What is the strongest rock in the world? It seems like a simple enough question but it’s not particularly easy to answer. I put on my researching hat and dove into the question of what the strongest rock is and why.
The strongest rock in the world is diabase, followed closely by other fine-grained igneous rocks and quartzite. Diabase is strongest in compression, tension, and shear stress. If mineral hardness is the determining factor of strength then diamond is technically the strongest rock in the world.
Rock strength is not always an easy thing to measure. It will vary from sample to sample even in the same rock type and depends on quite a few factors such as density, porosity, grain size and shape, and fracturing. Depending on what metric you use to determine rock strength the answer to this question might change, so let’s dive a little deeper.
How to Define Rock Strength
Crowning a single type of rock as the strongest in the world is more complicated than it might seem, mostly because there are many different ways to define strength. What are you doing to the rock to test its strength? Are you squeezing it? Pulling it apart? What if you’re trying to scratch it?
The answers to these questions will help determine the answer to the ultimate question of ‘What is the strongest rock?’. I’ll take a look at the most common ways to determine rock strength and which rocks are strongest using that definition.
For the purposes of comparing and quantifying some of these strength I’ll be talking in terms of Mega Pascals (MPa). If you’re like me, you probably have no idea how much force 1 MPa is. So just keep this in mind: 1 MPa is a little less than 150 psi (145 to be exact). Numbers are taken from a paper by Attewell & Farmer, 1976.
|Rock Type||Compressive Strength (Max MPa)||Tensile Strength (Max MPa)||Shear Strength (Max MPa)|
Strongest Rocks in Compression
Strength in compression is basically how hard you can squeeze something before it fails, or breaks. So when we talk about the compressive strength of rocks, we’re looking at how hard we can squeeze or press down on them before the break.
In general, rocks are incredibly hard in compression. Unless the rock is very thin (or you’re the Hulk) I doubt that you’ve ever been able to break a rock just by squeezing it. Just how strong a rock is depends on a few factors that we’ll get to in a bit.
The strongest rock in the world in compression is Diabase, which can stand up to 350 MPa. Diabase is a kind of fine-grained igneous rock. Close behind diabase in compressive strength come other fine-grained igneous rocks like Diorite, Gabbro, and Basalt. Interestingly, the one non-igneous rock in this super strong group is Quartzite.
Quick Conversion: 350 MPa is over 50,000 psi!
In my opinion, this is the best way to define how strong a rock is. In most applications, true rock strength is tested this way. All rocks will be stronger in compression than they are in any other mode, and I think it makes the most sense to compare their relative strengths when they are at their strongest.
Strongest Rocks in Tension
Strength in tension means how hard you can pull on something before it fails. In most settings, it is pretty rare that single rocks will ever be in tension, but on a larger level, rock layers are often in tension in fault settings. Entire rock units can be pulled apart due to enormous geological forces, causing them to break apart.
Rocks are much, much weaker in tension than they are in compression. In fact, most rocks are about 10% as strong in tension as in compression! The same basic trends emerge in relative strengths between types of rock, however.
As it turns out, Diabase is also the strongest rock in tension, able to withstand about 35 MPa of tensile force before it is torn apart. It is again closely followed by fellow igneous rocks diorite, gabbro, and basalt, and by the metamorphic Quartzite.
Strongest Rocks in Shear
Shear strength is how much force something can withstand when its being pushed in opposite directions. Think about grabbing a stick with two hands and then pushing one side while pulling the other (not bending) until it broke clean in two.
Rocks in shear stress tend to be about twice as strong as they are in tension, or 20% as strong as they are in compression. This is just a rough approximation, and it can vary quite a bit especially if the rock is foliated or layered and depending on the orientation.
If we’re choosing the strongest rocks based on their shear strength, some of the usual suspects come up again. Diabase, Basalt, and Quartzite all have top-end shear strengths of 60 MPa. Close runner ups are Granite and Limestone at 50 MPa.
Sometimes when people ask what the strongest rock is, they are really wondering what the hardest rock is. In geology, hardness is how resistant a mineral is to scratching.
Hardness is measured using Mohs Hardness Scale. This is a non-linear scale that ranks minerals based on their relative hardness. It runs from 1 to 10, and every mineral can scratch all the minerals below it on the scale.
Since all minerals are also rocks, diamond is the hardest rock. Rocks that contain more than one mineral can’t really have a single ‘hardness’ rating because each of the minerals they are comprised of will have a different hardness.
For example, most granite is made up of quartz, feldspar, and mica. The quartz grains will have a hardness of 7, while the feldspar is a 6 and mica is around 2.5 on the hardness scale. Depending on where you are scratching the granite you will observe a different hardness.
In my opinion, this is not the best way to determine how strong a rock is. Strength is a separate property from hardness, and since most rocks are comprised of more than one type of mineral it doesn’t even make sense to compare rocks in this way.
Why Are Some Rocks Stronger Than Others?
We have covered the different ways to quantify the strength of a rock, but what is it that makes some rocks stronger than others? The primary determining factor of rock strength is the contact surface area of its mineral grains. The more contact surface area there is between the grains, the stronger the rock will be. There are 5 attributes in a rock that contribute to this.
- Density – This is basically how heavy the individual grains of a rock are. A rock with heavier individual crystals is going to have a higher density than a rock with lighter grains. This is more technically referred to as ‘bulk density’ and it only takes into account the density of the indivual grains, ignoring any of the space in between.
Not surprisingly, Diabase has the highest bulk density (3.05 g/cm3) of any of the rock types in Table 1. The weakest rocks on the list are sandstone and shale, and they both have bulk densities of around 2.4 g/cm3.
- Porosity – To the surprise of many people, rocks often contain a lot of empty space. Porosity is a measure of that empty space between the grains of the rock. These pores are usually filled with fluid like water or oil, but especially on the surface, they can be filled with air. Think of rocks like pumice which are famous for being able to float in water. It can do this because it contains so many little air pockets – pores!
Rocks that are higher in porosity tend to be weaker. This is because the pores separate the rock grains, preventing them from forming bonds with one another. The less porosity there is, the more room there is for material which can essentially grab on to itself and hold the rock together. It’s like tearing apart a slice of swiss cheese compared to a same-sized slice of cheddar.
- Grain Size – The size of the grains of a rock can have a big impact on how strong it is. The larger the grains, the fewer points of contact there are between them in the same volume of rock. Think about if you filled a tin can with marbles. Then think about filling that same can with sand instead. The amount of surface area the sand grains are touching each other is much, much greater than that of the marbles.
The more surface area in a rock where the grains are touching each other, the more opportunity there is for the grains to bond with one another. This is why fine-grained rocks tend to be much stronger than coarse-grained rocks.
- Grain Shape – As in life, when it comes to rock strength, size isn’t all that matters! The shape and orientation of the individual grains are also a big determining factor of rock strength.
Especially in highly metamorphosed rocks, grains can develop an elongated shape and tend to point in the same direction. This typically ends up weakening the rock because it introduces structural weaknesses. However, in some rocks (like Quartzite) the rock grains have simply been ‘smushed’ more closely together which in turn makes them significantly stronger.
- Integrity – All of the numbers that I’ve given above apply to rocks that are intact and whole. In nature, however, it is more common than not that rocks will be fractured and weathered to some degree or another. Fractures can range in size from obvious and visible to the naked eye all the way down to the microscopic level.
If a rock has a significant degree of fracturing or weathering then it will lose a great deal of its strength. These fractures sometimes heal over time through geological processes but they will likely be the first points of failure if subjected to huge amounts of stress.
Strongest Sedimentary Rocks
So it seems like the fine-grained igneous rocks were the clear winner in just about all of our strength tests, but the sedimentary rocks are probably feeling left out. The igneous rocks at the top of our strength lists have the huge advantage of being very low in porosity and pretty fine-grained.
There are four types of sedimentary rock in Table 1: Sandstone, Shale, Limestone, and Dolomite. Of those four sedimentary rocks, Limestone and Dolomite tend to be the strongest sedimentary rocks, with the edge likely going to Dolomite. Speaking from my experience drilling hundreds of wells I can tell you that we hate drilling in Dolomite because it is super tough and drills very, very slowly.
Sedimentary rocks are highly variable so it’s very difficult to pin down just one number for any of these tests. There can be large differences in the porosity, grain size, and cementation levels of all of these rock types.
Limestone and Dolomite both have compressive strengths of about 250 MPa. This is quite a bit more than both Shale (100 Mpa) and Sandstone (170 Mpa) and is actually surprisingly in line with the strength of Granite.