Chapter 2: Rocks, Materials of the Solid Earth

What is the starting material of the rock cycle?

Molten rock called magma, created by melting in the crust and upper mantle.

What is lava?

Magma that reaches Earth’s surface and erupts.

What is crystallization (or solidification)?

The process in which molten rock cools and solidifies.

What are igneous rocks?

Rocks that form when molten rock solidifies either below or above the surface.

What is weathering?

The slow disintegration and decomposition of rocks by the atmosphere.

What is lithification?

The conversion of rock particles (sediment) into sedimentary rock.

How does a sedimentary rock become a metamorphic rock?

When it is subjected to great pressures and intense heat.

What is extrusive (volcanic) rock, and where is it abundant?

Molten rock that solidifies at the surface. Abundant in the western U.S. (Cascade Range).

What is intrusive (plutonic) rock, and where is it abundant?

Molten rock that solidifies beneath the surface. More common than extrusive rock. Examples: Mount Rushmore, Yellowstone.

How does cooling rate affect crystal size in igneous rocks?

Slower cooling produces larger but fewer crystals. Fast cooling produces small but many crystals.

What happens if molten material cools instantly?

It forms no crystals and becomes glass. This usually occurs in volcanic eruptions that produce tiny shards of glass called volcanic ash.

What materials mainly compose igneous rocks?

Silicate materials.

What are the two major groups of silicate minerals in igneous rocks?

Dark silicates and light silicates.

Describe dark silicates.

Rich in iron and magnesium but poor in silica. Examples: olivine, pyroxene, amphibole, and biotite mica.

Describe light silicates.

Contain potassium, calcium, and sodium; richer in silica than dark silicates. Examples: quartz, muscovite mica, and feldspars (feldspars are the most common minerals in igneous rocks, ~40%).

What is felsic (or granitic) composition?

Igneous rocks with more light minerals.

What is mafic (or basaltic) composition?

Igneous rocks with more dark minerals.

What is andesitic (intermediate) composition?

Rocks that have at least 25% dark silicate minerals, with the other dominant mineral being plagioclase feldspar.

What is ultramafic composition?

Rocks composed almost entirely of dark minerals. Example: peridotite.

What does igneous texture describe?

The size, shape, and arrangement of minerals in a rock.

What is fine-grained texture?

Produced by rapid cooling; crystals are so tiny they need a polarizing microscope to be seen.

What is coarse-grained texture?

Produced when large masses of igneous rock slowly cool at great depth; crystals are large enough to be seen without a microscope.

What is porphyritic texture?

Forms when magma solidifies for a long time, producing large crystals (phenocrysts) embedded in a matrix of small crystals (groundmass).

What is vesicular texture?

Forms in the upper zone of a lava flow and exhibits voids (vesicles) that represent gas bubbles trapped as the molten rock solidified. The openings are nearly spherical.

What is glassy texture?

Forms when unordered ions are frozen in place before they can rearrange into an orderly crystalline structure. Can occur during volcanic eruptions.

What is pyroclastic (fragmented) texture?

Composed of individual rock fragments ejected during volcanic eruptions. Fragments can be very fine ash, molten blobs, or large angular blocks. Example: welded tuff.

Describe granite.

Coarse-grained felsic (granitic) igneous rock that forms when large masses of magma solidify at great depths.

Describe examples where granite is found.

Examples: Pikes Peak, Mount Rushmore, Yosemite National Park.

Describe rhyolite.

Extrusive equivalent of granite; fine-grained; frequently contains glass fragments and voids (vesicles). Example: Yellowstone National Park.

Describe obsidian.

A common type of volcanic glass that exhibits conchoidal fractures.

Describe pumice.

Forms when large amounts of gas escape from molten rock; vesicular texture; can float in water.

Describe andesite.

Extrusive intermediate rock; can be fine-grained or porphyritic, often with phenocrysts of plagioclase feldspar or amphibole. Examples: Cascade Range and Andes Mountains.

Describe diorite.

Intrusive equivalent of andesite. Looks like granite but has few or no visible quartz crystals and contains more dark silicates.

Describe basalt.

Fine-grained volcanic (mafic) rock made mostly of pyroxene, olivine, and plagioclase feldspar. Forms the upper layers of the oceanic crust.

Describe gabbro.

Coarse-grained intrusive equivalent of basalt. Forms part of the oceanic crust.

What is Bowen’s Reaction Series?

As magma cools, certain minerals crystallize first at relatively high temperatures. At successively lower temperatures, other minerals begin to crystallize.

List the order of crystallization in Bowen’s Reaction Series.

First: olivine. Then: pyroxene and plagioclase feldspar. At intermediate temperatures: amphibole and biotite. In the last stage: muscovite and potassium feldspar may form. Quartz crystallizes from any remaining liquid.

What is magmatic differentiation?

The formation of one or more secondary magmas from a single parent magma.

How does crystallization change the composition of the remaining melt (example from notes)?

Crystallization of olivine and pyroxene removes iron and magnesium, leaving the remaining melt more felsic.

What is crystal settling?

If early-formed crystals are denser/heavier than the remaining melt, they sink toward the bottom of the magma chamber.

What is mechanical weathering?

A rock is broken into smaller pieces, each of which retains the original mineral properties.

Describe frost wedging.

Water expands about 9% when freezing. When water freezes inside a crack, it enlarges the crack and eventually produces angular fragments.

Describe salt crystal growth.

Sea spray or salty groundwater penetrates crevices in rock. As the water evaporates, salt crystals form and grow larger, weakening the rock by enlarging the crack.

Describe sheeting (and give examples).

A large igneous mass forms beneath the surface under high pressure. As erosion removes the overlying rock (unloading), the outer parts expand and separate. Continued weathering causes slabs to fall off, creating exfoliation domes. Examples: Stone Mountain (Georgia) and Half Dome (Yosemite).

How can biological activity cause mechanical weathering?

Plants, animals, and humans can cause weathering.

What is chemical weathering?

Processes that alter the internal structure of a mineral by adding or removing elements.

How does oxygen dissolved in water contribute to chemical weathering?

It oxidizes some minerals, especially those rich in iron.

What is carbonic acid and how does it form?

Carbon dioxide dissolved in water ionizes to form the very reactive hydrogen ion (H⁺) and bicarbonate (HCO₃⁻).

Give an example of a mineral easily attacked by even weakly acidic solutions.

Calcite (CaCO₃), found in marble and limestone.

What are the end products of chemical weathering?

Clay minerals.

Outline the basic process that forms sedimentary rocks.

Weathering begins the process → deposition transports the sediments → the sediment may be lithified (turned to rock).

How does compaction occur?

As piles of sediment accumulate, the bottom layers are compacted by the weight of the overlying layers.

Why are sedimentary rocks especially useful for studying Earth’s history?

They make up a majority of Earth’s surface layer and record many details of past environments.

Name three economically important sedimentary materials.

Coal, petroleum, and natural gas.

What are detrital sedimentary rocks?

Rocks formed from solid particles (detritus) that originate from weathered rocks.

What are the main minerals that compose detrital sedimentary rocks?

Clay minerals (main product of chemical weathering) and quartz (highly resistant to chemical weathering).

How are detrital sedimentary rocks primarily distinguished?

By particle size.

Describe conglomerate.

detrital sedimentary rock made of gravel-sized pieces of rounded sediment.

Describe breccia.

Detrital sedimentary rock made of gravel-sized pieces of angular sediment (indicates the sediment has not traveled far).

Describe sandstone.

Detrital sedimentary rock in which sand-size grains prevail.

Describe shale (mudstone).

The most common sedimentary rock; made of very fine-grained sediment composed mostly of clay minerals.

Describe siltstone.

Fine-grained detrital rock made of clay-sized sediment plus slightly larger silt-sized grains.

What does particle size indicate about the depositional environment?

Large particle size indicates a strong current. Silt and clay settle slowly and indicate a quiet marine environment.

What is limestone primarily composed of, and how is most of it formed?

Composed of calcite (CaCO₃). About 90% forms from biochemical sediments secreted by marine organisms.

Describe coquina.

A biochemical limestone that is a coarse rock made of loosely cemented shell fragments.

Describe chalk.

A soft, porous rock made almost entirely of the hard parts of microorganisms. Example: White Chalk Cliffs of England (plankton).

How does inorganic limestone form? Give an example.

Forms when chemical changes or high water temperatures cause calcite to precipitate. Example: travertine.

Name the microcrystalline rocks formed by precipitation of dissolved silica (SiO₂).

Chert (red), flint (dark), jasper (red), and petrified wood (multi-colored). These contain many fine-grained quartz crystals.

What is coal?

An organic sedimentary rock that consists mostly of organic matter and is the end product of the burial of plant matter.

What is the ideal environment for plant matter to accumulate without decaying (oxidizing)?

Swamps.

List the stages of coalification in order (and note energy content).

Peat → lignite → bituminous → anthracite (this is also the order of increasing energy content).

What is peat?

The product of the partial decay of vegetative matter.

Which type of coal is the most widely available and used?

Bituminous coal.

Classify the main types of coal as sedimentary or metamorphic.

Lignite and bituminous are sedimentary; anthracite is metamorphic.

What is compaction in the lithification of sediment?

The weight of overlying sediment compresses deeper sediments. It works better for fine-grained minerals.

What is cementation in the lithification of sediment?

Cementing materials (calcite, silica, or iron oxide) carried by water fill pore spaces and act as glue. It works better for sand and gravel.

How can you identify the common cementing agents?

Calcite will effervesce (fizz) with hydrochloric acid. Silica produces the hardest sedimentary rocks (because quartz is hard). Iron oxide produces an orange/red color.

What do all metamorphic rocks have?

Parent rocks.

How does metamorphism progress?

Incrementally from slight changes (low-grade) to substantial changes (high-grade). Example: low-grade metamorphism of shale produces slate.

What are the two main settings of metamorphism?

Contact metamorphism (heat from magma) and regional metamorphism (great pressures and temperatures associated with mountain building).

List the agents of metamorphism.

Heat, confining pressure, differential stress, and chemically active fluids.

What is confining pressure?

Pressure due to burial in which forces are equal in all directions.

What is differential stress?

Unequal forces associated with mountain building.

How do chemically active fluids affect rocks during metamorphism?

When two minerals are pressed together, they may recrystallize and grow longer in a direction perpendicular to compressional stresses. Hot fluids escaping during crystallization of an intrusive magma can significantly change the overall composition of surrounding rocks.

What is foliation?

A nearly flat arrangement of mineral grains in a rock, mostly caused by compressional stresses that produce nearly parallel alignments. Examples: micas and metaconglomerates.

Under what conditions do nonfoliated textures typically form?

Where deformation is minimal and the parent rock has a relatively simple composition.

Describe slate.

Foliated rock made of miniature mica flakes; has excellent cleavage; forms by low-grade metamorphism of shale or volcanic ash.

Describe phyllite.

Degree of metamorphism between slate and schist; has a glossy sheen and wavy surface.

Describe schist.

Moderately to strongly foliated rock (the term “schist” refers to the texture); often derived from shale; platy minerals are prominent.

Describe gneiss.

Banded metamorphic rock in which elongated and granular minerals dominate; mostly composed of quartz and feldspar.

Describe marble.

Coarse crystalline nonfoliated rock whose parent rock is limestone. Can be colored if the parent rock contains impurities.

Describe quartzite.

Very hard nonfoliated metamorphic rock formed from quartz sandstone.

How are intermediate- to high-grade metamorphic rocks often named?

Newly formed crystals are called accessory minerals. When a rock has easily recognizable accessory minerals, the mineral name is added as a prefix.