Rocks are made up of minerals. Most rocks contain more than a single mineral but some are monomineralic (meaning composed of a single mineral).

A mineral is essentially a chemical compound that has a specific composition and a particular set of physical and chemical properties . One of these properties is called the ‘hardness”. The hardness really depends on the nature of the chemical bonds between the elements that make up the mineral. So for example, the common mineral quartz, SiO₂ is a mineral that possesses rather strong Si-O bonds; that is electrostatic bonds between the cation Si⁺⁴ and the anion oxygen O ^— (minus 2).

Since ions with opposite charges attract, the Si-O forms a strong chemical BOND. In contrast a mineral like calcite CaCO₃ is relatively soft because the bonds between the Ca⁺⁺ ions and the carbonate polyion CO^3-- is not so strong. So a limestone, a rock type, is composed mainly of calcite which is soft mineral hence limestone is easily eroded.

Not only is calcite soft but also because rainwater is SLIGHTLY acidic, calcite will dissolve! So there are two reasons why limestones weather: calcite is mechanically soft (its easy to scratch) but it is chemically unstable when soaked with a slightly acidic solution like rainwater.

Quartz on the other hand is VERY INSOLUBLE in rainwater and quartz, as the principal mineral in the rock we call sandstone is also hard (not so easily mechanically scratched and turned into a powder that can be washed away by a stream of water).

So the bottom line is that rock that are composed of soft minerals are more susceptible to weathering and erosion compared to rocks made up of tough minerals.

Great question! There are two ways to think about this.

First, rocks are made of minerals, and mineral hardness is an intrinsic property like mineral density. So each mineral has a specific hardness associated with it. Hardness is qualified using Moh's Hardness , scaling from 1, the least hard, to 10, the most hard. Mineral hardness is determined by the nature of chemical bonds in the crystal structure (that is, ionic versus covalent) and the crystal morphology (or shape and size).

Diamonds and quartz are very hard because they have three-dimensional lattice structures of covalently bonded atoms. Native metals, like aluminum and gold, are soft because they have closely packed atoms with metallic bonds.

The morphology ties a mineral hardness into the rock hardness. For example, if you had a piece of feldspar, they have a Moh's hardness of 6, and the crystal could not be scratched with a fingernail. If you had some clay, which has some of the same chemical bonds as feldspar, it would easily be scratched because clay is made up of a bunch of tiny crystals. So the crystal size and grain boundaries change the apparent hardness. This can also be seen in a sandstone, which may be scratched easily with a knife, even though the individual little quartz grains are of greater hardness. Moh's scale also assumes a pure mineral. However, any breakdown of the crystal can change the mineral hardness. For example, feldspar crystals often break down or are replaced by clay minerals, causing the mineral to appear less hard. For further complexity, some minerals have different hardness for different sides of the crystal due to differences in how the atoms are bonded together. For example, kyanite has a hardness of 4 on one plane and 6 on the other. Depending on which plane you scratched, you might see a range of hardnesses.

To tie everything back to rocks, the hardness is determined by which minerals make up the rock, the size of the minerals, how the minerals are joined (this includes, sub-grain cement of some sedimentary rocks, crystals grown together of igneous rocks, crystals stretched and melted together of some metamorphic rocks, and a variety of other ways that rocks stay together as an assemblage of many or few minerals.

Some rocks may be harder than others for many reasons. Rocks made up of larger minerals don't break down as easily as those with smaller minerals (because there is less surface area for erosion to work at breaking down). Also, sedimentary rocks are generally less hard than igneous or metamorphic rocks - this is because the lithification process (how a sedimentary rock becomes a rock) does not involve heat or pressure, and sedimentary rocks are kind of just "smooshed" together. Another reason could be that the minerals in the rock themselves might be harder - for example, serpentinite (a metamorphic rock which can include very soft talc-like minerals) is likely to be softer than a quartzite (a metamorphic rock made of metamorphosed quartz grains). I hope that this has answered your question. Have a great day!

In geology hardness has a special meaning. It is the amount of force required to scratch a mineral. But most people who aren’t geologist don’t think of hardness this same way, so I will give you two answers. The first is the geology definition of hardness. The second is why some rocks are more solid than others.

Mineral hardness (the geology perspective)

Because it is really hard (and impractical) to measure force like this, we typically use a relative hardness scale: “This scratches quartz but not diamond. So it is harder than quartz, but softer than diamond.” We call this relative hardness scale the “Mohs Hardness Scale” and it is just a list of ten minerals in order of how difficult they are to scratch:

1) talc (baby powder)
2) gypsum (a kind of salt)
3) calcite (the same stuff some snails and clams make shells out of)
4) fluorite (another kind of salt)
5) apatite (your tooth enamel is made of a variety of apatite)
6) orthoclase
7) quartz
8) topaz
9) corundum (rubies and sapphires are what we call gem-quality corundum)
10) diamond

Most people don’t carry all these around with them, so a quick way to test hardness is this:

your fingernail is 2-3. Steel (like a pocket knife or nail) is usually 5-6, glass is about 7.

The reason minerals have different hardness is because the forces that hold their atoms together are different. In some minerals (like diamond) all of the atoms are held together so strongly, that it takes a lot of force to break them apart. In other minerals, some of the atoms are only weakly connected so it is very easy to break them apart; the graphite in your pencil is a good example. It breaks apart when you rub it against almost anything, which is why it is good for pencils!

Why some rocks are harder or more solid than others?

In general, how strong a rock is depends on how much empty space there is in the rock and how strongly all the pieces of the rock are held together.

Some rocks are very hard to break, because they are made of crystals that have all grown together with no space left between them. Geologist like simple names, so we call these kind of rocks “crystalline rocks”. In everyday life, most people call crystalline rocks “granite”, but to a geologist, granite is only one specific kind of crystalline rock.

Some volcanic rocks are hard to break, because they are solid glass. When lava freezes before crystals can grow, it makes a natural glass; obsidian is just one kind of lava-glass. You might think glass is weak, but that is because most glass (like windows) is very thin. Natural glass is not as hard as crystalline rocks though.

Some volcanic rocks are very weak, because they have lots of bubbles. Lava always has some gas dissolved in it. As it cools, the gas comes out of the lava to form bubbles. If the lava freezes before the bubbles can reach the surface, they get trapped in the rock. Just like you might expect, a rock with lots of holes in it is weaker than a solid one.

Sedimentary rocks are rocks that are made when pieces of gravel, sand, or mud get compacted and glued together by other minerals growing between them. We call the minerals that grow to hold all of the pieces together “cement”. How hard a sedimentary rock is depends on three things:

1) How much space is there between all of the pieces of gravel, sand, and mud? If there is a lot of space, then the rock will be weaker, just like a volcanic rock with lots of bubbles. If there isn’t much space then the rock will be stronger.

2) What is the cement made of? If the gravel, sand, and mud are held together by a mineral like quartz, the rock will be stronger than if they are held together by a weaker mineral like gypsum or halite (table salt).

3) What is the sediment made of? Sandstone (a rock made by cementing sand together) is usually stronger than mudstone (a rock made by cementing mud together), because sand is mostly made of quartz (strong and pretty hard), while mud is made of minerals similar to talc (“clays” which are pretty weak and soft).

Rocks are made of minerals, and different minerals have different crystal structures. The chemical bonds that hold atoms together in these minerals are stronger in some than in others, and the atoms themselves determine which bonds are stronger than others. Stronger bonds make for stronger minerals and, thus, harder rocks.