Volcanic mountains are formed either through the introduction and accumulation of magma over a crustal "hot spot" or through volcanic activity associated with the collision and subduction of a crustal plate, where the melting of the subducted crust creates magma, gases and pressures that are released in eruptions.

Continental crustal plate collisions, where neither plate subducts, create mountain ranges such as The Himalayan Mountain Range, where crustal material is thrust upward faster than erosion can wear it down.

Plates can stretch until they crack and slide, forming fault-block mountains.

In the ocean, great underwater mountains are formed when plates spread away from one another, and melted rock pushes up through the gap, creating hot, buoyant elevations of new crust (a simplification of the process).
Many mountains inclueding Everest were created by two of Earths tectonic plates pushing against each other.

This an area of common misconception. Most sedimentary rock, and the fossils contained therein cannot be dated directly with radiometric techniques, but can be dated indirectly. Here's the facts:

• Because most sedimentary rock is composed of particles of pre-existing rock of various types, each particle can be a different age.
• Carbon-14 or carbon dating techniques can only be used to date organic material which is of a maximum age of 70,000 years; this leaves out the vast majority of fossils found in rock, and only covering roughly .000016 percent of geologic time.
• The dating of fossils was originally done by their placement in a geologic column of rock strata. Because of the Law of Superposition, the deeper the stratum, the older it is (in an undisturbed body of rock or sediment). Fossils which were evident in a particular stratum, but missing from other strata were noted as "index" fossils. The presence of these fossils indicated the age of the stratum relative to other layers. The layers of stratum were placed in a geologic column, subdivided, and assigned names based on the presence of different types of fossil organisms. The use of fossils in this manner is known as biostratigraphy and is the basis of "relative dating" of rock layers and fossils, a dating technique that was in effect until the advent of radiometric techniques in the twentieth century.
• Radiometric dating of rock involves the measurement of the amount of decay in radioactive elements which are present in the rock. Radioactive elements decay at unique rates, depending on the isotope. This rate of decay is known as half-lives, it is the time necessary for ½ of the atoms to decay in a particular element. The decay follows a geometric scale, in that in the first half-life of an element, ½ of the atoms decay, yet in the second half-life, ½ of those remaining decay and so forth. By measuring this decay, and knowing the half life of an element, scientists can date a sample. Radiometric dating is particularly useful in dating igneous and metamorphic rock.
• Because of geologic events such as plate tectonics and volcanism, suitable material for radiometric dating, such as volcanic tuff, solidified lava, and igneous intrusions have been found as layers on, in, and cut through layers of sedimentary rock. Intrusions are always younger than the rock body they penetrate, meaning that the sedimentary rock in which intrusions are found will be older than a radiometrically dated sample of the intrusion. Lava flows and volcanic ash which form layers in rock will be younger than the rock below and older than the rock above. The solidified intrusions and lava flows can be dated with radiometric techniques.
• The radiometric dating of suitable rocks (i.e. igneous or metamorphic) in proximity to their sedimentary counterparts, therefore allows the sedimentary rock to be dated as well.

No, far from it. Limestone is actually quite soft, rating only a 3 or 4 on the Mohs scale.

Care should be taken when making generalisations about rock properties.

Hardness is a function of a material's elastic modulus (how much it deforms under a given load).

Limestone (like all rocks) is a natural Earth material. As such it can display a very large variability in properties.

For example limestones can have Young's modulus values ranging from 9 to 70 MPa.[A] This is clearly very variable, but the upper bound values are comparable with values derived for "hard" rocks such as metamorphic hornfels and the coarser grained igneous rocks such as granite, whereas the lower bound values are very soft indeed by rock standards.

So to answer the question, no, limestone is not the hardest rock, however some types of limestone can be classed with some of the hardest rocks, whereas other types may be some of the softest.

[A] Bell, F. G. (2007). Basic Environmental and Engineering Geology. Dunbeath, Whittles Publishing Limited.

1. Used as aggregate or base for roads and foundations.
2. Used in the purification of molten glass.
3. Used to remove impurities from molten iron.
4. Used as filler and abrasive in toothpastes.
5. Used in the production of Portland cement.
6. Used as an aggregate in concrete.
7. Used as a soil conditioner for neutralizing acidic soils.
8. Used as a source of calcium in supplements and food additives.
9. Used to make paper white.
10. Used in the purification of sugar.
11. Used in the manufacture of brake pads.
12. Used in the manufacture of medicinal antacids.
13. Used in the preparation of wools and dyes.
14. Used as a construction stone in buildings.
15. Used as counter tops and flooring.
16. Used as a water neutralizer.
17. Used as a paint additive.
18. Used as landscaping rock.