Unit 2.2 Earth Flashcards
Describe how igneous rocks are formed.
Cooling of magma, underground (intrusive) or on the surface (extrusive).
List the principal elements that make up the Earth’s crust.
- oxygen (45%)
- silicon (27%)
- aluminium (8%)
- iron (6%)
- calcium (5%)
- sodium
- potassium
- magnesium
Describe how fragmentary (sedimentary) rocks are formed.
Accumulation of sediments, which undergo lithification through compression and cementing.
Describe how metamorphic rocks are formed.
Igneous or sedimentary rocks which are subjected to extreme heat and pressure, to the point at which their constituent minerals reorganise.
What are the distinguishing features of igneous rocks?
Interlocking crystals with random orientation.
What are the distinguishing features of sedimentary rocks?
Grains which may be cemented but not interlocking.
The presence of fossils would suggest fragmentary rock, but may not be present.
What are the distinguishing features of metamorphic rocks?
Interlocking crystals arranged into bands / oriented in the same direction.
Name the major rock forming silicate minerals, describe their structures at a molecular level, and account for the place of metal ions in the crystal structure.
Mafic minerals have simpler structures and contain Mg, Fe (and Ca), while felsic minerals are more complex and may contain Al, K and Na.
Quartz comprises silicon and oxygen molecules arrange in a 3D structure.
Feldspar also has a 3D structure. All feldspars contain Al, but orthoclase feldspar is rich in K. Plagioclase feldspar ranges from Na- to Ca-rich (mafic).
Mica has a 2D sheet structure. It ranges from Al-rich to Mg- or Fe-rich.
Amphibole contains Mg, Fe, Ca and Al, arranged in a 2D double chain.
Pyroxene contains Mg, Fe and Ca in a 1D singe chain structure.
Olivine contains Mg and Fe in isolated groups.
Place the major silicate-based rock-forming minerals in order of melting.
Felsic rocks have lower melting points, and so partial melting produces a magma which is more felsic than its source.
- quartz
- feldspar
- mica
- amphibole
- pyroxene
- olivine
Place the major silicate-based rock-forming minerals in order of formation from magma.
Mafic minerals crystallise at lower temperatures than felsic minerals, so forming magma earlier.
- olivine
- pyroxene
- amphibole
- feldspar
- mica
- quartz
Place the major silicate-based rock-forming minerals in order of susceptibility to weathering.
Mafic minerals, with their simpler structures, are the most susceptible to weathering.
- olivine
- pyroxene
- amphibole
- feldspar
- mica
- quartz
What is orthoclase feldspar?
Feldspar which is rich in potassium.
What is plagioclase feldspar?
Feldspar which ranges from Na-rich (felsic) to Ca-rich (mafic).
What rock is formed as a mafic magma cools within the Earth?
Gabbro.
What rock is formed as a mafic magma cools at the surface?
Basalt.
What rock is formed as an intermediate magma cools within the Earth?
Diorite.
What rock is formed as an intermediate magma cools at the surface?
Andesite.
What rock is formed as a felsic magma cools within the Earth?
Granite.
What rock is formed as a felsic magma cools at the surface?
Rhyolite.
Describe conglomerate in relation to fragment size and composition.
Rounded, pebble-sized fragments of rock, above 2mm in diameter.
If the rock is composed of angular fragments, it is termed breccia.
Describe sandstone in relation to fragment size and composition.
Feldspar, quartz grains and mica flakes, between 2mm and 0.63mm.
Describe mudstone (shale) in relation to fragment size and composition.
Grains below 0.063mm in diameter. Predominantly clay minerals.
Describe the different types of limestone and explain the origins of their constituents.
Oolitic limestone is formed of lithified ooids: tiny nuclei, potentially grains of sand or shell fragments, which attract a build-up of calcium carbonate.
Shelly limestone is formed from the calcium-rich shells of marine creatures.
Crinoidal limestone is formed from crinoids, a marine animal.
Outline the general structure of kaolinite, and (in general terms) its formation from other silicate materials.
Kaolinite is a 1:1 clay. Every silicon-oxygen tetrahedral sheet (SiO4) is stacked on an aluminium-oxygen octahedral sheet. The interlayer between each pairing is negligible.
Kaolinite is the most widespread clay, and is the residual end product of weathering: when feldspars are dissolved by aqueous carbon dioxide, they release metal ions and soluble silica, leaving behind kaolinite.