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The Rock Cycle
What is a Rock?
1. A rock is a hard substance composed of one or more minerals
2. They are the building blocks of the Earth
3. Man has used rocks for tools, weapons, and for buildings for millions of years
4. Rocks are placed into three groups according to how they form: igneous, sedimentary, and metamorphic
Igneous Rocks
Originally were hot, fluid magma inside the Earth
Ignis, comes from Latin, it means fire
Come from sediments that were carried along by wind and water
Over time these sediment become cemented together to form rock
When chemical changes occur in existing rock either by tremendous heat and/or pressure rocks metamorphosis, or
Meta means change, morph means form
Metamorphic = to change form
The Rock Cycle
Rocks are subject to change
Igneous and sedimentary rocks can change into metamorphic by heat and pressure
Sedimentary and metamorphic rocks can change into igneous rocks by complete melting and solidifying
Igneous and metamorphic rocks can change into sedimentary rocks through erosion
Igneous Rock
Igneous rocks form when magma cools and makes crystals.
1. INTRUSIVE: forms underground (IN the ground), where the magma cools slowly resulting in large crystals.
2. EXTRUSIVE: forms above ground (EXIT the earth), where the magma cools quickly resulting in small crystals
3. Volcanic igneous rocks: formed from molten rock that cooled quickly on or near the earth's surface.
4. Plutonic igneous rocks: are the result of the slow cooling of molten rock far beneath the surface of the earth.
classified according to their shape, size and position in the surrounding rock
Igneous rocks are classified according to their composition and texture
Composition: minerals from which the rock formed
Texture: the shape, size, arrangement and distribution of the minerals that make up the rock
Glassy: Textures are referred to as glassy (obsidian) when you can not see the crystals
Aphanitic: fine grained (basalt) very small interlocking crystals, very difficult to see with the “naked” eye
Phaneritic: coarse grained (granite) interlocking crystals roughly the same size
Porphyritic (Trachtye) large crystals scattered on a background of smaller crystals
Felsic: light colored, high silica content, contains quartz and feldspars
Mafic: dark colored, lower silica content, contains iron and magnesium
Ultramafic: low silica content, very high levels of iron and magnesium, very rare
The more silica the lighter the color
Magma composition
All igneous rocks are made of magma.
The silica content affects melting temperature and impacts how quickly the magma flows
BASALTIC: contains 50 % silica (makes basalt)
ANDESITIC: contains 60 % silica (makes andesite)
RHYOLITIC: contains 70 % silica (makes rhyolite)
Main factors involved in the formation of magma are temperature, pressure, water content and mineral composition
At the end of the crystallization process mineral rich fluid fills voids and cracks in the surrounding rock. The fluid
solidifies and can form veins of gold (or other rare minerals)
Pegmatites: Veins of extremely large grained minerals. This is where most of the gem stones are found
Kimberlites: Found in rare ultramafic rock, can only form extremely deep and under very high pressure. Diamonds are
only found in these deposits (mostly in S. Africa)
What characteristics are used to classify igneous rocks?
How are intrusive rocks similar to extrusive rocks? How are they different?
What determines the size and types of crystals in rocks?
What is the relationship between a rock’s texture and where it was formed?
What feature is most likely to contain diamonds?
Sedimentary Rocks: Slowly Built Layers
Sedimentary rocks are classified according to the origin of the material that they are made from:
Clastic, Organic, Chemical
Clastic sedimentary rocks are made from the fragments of existing rocks
They are further classified according to the size and shape of the fragments they are made of
Conglomerate / Breccia
These are made from pebbles cemented together with mud, clay, or sand
Over one third is made of pebbles
Made from particles the size of sand
At least half of the material must be sand to be classified as sandstone
Grains are cemented together by minerals
Most common sedimentary rock
Shale / Mudstone
Made from particles smaller than sand
Made from small particles of mud and clay
Most can be split into flat pieces
Organic Rocks
Organic sedimentary rocks are made either directly or indirectly from material that was once living
May be produced by the shells of once living organisms
Living organisms may produce limestone directly as in coral reefs
Fine grained limestone made from microscopic shells
Formed from the remains of plants that lived millions of years ago
Chemical Rocks
Formed when a sea or lake dries up leaving behind large amounts of minerals
Minerals can be deposited in caves
This category includes rock salt, gypsum, and some limestone
The Making of Sedimentary Rocks
Erosion and weathering of parent rock
Transportation of sediments
Deposition of sediments
Compaction and cementation of sediments
Lithification which turns the sediments into sedimentary rocks
Metamorphic Rock
Metamorphic rocks are rocks that have "morphed" into another kind of rock.
These rocks were once igneous or sedimentary rocks.
The rocks are under tons of pressure, which fosters heat build up, and this causes them to change.
Formed from older "parent" rock (either igneous or sedimentary) under intense heat and/or pressure at considerable
depths beneath the earth's surface.
Types of Metamorphism
Large areas of the Earth’s crust has been metamorphosed
Contact: When molten rocks come in contact with solid rock, metamorphic effects decrease farther away from the
intrusion of molten rock
Hydrothermal: Very hot water reacts with rock, dissolves some minerals, breaks down minerals and forms new minerals
Foliated: wavy layers and bands, most common is schist
Nonfoliated: blocky crystal shapes, quartzite and marble
Porphyroblasts: a large mineral crystal in a metamorphic rock, found in both regional and contact, some crystals grow
much larger than the surrounding crystals
Common Metamorphosed Rock
Deeper in the Earth with higher pressure metamorphic can morph into new metamorphic
EX: Schist can become Gneiss
•What does a rock want to be when it gets older?
1. How does the chemical composition of a rock change during metamorphism?
2. Name the three main types of metamorphism.
3. What causes foliated textures?
4. What parts of the rock cycle occur deep within the Earth?