9A6-ROCKS

ROCKS

What is a Rock?

Naturally-occurring mixtures of minerals, mineraloids, glass or organic matter.
- The crust of the Earth is made up of rocks of various types.
Term ‘rock’ refers not only to hard materials like granite but also to soft and loose particles like sand, silt and clay .

In other words:-

- Minerals are generally crystalline in appearance.
- Are homogeneous in form while rocks are heterogeneous in their composition.
- Are made up of chemical elements.
- About 2,000 minerals are known to exist in the crust
- These combine in different proportions to form various kinds of rocks.

ROCKS

INTRODUCTION:

-Major part of earth’s crust – formed by rocks and minerals
-Minerals have definite chemical composition by which they can be identified.
-ROCK – mixture of various minerals
- An aggregate of minerals that forms a more or less definite unit of the lithosphere.
-Rock is a consolidated and compact mass of more than two minerals

TYPES OF ROCKS

1. Igneous Rocks (made by “fire”) - Solidified from molten rock (i.e., magma).
2.Sedimentary Rocks - Deposited and buried at Earth’s surface.
3. Metamorphic Rocks (“changed form”) - Transformed from preexisting rocks under high pressure and temperature.Distinguishing Characteristics
Mineralogy - Constituent minerals and their relative proportions.
Texture - Sizes, shapes, and arrangements of minerals within the rock, e.g.,
–Course-grained
–Fine-grained
–Foliated (planar fabric)
All are clues to a rock’s origin and history.

- Magma is molten rock

–Lava is magma that is on the Earth’s Surface

An igneous rock is a rock that is created by a volcano. Some rocks cool quickly and are very shiny.

-Other rocks cool as they fly through the air and release lots of air, causing lots of holes.

IGNEOUS ROCKS ARE:
-Are of thermal origin and are formed by the process of solidification of molten rock material called magma.
- Such magma erupts during volcanic eruptions and on reaching the Earth’s surface, gets solidified by cooling.
- So known as primary rocks
- Magma may get solidified below the surface of the earth.
- COOLING and SOLIDIFICATION are 2 processes involved in the formation of igneous rocks.
They are:-
- Form by solidification (crystallization) of melted minerals
- At the surface, LAVA hardens to form EXTRUSIVE rocks with tiny (FINE-GRAINED) crystals or GLASSY (no crystal) TEXTURES
- Beneath the surface, MAGMA hardens to form INTRUSIVE rocks with easily visible (COARSE-GRAINED) crystal texture.

- Below ground = from magma (intrusive igneous rock)

- Usually have LARGE crystal grains (they cooled slowly)

Where are igneous rocks formed?

Igneous rocks form when magma pours out of a volcano as lava and cool.

TYPES OF IGNEOUS ROCKS

Minerals crystallize from melt, derived from deep within Earth’s crust or mantle
–High temperatures, up to 700° C or more!!
–Crystal size depends on cooling rate.
uIntrusive rocks cool slowly within deep magma chambers:
–Course, interlocking crystals
uExtrusive rocks cool rapidly at (or near) the surface of the earth:
–Fine-grained, often “glassy”

1. INTRUSIVE IGNEOUS ROCKS:

- When molten materials of earth’s interior (magma) do not reach earth’s surface, they cool and solidify below the surface – are called intrusive igneous rocks

Eg. Granite, diorite, gabbro
- Called Plutonic rocks
- Have larger crystals, compact, resisten and glassy in appearance
- E.g. Quartz, batholith
Rate of cooling is slower.
Been formed at great depths..
A. SILLS: some of the lava may push its way through passages in the form of sills or dykes.
uWhen magma is being forced upwards, some of the lava may push its way in between two layers of rock and get solidified there.
uThe intrusive rock formed – called sill – almost horizontal
B. DYKES: Magma also forces its way before reaching the main fissure, trying to go up but gets solidified before reaching the surface.
- These intrusive rocks lying in a slanting direction as an off shoot are called dykes.

2. EXTRUSIVE IGNEOUS ROCKS:
- When lava pours out on Earth’s surface through volcanic eruptions or fissures, it cols and soldifies to form extrusive igneous rocks.
- Rate of cooling is more rapid than in the interior. So crystals are finer.

examples of extrusive igneous rocks:











































According to chemical composition:
1.ACIDIC IGNEOUS ROCKS: acidic rocks are light coloured , less dense. Granite , crystals, feldspar, mica
2.BASIC IGNEOUS ROCKS: contain lower percentage of silica, higher percentage of oxides of denser elements – iron, aluminium, or magnesium. Basic rocks are denser, darker in colour than acidic rocks.
3.ULTRA BASIC ROCKS: consists of ferre, magnesium

–Does a rock melt like an ice cube, all at the same time?Rocks melt according to their melting points.
–Example: Ice cube with wax

Which would melt first ice or wax?
This example is known as partial melting.
–Partial Melting: some minerals melt at lower temperatures and other minerals remain solid
–Think of “stew”


Igneous rocks

SEDIMENTARY ROCKS

Derived from Latin word – sedimentum=settling down
Denudation is the disintegration & decomposition of rocks, as well as the wearing away of the surface of the land.

Transportation of sediments and formation of sedimentary rocks by mode of river water- deposition on the continent and on the ocean floor.

SEDIMENTARY ROCKS:

In other words, SEDIMENTARY ROCKS are the secondary rocks which are formed from the loose fragments detrital or clastic sediments produced by weathering of older rocks.
•Almost 90% of earth crust is made up of igneous rocks
•75% of land surface on the earth is covered by thin veneer of sediments or sedimentary rocks.
•These sediments are transported and deposited by river water, wind or by movement of glacial ice. Transportation is either in suspension or in solution.
•When settle down on the beds of ocean, river and lakes undergo compaction/cementation for millions of years to form SEDIMENTARY ROCKS
•These consolidated layered strata are known as stratified rocks.
•The strata varies in thickness and may be tilted or horizontal.
•Sediments consist of loose particles or gravel, sand, silt and clay in various proportions – hence – porous and permeable.
•Loose particles get consolidated or compacted into hard rocks by presence of cementing substances like lime or the presence of overlying deposits.They are separated by bedding planes.

LITHIFICATION:
•Lithification is the cementation, compaction and hardening of sediments into sedimentary rocks
•Called secondary rocks – they are derived by denudation of other pre-existing primary or parent rocks.
•Called stratified rocks – as sediments are deposited in waterbodies, they get sorted out according to their size.
•Sediments accumulate in different layers or stata arranged one above the other
•Each layer or stratum has particles of a given size.
•Presence of remains of plants and animals between layers of sediments
•These remains get preserved between strata of sedimentary rocks.
•These organic remains or their skeletal impressions are called fossils.
•Fossils help us in fixing the relative ages of rocks.

IMPORTANCE OF SEDIMENTARY ROCK:
“Present is the key to the past”
•Helps in knowing depositional environment viz. marine (ocean deposits), fluvial (river deposits), aeolian (wind deposits), glacial, estuarine, Lacustrine (lake deposits) etc.
•Helps in knowing the provenance (i.e. source area of the sediments); change in climatic conditions i.e. in knowing and understanding old climate=paleoclimate.
•Sedimentary rocks are subject-divided on the basis of the nature of sediments, origin, composition and mode of formation.
Sedimentary rocks may be made of rock fragments—sediments—or by chemical reactions. The classification of sediments is shown below.

Sedimentary Rocks

How They are Made?

Wind and water break down the earth
* Bits of earth settle in lakes and rivers
* Layers are formed and build up
* Pressure & time turn the layers to rock
Formed from sediments (rock fragments, mineral grains, animal & plant remains) that are pressed or cemented together or when sediments precipitate out of a solution.These sediments are moved by wind, water, ice or gravity.
Sedimentary rocks represent 7% of the Earth’s crust, but they cover 70% of the Earth’s surface.
Sedimentary rocks are fossil-carrying rocks.

What turns sediments into solid rock?
Water or wind breaks down and deposits sediment (erosion & deposition)















The heavy sediments press down on the layers beneath (compaction)

Dissolved minerals flow between the particles and cement them together (cementation)

How can sedimentary layers help us understand the age of fossils?


As sedimentary rocks are deposited, they form horizontal layers
Scientists know that the layers on top (and the fossils in the top layer) are YOUNGER than the fossils in lower layers.











Examples
Limestone made when calcite mineral precipitates from sea water






Rock Salt—made from evaporation of sea waters


Loose particles (sand, silt, marine shells) accumulate on shorelines, basins, rivers, etc.,

–Clastic Sediments
Minerals precipitate from dissolved chemicals in water
–Chemical & Biochemical Sediments
All are the products of Weathering - that breaks up and decays rocks, and Erosion - that transports from source to point of deposition

What is a sedimentary rock?
A sedimentary rock is a rock that is made of layers of sediment (sand, clay, mud) close to the earth’s surface.

Where are sedimentary rocks formed?
Sedimentary rocks are formed under rivers, lakes, oceans, or streams.

TYPES OF SEDIMENTARY ROCKS (EXAMPLES)

METAMORPHIC ROCKS

Rocks that have changed
They were once igneous or sedimentary
Pressure and heat changed the rocks
Rocks that have changed due to intense temperature and pressure
“Meta” means “change” and morphosis means “form” in Greek
Igneous, sedimentary and other metamorphic rocks can change to become metamorphic rocks
High temperatures and pressures at depth cause changes in mineralogy, texture, and composition
–Changes take place in Solid State by recrystallization and chemical reactions
–Temperatures greater than 250°, less than 700°
Regional Metamorphism - High pressures and temperatures derive from regional collision, deformation and mountain building.
Contact Metamorphism - Locally high temperatures, adjacent to intrusions.

TYPES OF METAMORPHIC ROCKS:

What is a metamorphic rock?
A metamorphic rock is any sedimentary or igneous rock that has been changed, or morphed, because of pressure and heat.
These rocks are made deep inside the earth where heat and pressure change the rocks .

Examples:

* Marble

* Schist

What occurs in the Earth to change these rocks?
* Pressure from overlying rock layers
* High heat, but not enough to melt the rock
* Rocks may be flattened or bent or atoms may be exchanged to form new minerals.

How can Starbursts represent the Rock Cycle? Which rock form does your stacked Starbursts represent?
Now press your sedimentary rock in the palm of your hands for at least 2 minutes (do not twist) Some have large & small crystals (called porphyritic)













* Above ground = from lava (extrusive igneous rock)
* Usually have SMALL or NO crystals (they cooled too quickly)




























Basalt Igneous Rocks —made from lava/magma that is low in silica, rich in iron and magnesium. Rocks are dark-colored.



















Granite Igneous Rocks—made from magma/lava high in silica and oxygen. Rocks are light-colored.




















Andesitic Igneous Rocks—have a composition between basaltic and granitic.
















Practice Classifying Igneous Rocks according to their composition:

Diorite

Granite

ROCK CYCLE
Once a rock is formed, does it stay the same rock forever?
NO

- Rocks are continually changed by many processes, such as weathering, erosion, compaction, cementation, melting, and cooling
- Rocks can change to and from the three types
What is the process through which rocks change?
The Rock Cycle—earth materials change back and forth among the different types of rocks
The parent material of all rocks are igneous rocks. As soon as the igneous rocks come out of the surface of the Earth, they are eroded by different agents of eroison. The rock material changes into sediments. The sediments deposit in layers at some place to sorm sedimentary rocks. These sedimentary rodks are again buried into the Earth due to forces of the Earth. If these sedimentary rocks go very deep in the Earth they melt, change into lava and again come out as igneous rocks. If these sedimentary rocks donot go very deep, they may change into metamorphic rocks or igneous rocks which are again converted into sediments whenever they come out of the surface of the Earth. In this way, a cyclic process of changing the form of rocks is formed. This is called Rock Cycle.