Weathering rates of rock vary widely regional based on the following factors:

• Mineral composition of the rock (some minerals are less stable at surface conditions than others)
• Amount of plant life growing on the rocks and mountains. (Plant roots can be powerful chemical and physical weathering agent)
• Amount of precipitation (Chemical dissolution due to rainfall is a significant weathering factor for some rock types (carbonate rocks)
• Seasonal freezing periods. (Ice expansion in cracks and fractures can weaken rocks over very short periods of time.
• Temperature range of the region. (I believe diurnal temperature differentials specially in high mountain deserts has a specific weather impact on some rock types).
• Local earthquakes and volcanic activity.
• Erosion due to water (streams, rivers, lakes, and oceans).
• Erosion due to glaciers and ice flows.

You can get an estimate of weathering rates of different rock types for a local region by careful examination of cemetery gravestones.

• Metamorphic

• Igneous

• Sedimentary

The process these rocks go through is called the rock cycle.

There are also some substages:

• Magma (Lava; molten rock)

• Sediment or soil (microscopic rocks we cannot see; sorta like body cells)

After it decomposes, what does it turn into?

Well, more rock. Just in another stage of the rock cycle.

(usually sedimentary)

As mentioned above, rocks are always changing form, and they do go through a number of substages. But after a rock decomposes, it will become sediment, then after a number of years will compress into a form of sedimentary rock (e.g. Sandstone, shale, etc.).

And as @jamesqf mentioned above, some sedimentary rock can be crushed with your bare hands.