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ROCKS – Unit Notes
Rocks
• A rock is any solid mass of mineral or mineral-like matter that occurs naturally as part of our
planet.
• The three major rock types are igneous rocks, sedimentary rocks, and metamorphic rocks.
Rocks vs. Minerals
• Some rocks are composed of just one mineral but many rocks, such as granite, occur as solid
mixtures of minerals.
• A key characteristic of rocks is that each of the component minerals retains their properties in the
mixture.
• Unlike minerals, some rocks are composed of organic matter, such as coal.
• Also, some rocks, such as obsidian and pumice, do not have a crystalline structure.
The Rock Cycle
• Interactions among Earth’s water, air, and land can cause rocks to change from one type to
another. The continuous processes that cause rocks to change make up the rock cycle.
• Rock Cycle Key Events
• Magma is molten material that forms deep beneath the Earth’s surface.
• When magma cools and hardens beneath the surface or as the result of a volcanic eruption,
igneous rock forms.
• Magma that reaches the surface is called lava.
Rock Cycle Key Events
• Weathering is a process in which rocks are physically and chemically broken down by water, air,
and living things.
• These weathered pieces of earth materials are sediments which are often moved by water, gravity,
glaciers, or wind.
• Eventually sediments are compacted and cemented to form sedimentary rock.
Rock Cycle Key Events
• Rocks buried deep within the Earth are subjected to extreme temperature and pressure.
• Under these extreme temperature and pressure conditions, sedimentary rock will change into
metamorphic rock.
Alternate Paths
• The rock cycle is not a circular path.
• Any rock can be buried and become a sedimentary rock. Igneous rocks can be put under pressure
and become metamorphic rock. As just reviewed, we saw one possible path for rocks. There are
many.
What causes The Rock Cycle?
• Processes driven by heat from Earth’s interior are responsible for forming both igneous and
metamorphic rocks.
• Weathering and the movement of weathered materials are external processes powered by energy
from the sun. External processes produce sedimentary rocks.
Igneous Rocks
• Igneous rocks form from when magma or lava cools and hardens.
Lava is on the surface and magma is still inside the Earth
Different kinds of igneous rocks form depending on where they are formed
Formation of Igneous Rocks
• The word igneous comes from the Latin word ignis, which means “fire”. Perhaps that is why
people often associate igneous rocks with fiery volcanic eruptions like the one shown below.
Intrusive Igneous Rocks
• Rocks that form when magma hardens beneath Earth’s surface are called intrusive igneous rocks.
ROCKS – Unit Notes
This is because they intrude into the existing rocks.
These rocks are only visible because erosion strips away overlaying rock.
Intrusive Igneous Rocks
• Magma contains many various elements (such as silicon and oxygen like Silicate Minerals) as well
as various gases (such as H2O).
• These gases are trapped within magma as it cools and rises (it rises because it is less dense than
the surrounding rock)
• As it rises it also cools which allows minerals to form and to slowly grow in size.
• Large Mineral Size - Granite
Extrusive Igneous Rock
• One major difference between lava and magma is that in lava, most of the gases have escaped.
• When lava hardens, the rocks that form are called extrusive igneous rocks.
This is because they are extruded onto the surface.
• These rocks cool very quickly so minerals do not grow large in size.
• Small Mineral Size - Rhyolite
Classification of Igneous Rocks
• Recall the images of granite and rhyolite.
Granite contains large mineral grains.
Rhyolite contains small mineral grains, only a few of which can be seen with the unaided eye.
• Texture and composition are two characteristics used to classify igneous rocks.
Texture describes the appearance of the size, shape, and arrangement of interlocking crystals.
Composition classes of igneous rocks are based on the proportions of light and dark minerals in the
rock.
Coarse and Fine Grained Texture
• Rate of cooling strongly affects the textures of igneous rocks.
• Slow cooling results in the formation of large crystals which creates a coarse-grained texture.
• Rapid cooling of magma or lava results in rocks with small, interconnected mineral grains which
creates a fine-grained texture.
Glassy and Porphyritic Texture
• Lava may cool so rapidly that the ions (positively or negatively charged atoms) may be unable to
arrange themselves into crystals.
This may result in a glassy texture such as obsidian
• When magma slowly cools deep within the Earth, some crystals may become quite large before
others even begin to form, creating porphyritic textures.
Granitic and Basaltic Composition
• One group of igneous rocks that is made of mostly light-colored silicate minerals of quartz and
feldspar is said to have granitic composition.
These are the major kinds of rocks that make up continental crust.
• Igneous rocks that contain mostly dark silicate minerals are considered to have basaltic
composition.
Other Compositional Groups
• Rocks with a composition between granitic and basaltic rocks have an andesitic composition.
• Rocks composed almost entirely of dark silicate minerals are referred to as ultramafic.
Ultramafic rocks are composed of heavier elements and are relatively rare at the Earth’s surface.
Sedimentary Rocks
• As we saw previously, sedimentary rocks begin to form when existing rocks are broken down into
sediments.
• Sediments, which consist mainly of weathered rock debris, are often transported to other places.
ROCKS – Unit Notes
When sediments are dropped, they eventually become compacted and cemented to form
sedimentary rocks.
Formation of Sedimentary Rocks
• The word sedimentary comes from the Latin word sedimentum, which means “settling.”
• Sedimentary rocks form when solids settle out of a fluid such as a water or air.
Weathering, Erosion, and Deposition
• Chemical Weathering – this takes place when the minerals in rocks change into new substances
through chemical change
• Physical Weathering – this takes place when physical forces (smashing, breaking, grinding down)
break rocks into smaller pieces.
Living things can cause physical and chemical changes, too!
Weathering, Erosion, and Deposition
• Weathered sediments don’t usually remain in place.
• Erosion involves weathering and the removal of rock.
When an agent of erosion (water, wind, ice, or gravity) loses energy, it drops the sediments.
This process is called deposition.
Compaction and Cementation
• After sediments are deposited, they often become lithified, or turned into rock.
Compaction and Cementation change sediments into rock.
• Compaction is a process that squeezes, or compacts, sediments.
• Cementation takes place when dissolved minerals are deposited in the tiny spaces among the
sediments.
Compaction and Cementation
• Classification of Sedimentary Rocks
• Just like igneous rocks, sedimentary rocks can be classified into two main groups according to the
way they form.
• Clastic Sedimentary Rocks – rocks that are made of weathered bits of rocks and minerals
• Chemical Sedimentary Rocks – rocks that form when dissolved minerals precipitate from water
solutions
Clastic Sedimentary Rocks
• Many different minerals are found in clastic rocks.
• Clastic sedimentary rocks can be grouped according to the size of the sediments in the rocks.
Chemical Sedimentary Rocks
• Chemical sedimentary rocks form when dissolved substances precipitate, or separate, from water
solution.
• This precipitation generally occurs when the water evaporates or boils off leaving a solid product.
• About 90% of limestones are formed from shells and skeletal remains of organisms that settle on
the ocean floor.
Features of Sedimentary Rocks
• The many unique features of sedimentary rocks are clues to how, when, and where the rocks
formed.
Layers of undisturbed sedimentary rocks record specific periods of sediment deposition with older
rocks in layers as you go down.
Fossils are also found uniquely in sedimentary rocks giving clues to climate based on the kind of
fossils found.
Metamorphic Rocks
• Recall that metamorphic rocks form when existing rocks are changed by heat and pressure.
• Metamorphism is a very appropriate name for this process because it means to change form.
• Rocks produced during metamorphism often look much different from the original rocks.
•
ROCKS – Unit Notes
Metamorphism
• Formation of Metamorphic Rocks
• Most metamorphic changes occur at elevated temperatures and pressures.
• These conditions are found a few kilometers below Earth’s surface and extend into the upper
mantle.
Most metamorphism occurs in one of two settings: contact metamorphism or regional
metamorphism.
Contact Metamorphism
• When magma intrudes (recall intrusive igneous rock) contact metamorphism may take place.
• During contact metamorphism, hot magma moves into rock.
• Such low grade metamorphism can cause limestone to change into marble.
Regional Metamorphism
• During mountain building, large areas of rocks are subjected to extreme pressures and
temperatures.
• Regional metamorphism results in large-scale deformation and high-grade metamorphism.
Agents of Metamorphism
• The agents of metamorphism are heat, pressure, and hydrothermal solutions.
• Heat
• The most important agent of metamorphism is heat.
Heat provides the necessary energy to drive chemical reactions.
• This heat comes from two sources: magma and the change in temperature with depth
Magma essential bakes rocks
Temperature increases between 20 C - 30 C per km
How hot is it deep in the crust?
• The deeper a person goes beneath Earth’s surface, the hotter it gets. In one of the deepest mines
in the world, the Western Deep Levels in South Africa, temperatures can be high enough to scorch
human skin.
• Miners often work in groups of two; one miner mines the rock and the other operates a large fan
that keeps the worker cool.
Pressure (Stress)
• Pressure, like temperature, also increases with depth. Like the water pressure you might have
experienced at the bottom of a swimming pool, pressure on rocks within the Earth is applied in all
directions.
• Increases in temperature and pressure cause rocks to flow rather than fracture (recall cleavage vs.
fracture)
Reactions in Solution
• Solutions of minerals that surround mineral grains in rocks aid in recrystalization but making it
easier for ions to move.
• When solutions exist in very hot water it increases the speeds reactions can take place.
• Hydrothermal solutions exist when hot, water-based solutions escape from a mass of magma.
Classification of Metamorphic Rocks
• Like igneous and sedimentary rocks, metamorphic rocks can be classified by texture and
composition.
• Metamorphic rocks can be foliated or nonfoliated.
Foliated Metamorphic Rocks
• When rocks undergo contact metamorphism, they become more compact and thus more dense.
• Under extreme conditions, certain minerals will recrystallize, some in a preferred orientation
which is at right angles to the direction of the force.
• This gives the rock a layered or banded appearance.
ROCKS – Unit Notes
• Foliated Metamorphic Rocks
Nonfoliated Metamorphic Rocks
• A metamorphic rock that does not have a banded texture is called a nonfoliated metamorphic
rock.
• Most nonfoliated rocks contain only one mineral.
• Marble, a metamorphized form of limestone, contains rearranged calcite crystals which forms a
stronger interlocked pattern.