- Sedimentary rocks are rocks formed by the deposition, compaction and cementing of small particles (such as silt, sand and pebbles). These particles are deposited in layers or strata.
- The compaction of these sediments is caused by the weight of more sediments laid down on top of them.
- The cementation is caused by the hardening of calcium carbonate which is the chemical that makes up shells and coral.
|TYPE OF SEDIMENTARY ROCK
||PARTICLES OF WHICH ROCK IS COMPOSED
||Sand, pebbles (similar to concrete)
|Shale (or Mudstone)
||Silt or Mud
||Crushed shells or Coral
- Igneous Rocks are formed by heat from magma (which is molten rock beneath the earth's surface), and are usually associated with volcanoes. There are 2 types of igneous rocks - plutonic and volcanic.
- Plutonic rocks are igneous rocks that from deep beneath the earth's surface. The molten magma cools very slowly forming large crystals in rocks such as granite.
- Volcanic rocks are igneous rocks that form above the earth's surface. They cool quickly and form small crystals. Example are basalt (made from lava) and pumicestone (formed when lava is forcefully ejected from a volcano and air bubbles form within the rock).
- Metamorphic rocks are any rocks that have been changed by heat and pressure of volcanic or other earth movements.
- Metamorphic rocks may have originally been either sedimentary or igneous or metamorphic rocks.
|NAME OF ORIGINAL ROCK
||TYPE OF ORIGINAL ROCK
- The main colour
- Colour is not a reliable guide as some colours are caused by small amounts of impurities
- The shininess or dullness of the surface
- Lustre can be earthy (dull), vitreous (glassy), brilliant (shiny), pearly (shiny but not glossy), metallic (like a shiny metal)
- The colour of the powdered mineral
- Scratch the mineral on a tile and observe the colour left on the tile
Cleavage and Fracture
- This is how the mineral breaks. Fracture can be uneven (rough), even (nearly smooth), and conchoidal (curved lines where the mineral was hit).
- If it breaks along a smooth surface, this is due to cleavage. Different minerals show either none, one, two or three cleavages.
- A mineral can be scratched by something harder than it is. Hardness can be of 5 types - very soft (scratched by a fingernail), soft (scratches fingernails but not coins), medium (scratches a coin but not a knife blade), hard (scratches a knife blade but not glass), and very hard (scratches glass)
MOH'S SCALE OF HARDNESS
Moh's Scale of Hardness is a scale of 10 minerals from very soft talc to very hard diamond: