Rocks

Lesson 6: Rocks

Content Standard

The learners demonstrate an understanding of the three main categories of rocks, and the origin and environment of formation of common minerals and rocks.

Learning Competency

The learners shall be able to make a plan that the community may use to conserve and protect its resources for future generations. The learners will be able to classify rocks into igneous, sedimentary and metamorphic (S11/12ES- Ib-10).

Specific Learning Outcomes

At the end of the lesson, the learners will be able to

1.  Classify and describe the three basic rock types;

2.  Establish relationships between rock types and the origin and environment of deposition/formation;

3.  Understand the different geologic processes involved in  rock formation

ROCKS

Rocks are aggregate of minerals. It can be composed of single mineral (e.g. Quartzite, a metamorphic rock composed predominantly of Quartz) or more commonly, as an aggregate of two or more minerals. A mineral name can be used as a rock name (e.g. Gypsum Rock which is composed predominantly of the mineral Gypsum (CaSO4)).

A.      Igneous rocks - rocks that are formed from the solidification of molten rock material (magma or lava).  Molten rock material can solidify below the surface of the earth (plutonic igneous rocks) or at the surface of the Earth (volcanic igneous rocks).  Minerals are formed during the crystallization of the magma. Note that the rate of cooling is one of the most important factors that control crystal size and the texture of the rock in general.

Magma is a molten rock material beneath the surface of the earth. Lava is molten rock material extruded to the surface of the earth through volcanic or fissure eruptions.

Plutonic or intrusive rocks

·         from solidified magma underneath the earth

·         gradual lowering of the temperature gradient at depth towards the surface would cause slow cooling/crystallization

·         Phaneritic texture

·         Examples: granite, diorite, gabbro

Volcanic or extrusive rocks

•        from solidified lava at or near the surface of the earth

•        fast rate of cooling/crystallization due to huge variance in the temperature between Earth’s surface and underneath

•        common textures: aphanitic, porphyritic and vesicular

•        examples: rhyolite, andesite, basalt

•        pyroclastic rocks: fragmental rocks usually associated with violent or explosive type of eruption. Examples tuff and  pyroclastic flow deposits (ignimbrite)

Igneous rocks are also classified according to silica content: felsic, intermediate, mafic and ultramafic.

•        felsic: also called granitic; >65% silica, generally light-colored

•        intermediate: also called andesitic; 55-65% silica; generally medium colored (medium gray)

•        mafic: also called basaltic; 45-55% silica; generally dark colored

•        ultramafic: <45% silica; generally very dark colored; composed mainly of olivine and pyroxene which are the major constituents of the upper mantle

B.      Sedimentary rocks- These are rocks that formed through the accumulation, compaction, and cementation of sediments.  They generally form at surface or near surface conditions.

•        Sedimentary processes at or near the surface of the Earth include: weathering of rocks, sediment transport and deposition, compaction and cementation

•        Factors in sedimentary processes: weathering and transport agents (water, wind ice)

•        Common sedimentary features: strata and fossils

•        Strata: >1cm is called bedding and anything less is called lamination; layering is the result of a change in grain size and composition; each layer represents a distinct period of deposition.

•        Fossils: remains and traces of plants and animals that are preserved in rocks

Non-clastic / Chemical/Biochemical – derived from sediments that precipitated from concentrated solutions (e.g. seawater) or from the accumulation of biologic or organic material (e.g. shells, plant material).  They are further classified on the basis of chemical composition.

Clastic/terrigenous - form from the accumulation and lithification of sediments derived from the breakdown of pre-existing rocks.  They are further classified according to dominant grain size.

C.      Metamorphic rocks - rocks that form from the transformation of pre-existing rocks (igneous, sedimentary, or metamorphic rocks) through the process of metamorphism.  Metamorphism can involve changes in the physical and chemical properties of rocks in response to heat, pressure, and chemically active fluids. They are commonly formed underneath the earth through metamorphism.

Contact metamorphism

•        Heat as the main factor: occurs when a pre-existing rocks get in contact with a heat source (magma)

•        Occurs on a relatively small scale: around the vicinity of intruding magma

•        Creates non-foliated metamorphic rocks (e.g. hornfels)

Regional metamorphism

•        Pressure as main factor: occurs in areas that have undergone deformation during orogenic event resulting in mountain belts

•        Occurs in a regional/large scale

•        Creates foliated metamorphic rocks such as schist and gneiss

•        Non-foliated rocks like marble also form thru regional metamorphism, where pressure is not intense, far from the main geologic event.

THE ROCK CYCLE

The rock cycle illustrates how geologic processes occurring both at the surface and underneath the Earth’s surface can change a rock from one type to another.

DOWNLOAD THE DOCX VERSION HERE

ALTERNATIVE LEARNING RESOURCE

Watch the Youtube video regarding Rock Cycle: