Because there is more open space between the individual sediment grains than between the minerals in a crystallized rock.
The sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Most sedimentary rocks contain either quartz (especially siliciclastic rocks) or calcite (especially carbonate rocks). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. Sedimentary rocks tend to be more porous than igneous rocks because there is more open space between the individual sediment grains than between the minerals in a crystallized … rock. It was originally a clay (sedimentary rock) which became a shale through intens compaction (most often because of burial) which became a slate (metamorphic rock) t … hrough low-grade regional metamorphism. In general, compaction and low-grade regional metamorphism reduces porosity, so it is not really porous. Porous rocks. Rocks with rounded grains are more likely to absorb water than rocks with interlocking grains. This is because the water can get into the gaps between the grains. Rocks that absorb water are described as being porous. Rocks with rounded grains are usually softer and more crumbly than rocks with interlocking grains. So porous rocks tend to be softer than non-porous rocks. Porous rocks. Rocks with rounded grains are more likely to absorb water than rocks with interlocking grains. This is because the water can get into the gaps between the grains. Rocks that absorb water are called porous. Rocks with rounded grains are usually softer and more crumbly than rocks with interlocking grain Sedimentary rocks are types of rock that are formed by the deposition of material at the Earth 's surface and within bodies of water. Most sedimentary rocks contain either quartz (especially siliciclastic rocks) or calcite (especially carbonate rocks). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. Rocks are made of grains that fit together. Each grain in the rock is made from a mineral, which is a chemical compound. The grains in a rock can be different colours, shapes and sizes. Granite has interlocking grains. Sandstone has rounded grains. Some types of rock have interlocking grains that fit tightly together. Rocks with rounded grains are more likely to absorb water than rocks with interlocking grains. This is because the water can get into the gaps between the grains. Rocks that absorb water are called porous. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers which are called agents of denudation. Most sedimentary rocks contain either quartz (especially siliciclastic rocks) or calcite (especially carbonate rocks). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. Sedimentary rocks tend to be more porous than igneous rocks because there is more open space between the individual sediment grains than between the minerals in a crystallized … rock. Porosity of rocks  Consolidated rocks (e.g. sandstone, shale, granite or limestone) potentially have more complex dual porosities, as compared with alluvial sediment. Sandstone is a clastic sedimentary rock that forms from the cementing together of sand sized grains forming a solid rock. Quartz is the most abundant mineral that forms sandstone. Calcium carbonate, silica, or iron has been added to the water that is in contact with the sand grains. Sedimentary rocks are formed in three ways from these different sized sediments. A sedimentary rock is a layered rock that is formed from the compaction, cementation, and the recrystallization of
https://en.wikipedia.org/wiki/Sedimentary_rock http://www.answers.com/Q/Are_sedimentary_rocks_porous http://www.bbc.co.uk/education/guides/zgb9kqt/revision http://www.bbc.co.uk/bitesize/ks3/science/environment_earth_universe/rock_cycle/revision/2/ https://en.wikipedia.org/wiki/Sedimentary_rock http://www.bbc.co.uk/bitesize/ks3/science/environment_earth_universe/rock_cycle/revision/2/ https://en.wikipedia.org/wiki/Sedimentary_rock http://www.answers.com/Q/Why_sedimentary_rocks_are_often_porous http://volcano.oregonstate.edu/book/export/html/1056
why sedimentary rocks are porous