Rocks : Materials of the Lithosphere

1. Rocks : Materials of the Lithosphere

2. Introduction • Earth as a System: The Rock Cycle • Basic cycle • Igneous Rocks: “Formed by Fire” • Magma crystallization to form rocks • Classifying and naming igneous rocks • Sedimentary Rocks: Compacted and Cemented Sediment • Classifying sedimentary rocks • Lithification of sediment • Features of sedimentary rocks • Metamorphic Rocks: Changed in Form • Three metamorphic agents • Metamorphism changes texture • Classifying metamorphic rocks • Resources from Rocks and Minerals • Metallic mineral resources • Nonemetallic mineral resources

3. Why study rocks? • Great economic value • Basic knowledge is essential to understanding Earth phenomena • Contain clues about the environments in which they formed

4. Earth as a System: The Rock Cycle • Earth consists of many interacting parts comprising the whole • Rock Cycle helps us understand the origin of igneous, sedimentary, and metamorphic rocks and how each is linked to the other.

5. The Basic Cycle • Magma is molten material inside Earth • Cools and solidifies – crystallization • May occur in the subsurface, near the surface or on the surface • Resulting rocks are called igneous rocks • If they are exposed on the surface, they will undergo weathering which will decompose or disintegrate rocks to form smaller particles • These particles are called sediment

6. These sediments may wind up in the ocean but some will be deposited in river floodplains, desert basins, swamps and dunes • They may be carried by wind, water or ice. • These sediments are thenlithified, turned into rock, to form sedimentary rocks • If these rocks are buried deep inside the Earth and are subjected to intense heat and pressure but do not melt, they will become metamorphic rocks. • If these rocks are subjected to further heat and pressure and undergo melting, they will solidify and form igneous rocks.

7. Sedimentary rocks can under melting if close to a heat source and become igneous. • Igneous and metamorphic rocks can undergo weathering and erosion and become sedimentary rocks. • Igneous and sedimentary rocks can under intense heat and pressure without melting and become metamorphic rocks. • And the cycle continues.

8. Igneous Rocks – “Formed by Fire” • Molten rock can originate as deep as 200 km within the Earth and consists primarily of Si, O, Al, Fe, Ca, Na, K, Mg as well others • It is called magma because it is found in the subsurface and it also carries much water vapor, along with CO2 and SO2. • It is less dense than surrounding rock so it tends to move upward over millions of years and causes a volcanic eruption

9. Molten rock that reaches the surface or is ejected outward is called lava because it has lost most of its gaseous content • Rocks that result from extruded lava are called volcanic or extrusive igneousrocks. • Magma that doesn’t reach the surface and solidifies at great depth are called intrusive or plutonic igneous rocks.

10. Crystallization of Magma • Magma, or melt, is a very hot, thick fluid containing solids and gases • Solids are mineral crystals • Liquid portion contains ions that move freely • As magma cools ions slow down and begin to arrange themselves into crystal patterns – crystallization • Usually not all molten material solidifies at the same time • As it cools, small crystals develop in a systematic fashion • When crystals are large enough for their edges to meet, their growth stops for lack of space and crystallization continues elsewhere • Eventually all the liquid is transformed into a solid mass of interlocking crystals.

11. Rate of Cooling • Influences crystal size • Slow cooling results in large crystals – can be seen with naked eye • Rapid cooling results in small intergrown crystals • If cooling is extremely rapid where ions had no time to arrange themselves, volcanic glass forms, which occurs during violent eruptions forming tiny glass shards called volcanic ash

12. Classification of Igneous Rocks • Igneous rocks are classified according to texture and mineral content • Texture refers to the size and arrangement of mineral crystals • Reveals how rapid or slow cooling occurred • Texture can be either: • Glassy – cooled extremely rapid with no crystals • Fine-grained (Aphanitic) – cooled close to the surface somewhat rapidly so only small crystals form • Coarse-grained (Phaneritic) – cooled very slowly deep inside • Porphyritic– started cooling deep inside with large crystals forming, but before all magma is solidified it is ejected close to the surface where rapid cooling occurred and small crystals formed. 2 sizes of crystals

13. Mineral composition is determined by what the magma is carrying in the melt • Large variety of igneous rocks occur from the same volcano • N.L. Bowen discovered that as magma cools in the laboratory, certain minerals crystallize first, at very high temperatures • Then as the temperature lowers, other minerals crystallize • If the mineral remains in the molten solution after crystallization, it will react with the remaining liquid to produce the next mineral in the sequence which is known as Bowen’s Reaction Series

14. Classifying Igneous Rocks • Granitic (felsic) refers to light colored igneous rocks with the dominant minerals being quartz, potassium feldspars and only less than 15% dark minerals (ferromagnesians) • Andesitic (intermediate) refers to medium color igneous rocks with the dominant minerals being amphibole, and intermediate plagioclase feldspars and 15-40% dark minerals • Basaltic (mafic) refers to dark colored igneous rocks with the dominant minerals being pyroxene and calcium rich plagioclase feldspars with over 40% dark minerals • Ultramafic refers to very dark colored igneous rocks with the dominant minerals being pyroxene and olivine which is 100% dark minerals

15. Granitic rocks that are coarse grained are called granite, if fine-grained they are called rhyolite • Andesitic rocks that are coarse-grained are called diorite, if fine-grained they are called andesite • Basaltic rocks that are coarse-grained are called gabbro, if fine-grained they are called basalt • Ultramafic rocks that are coarse-grained are called periodotite, if fine-grained they are called komatite • Glassy igneous rocks are named obsidian if glassy and has conchoidal fracture, pumice if frothy glass and full of holes that formed from escaping gases during a violent eruption, or scoria if rough with holes.

16. Sedimentary Rocks:Compacted and Cemented Sediment

17. Weathering begins the process of forming a sedimentary rock by breaking them down either by disintegration (smaller pieces) or decomposition (chemical changes) • Next, gravity and erosional agents (wind, water, ice) remove the products of weathering and carry them to new locations • While in transport they are broken down more • There they are deposited as sediment which is then compacted and cemented in a process called lithification, thus forming a sedimentary rock.

18. The weathering of bedrock is a continuous process • Piles of sediment accumulate, the materials at the bottom are compacted by weight of overlying sediment - compaction • Over time, sediment is cemented by materials carried by water in the spaces between the sediments - cementation • Sedimentary rocks account for only 5% by volume of the Earth’s outer surface • They form on the surface and later buried to form other types of rock depending on processes within the Earth like heat and pressure

19. What do sedimentary rocks tell us? • They give details of Earth’s history. • They hold clues to past environments. • They may exhibit characteristics about the method and distance of sediment transport. • They may contain fossils that tell much about past life forms and their evolution • Many are important economically such as: • Coal, oil and natural gas (fossil fuels) • Others are sources of important minerals such: • Aluminum, iron, manganese, fertilizer, sand, gravel

20. Classification of Sedimentary Rocks • Materials accumulating as sediment have two principal sources: • First, sediment may originate as solid particles from weathered rocks • These sediments are known as detritus • Rocks that form from these are called detrital, or clastic sedimentary rocks • Second, sediment formed from soluble material produced largely by chemical weathering and precipitated back as solids are called chemical or nonclastic sedimentary rocks

21. Detrital or Clastic Sedimentary Rocks • When a granite is weathered, the quartz is set free because it is so resistant • The clay minerals are the most abundant products of chemical weathering of the feldspars. • Geologists use particle size to distinguish among the sedimentary rocks

22. The four sizes are: • Clay <1/256 mm • Silt 1/16 – 1/256 mm • Sand 2 - 1/16 mm • Gravel – between 2 mm and 256 mm • Granule 2-4 mm • Pebble 4-64 mm • Cobble 64-256 mm • Boulder > 256 mm

23. When gravel-sized particles dominate, the rock is called a: • Conglomerate if particles are well-rounded, which means they have been transported long distances • Brecciaif particles are angular, indicating that they were not transported long distances because angles haven’t been worn off • If particles are sand-sized, the rock is called a sandstone • If particles are silt-sized, the rock is called a siltstone • If particles are clay size, which is very fine-grained, the rock is called a shale.

24. The size of the particle provides information about the environment in which the sediment was deposited • Currents of water and air can sort the particles by size • The stronger the current, the larger the size it can carry • Gravels are moved by swiftly moving flowing rivers, rockslides, and glaciers • Less energy is needed to transport sand, which is common in windblown dunes, river deposits, and beaches • Because silts and clays settle very slowly, accumulations of these materials are associated with very quiet waters of lakes, lagoons, swamps, or deep oceans.

25. Mineral composition can also be used to classify a clastic sedimentary rock • Most sandstones are mainly quartz because quartz is to resistant to chemical weathering so they may be called a quartz sandstone • A sandstone that is predominately orthoclase feldspar is called arkose and indicates that is hasn’t been transported a long distance because orthoclase weathers chemically easily. • Rocks consisting of only one size of particle is rare, so rocks containing both silt and sand can be classified as a silty sandstone or a sandy siltstone depending which particle dominates

26. Chemical or Nonclastic Sedimentary Rocks