In eastern Ontario, Canada, there's a place where, when walking and stomping, you can hear a hollowness in the ground.

The area is primarily igneous rock with many large boulders, and the spot in question is directly over a very large buried boulder.

The boulder is under 15cm of gravel and 30cm of sand.

There are caves in this area.

Is the hollow sound an indication of a cavern/caves, sinkhole, geode or something else?

  • $\begingroup$ Can you specify the location more precisely? What sort of caves are these? Would the boulders be glacial deposits or landslides? $\endgroup$ Aug 8 '19 at 4:17

It is extremely unlikely to be a sink hole. They are characteristic of waterlogged limestone country such as is found in Florida. Hollows are sometimes found in flows of igneous rock where there has been volcanic activity. Some hollows are large gas bubbles which were trapped when the rock around them solidified, but probably the most common hollows in volcanic places are lava tubes, formed when a stream of lava solidified in its outer layer while the still molten interior drained away. Your talk of boulders and gravel leads me to believe there has been glacial activity in the area. It isn't possible to say what is causing this hollow sound without a personal, on-the-spot investigation, but I think we can say it's not a sink hole, but might be a volcanic gas hollow or an ancient lava tube. On the other hand it might be something else, it's impossible to say without digging down to it. If the site is limestone and not volcanic, that would put a whole new perspective on it. The caves you refer to might give a clue. It won't be a geode; they are tiny things, usually about the size of a cricket ball and with the hollow half-filled with quartz crystals. Geodes are characteristic of limestone geology, not igneous rock.

  • $\begingroup$ I agree sinkholes to be unlikely. The crust covering a lava tube is interesting, but of all the sources I've found, they claim geodes are often found in igneous rock as a result of lava cooling. Also, the geode in Put-in-Bay, Ohio measures 35 feet in diameter, so I don't see a 6 foot one to be impossible. It all comes back to that hollowness for me and how it increases as I get to the center. That, to me, suggests a hollow sphere or dome. $\endgroup$ Aug 6 '19 at 19:01
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    $\begingroup$ You may be right, but the only kind of geode I have come across is the small kind I described. Its probably just a matter of terminology. What I have called gas bubbles are probably what some other people call geodes, but that leads to confusion because what many people, including myself, usually understand by the term are the small, hollow pebbles I told you about.. $\endgroup$ Aug 6 '19 at 19:15
  • $\begingroup$ @Chimera.Zen The nearest volcanism (very ancient) seems to be on the Quebec border by Noranda. $\endgroup$ Aug 8 '19 at 4:21

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