Lecture 2 - Rock & the Rock Cycle

Rock

  • Any naturally occurring solid mass or aggregate of minerals or mineraloid matter

  • It is categorized based in minerals included, chemical composition and its genesis

  • Generally classified into three: Igneous, Sedimentary, and Metamorphic

Rock Formation

  • A mappable unit of rock consisting of an initially connected volume deposited in one interval of time

  • A formation may contain an assortment of different kinds of rocks but there is a prevailing character in the mix of rocks.

  • The distinctive character of a formation may relate to the engineering properties

Soils

  • Formed by weathering of rocks

  • The soil’s physical property is dictated primarily by the minerals that consists the weathered rock

The Rock Cycle

  • Magma is rock that is hot enough of being molten (800 - 1300°C)

  • Igneous rocks form from the cooling of magma

  • Extrusive (forms above surface; seconds to years to form)

  • Intrusive (forms within crust; centuries - millions of years to form)

  • Outcrop: Part of bedrock exposed at the surface. All exposed rocks are subject to some form of weathering (physical and chemical)

  • The weathering products are eroded, transported, and deposited as sediments. Sediments are the smaller particles from the rocks after weathering, erosion, transportation, and deposition process

  • Unless they are moved, they will be buried by more sediments or other materials

  • At depths of hundreds of meters, they are compressed and cemented to become sedimentary rocks

  • When rocks are buried deeper and heated up and squeezed, they form metamorphic rocks

  • Uplift - process of bringing material buried under the surface onto the surface

  • Burial - getting material into deeper depths

  • Uplift and Burial can happen to any kind of rock

  • Weathering, erosion, transportation, and deposition - are processes involved in breaking down rocks and transferring to another place until it settle in a place

  • Any rock can be brought up to the surface and part of it becomes an outcrop and be exposed to weathering and become sediments

  • Sedimentary rocks come from sediments which may come from any kind of rock. The rocks just need to be exposed and this usually means that these rocks are brought above the surface

  • Igneous rocks come from magma. Magma can form from metamorphic rocks

  • Metamorphic rocks can come from any kind of rock as long as they are buried deep enough to experience great temperature and pressure

Minerals

  • Naturally occurring inorganic compound with a definite composition

  • Make up rocks, although most of the rocks will have several different minerals in it

  • Only a small set of minerals are rock-forming: mostly silicates

Mineral Properties

  • Color - some minerals may have variations of color so it is not a reliable way to describe minerals by itself

  • Streak - color of the mineral in its powdered form

  • Luster - the way the mineral reflects light

    • Metallic

    • Dull

    • Pearly

    • Vitreous (glassy)

  • Hardness - measures the ability to scratch or be scratched

    • Mohs Scale of Hardness is used to determine how hard a mineral is by scratching it against reference minerals

  • Cleavage - The way a mineral breaks along a plane easily and smoothly based on its lattice. The plane of breakage is called the cleavage plane

  • Fracture - The way a mineral breaks if there are no cleavage planes

  • Density - Mass/Volume of a mineral. Specific weight (Weight/Volume) is simply the density multiplied by g.

Mineral Identification - Identification of minerals contribute to identification of rocks and their properties

Mineral Groups - minerals are grouped based on the predominant anion or anion complex

Mineral Formation

  • Most minerals forming the rocks are from the process of magma cooling

  • As magma rises, it cools, and minerals can crystallize

    • If cooling is fast (minutes to years), minerals may not have time to become ordered, and only small crystals are formed

    • If cooling is slow (decades to millions of years), higher level of ordering may be achieved, and larger crystals can form

    • If cooling is too rapid (seconds), the rock will be glassy, meaning no crystals were formed.

  • Minerals can also be formed via:

    • Precipitation from aqueous solution or gaseous emanations

    • Metamorphism

    • Weathering

    • Organically

Use of Minerals

  • Since minerals are found in rocks, a lot of the characteristics of minerals matter in our usage of rocks

  • Some minerals are mined to be used as additives or to be processed to make construction materials like gypsum, cement, and aggregates (sand and gravel used in concrete making)

  • Certain minerals that decay or become weak in certain conditions may be dangerous for certain sites/projects.