a monster volcano: 27 km high and 550 km across. This image shows the outline of the entire state of Hawaii, superimposed on the enormous Olympus Mons.

Here’s another view, comparing the heights of Mt. Everest on the Earth, Maxwell Montes on Venus and Olympus Mons. (The horizontal scale has been drastically squashed; the slopes of Olympus mons should only be a few degrees.)

Why is it so big? Two reasons. First, plate tectonics doesn’t seem to work on Mars, so the hotspot underneath Olympus Mons just keeps feeding the same volcano, rather than constantly creating new ones. Second, the gravity on Mars is only about 1/3 that on Earth and the crust is much thicker and stronger. On Earth, when mountains get too big, the crust actually flexes and sinks. Much like an iceberg in the ocean, a mountain on earth is just the tip of a huge mass of rock that extends deep down. Mt. Everest is about as tall as a mountain can be on the Earth. Add any more rock to Everest, and it will just sink a little more. On Mars, with lower gravity and a stronger crust, mountains can be built up to a much greater height. That’s why Olympus Mons is about three times as tall as Mount Everest.

Explore posts in the same categories: Earth, Field Work, Geology, Volcanoes

26 Comments on “Shield Volcanoes”

  1. Obbedienza Nazi Says:

    succhiate il minchio

  2. Raphael Rosa Says:

    Given the strength of the Martian Crust and relatively (Compared to Earth) light gravity, it would seem reasonable to assume that the same kind of lava tubes and caves associated with volcanos here on Earth would be present on the slopes of Olympus Mons. With few quakes to destroy them, or water to erode them, there may be an extensive system of lava tubes and caves throughout the Martian volcano. In addition, we know that their is some (albiet very small) quantity of water vapor in the Martian atmosphere that could very well precipitate out as clouds and weather systems pass around and over Olympus Mons similar to the way that our own mountain ranges cause rainfall on the windward sides. This condensation could end up in the lava tubes, where protected from solar radiation, it remains as ice. The volcano might generate some internal heat? Protection from UV radiation + water + heat = life?

    • Nicole Says:

      I AGREE. However, there is also evidence that perhaps volcanic activity has ceased on Mars, but it isn’t conclusive. I wish the space program had tons more funding. I want a manned mission to Mars in the next ten years!

  3. Darcy Says:

    can u compare Olympus Mons to mauna loa

  4. Ryan Says:

    Mauna Loa is about 60 miles by 30 miles and its peak is 9,170 m from the ocean floor. Olympus Mons is about 340 miles across and is 27,000 m tall. In other words, even counting the part of Mauna Loa that is underwater, Olympus Mons is almost 3 times taller and almost six times as broad.

  5. Jackson F. Says:

    Another reason for Olympus Mons’ success is the lack of liquid water on mars. This means that much of the major erosion processes (frost wedging, chemical weathering, etc) are unable to take place, because they lack water. On Earth, there have been other mountains as tall as Everest, but over millions of years they were eroded away, because of the large amount of liquid water on the planet, as well as the large range of temperatures each year (freezing/thawing cycle).

  6. Me ( 10 years old ) Says:

    sir i do not understand very well
    what will happen if it explodes ( the olmpus mons )
    will lifes be generated on that planet again ??

  7. Ryan Says:

    Olympus Mons is not the sort of volcano that explodes. Its lava is too runny, so instead of exploding, the lava just oozes out and flows downhill.

    An eruption probably would not help life on Mars very much. It is possible that the extra heat in the ground would thaw any ice there and make it a nicer place for microbes to live, but it would not make the whole planet habitable.

  8. Joseph Says:

    Can you please tell me where you found the information regarding the strength of the Martian crust?

  9. Blogengeezer Says:

    Earth is most assuredly a ‘Special’ unique place no doubt. The fact that Plate Tectonics, so vital to Earths environmental consistency, do not function on Mars, is even more intriguing.

  10. Stu Says:

    I’m thinking of climbing it. I dont think it would be too difficult if the angle of the slopes is so slight. Maybe just taking a vehicle would be easier. It does look really impressive although you would never be able to compute the enormity it until you were stood looking at it wherby you were in a position to notice the scale. No amount of images compring sizes with Everest could do that.

  11. bob yjy Says:

    shield volcanoes are wierd

  12. Hampton Says:

    Does any one have an explanation for the apparent steep cliffs at the base of Olympus Mons? It looks like erosional forces may have caused this feature but could it also have been from a high pressure magma chamber lifting the entire structure from below? Thanks for any insight. J

    • Bill Says:

      There has been much speculation to nature of the scarps surrounding Olympus Mons.

      1. It is mass wasting (Landslides of the basal cliffs), the evidence lies in the tectonic/etched terrain called Olympus aureole.

      2. It is evidence of plate tectonic movements on Mars

      3. Structural collapse of the flanks due to the size of the volcanic edifice. Bill

  13. Garry Maxfield Says:

    Olympus Mons is not a volcano. It is only because we do not have another frame of reference that we say it is a volcano. When does a volcano have a smooth caldera? No sign of lava outflowing as well.

    • Ryan Says:

      If it’s not a volcano, what is it?

      The caldera of Olympus Mons isn’t smooth, it is made of multiple blocks of material that have dropped down to different levels and fractured and faulted in the process. There are plenty of volcanoes on earth that have done the same.

      And the surface of Olympus Mons is covered with lava flows, which are pretty good signs of lava outflowing…

  14. Ben Says:

    thank you for posting this… this will be a big help with the report im writing! And i was wonder if you now what matreails are on the mountain next time. Thanks!

  15. Elliot Bastien Says:

    Dear Michael Martine,
    Your image of the shield volcano was brought to my attention by Jay Robinson of Hawai’i Volcanoes National Park. He was responding to our request for a high res image of a shield volcano. Would you have a high resolution image that we could use in Montserrat’s Volcano Interpretation Centre?

  16. josh Says:

    was olmpus erupted?

  17. Joe Labriola Says:

    Makes you wonder between such a dynamic planet such as our Earth and the much less dynamic but still interesting Mars, what else must be out there in the universe…

  18. Rob Dunstan Says:

    Very informative site. Thank you for posting this.
    How tall (deep) are the walls of the caldera? Are there any mountain ranges on Mars? Or are there only extinct volcanoes and gorges and canyons for impacts? Even without plate tectonics, the surface still has an amazingly diverse structure.

  19. Rob Dunstan Says:

    .. Also, the panoramic view of the Hawaiian volcano (above) shows the typical shallow slope of a shield volcano. Is Olympus Mons shallower than this?

    • Ryan Says:

      I don’t know offhand the exact number, but the caldera is quite deep, possibly a kilometer or two. There are no mountain ranges on Mars like the ones on Earth that are formed by tectonics; most of the “mountain ranges” are impact crater rims.

      And yes, Olympus Mons is quite shallow. It is actually so broad that if you were standing on the rim, you couldn’t see the base because it would be below the horizon.

  20. spirit Says:

    G-sus Christ the last image was really good for the imagination! O.O!

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