Rocks formed from thermal metamorphism are very hard. An example is hornfels. Just search thermal metamorphism and/or hornfels and you'll see some nice explanations on the web.
The rocks are formed due to the erosional activity of air. The hard part remaining are the balancing rocks and the soft sediments around the rocks got eroded by the wind.
hard strong rocks
Sedimentary rocks are formed from buildup of sediment, or particles of dirt and organic debris and things like that. Think of sedimentary rocks like the bottom of a river bed- stuff builds up on top until it becomes hard and solid- a rock, so pressure is a factor, but not necessarily heat. It is usually metamorphic rocks that involve both heat and pressure.
Both igneous and metamorphic rocks are formed by pressure and some kind of heat
Metamorphic rocks are formed from sedimentary and igneous rocks. When sedimentary or igneous rock is subjected to heat and pressure, the rock gets compressed. Therefore, if you had the same amount of metamorphic rock and sedimentary or igneous rock, the metamorphic rock would be heavier, because there is more 'stuff' in the metamorphic rock. ---------------------------------------------- However please note: There are types of igneous rocks that are hard (eg granite) and others that are soft (eg volcanic ash). There are types of metamorphic rocks that are hard (eg gneiss) and others that are soft (eg marble). Thus "hardness" is not a characteristic that can be used to distinguish between the two.
IGNEOUS refers to rocks that originate in the interior of the Earth, and include the various volcanic types and the granite group of rocks. There is a 1:1 correspondence between the granite types of rock, and the volcanic ones - it depends upon whether they have been melted or not. Granites usually have well differentiated crystals, and are hard. SEDIMENTARY rocks are those that have result from the weathering of the above igneous rocks, and in general are deposited as fine-grained sediments at sea, and this will eventually form rocks. METAMORPHIC rocks are generally formed from sedimentary rocks that have been subjected to heat and pressure. Consequently, the mineral assembly will differ from that of the sedimentary, new minerals will be formed, and shear and folding are common. The degree of metamorphism varies depending on the burial pressure and temperatures. In the extreme of melting, then granites will result if the cooling is very slow, or volcanics will form if the rock is ejected in the molten state. Marble is a curious metamorphic rock in that it may be formed from the hard bodies of tiny marine animals, but with a low degree of metamorphism, often fossils may still be recognized. At extreme metamorphism, it will become a matrix mainly of CaCO3.
The Earth's hard surface is primarily composed of rocks and minerals. These rocks can be classified into three main groups: igneous, sedimentary, and metamorphic rocks. Igneous rocks are formed from the cooling of molten magma, sedimentary rocks are formed from the accumulation and cementation of sediments, and metamorphic rocks are formed through the alteration of existing rocks under high pressure and temperature.
The rocks are formed due to the erosional activity of air. The hard part remaining are the balancing rocks and the soft sediments around the rocks got eroded by the wind.
Generally speaking, when a geologist talks of "soft rocks" and "hard rocks" what he or she means is "sedimentary rocks" or "igneous and metamorphic rocks". Limestone is a sedimentary rock, and compared to most other rocks, is relatively soft. lol Different limestones have different hardnesses. Most "marbles" are actually hard limestones, because they have not experienced the high pressures and temperatures required for metamorphism.
igneous rocks form
hard strong rocks
quartzite is made from quartz. in the case of metamorphic rocks, quartzite is made from regional metamorphism. in the process of regional metamorphism, pressure is applied making the rock hard and dense.
Erosion is when rocks or other hard materials are grinded away over years by wind, snow, ice, water, sand, etc. Deposition is the silt that is deposited by a river. Metamorphism is when a rock changes stage such as from igneous to metamorphic or sedimentary to igneous.
Quartzite is a nonfoliated metamorphic rock that formed by the metamorphism of pure quartz sandstone. The intense heat and pressure of metamorphism causes the quartz grains to compact and become tightly intergrown with each other, resulting in very hard and dense quartzite. I found this on Minerals Education Coalition
Fossils are formed in rocks when the hard body parts of an organism get buried in sediment and are preserved when it turns to rock.
Metamorphic rock is generally hard, as it forms through the intense heat and pressure of metamorphism, which causes the minerals in the rock to recrystallize and align in a more compact and dense structure. This results in a rock that is usually more resistant to weathering and erosion compared to softer sedimentary rocks.
While rocks are not flammable, extreme heat can cause certain rocks to melt or crack due to thermal expansion. However, rocks do not undergo combustion or burn in the same way that organic materials do.