GEO Exam 1

Uniformitarianism

The principle that "the present is the key to the past" — the same geologic processes acting today (erosion, deposition, volcanism, plate movement) operated in the past at similar rates, so we can use present-day processes to interpret the rock record and Earth's history.

Solar Nebula Theory

The solar system formed from a rotating cloud of gas and dust (a nebula) that collapsed under its own gravity. Most material concentrated at the center to form the Sun; the remaining material flattened into a spinning disk in which planets formed by accretion.

Process by which planets formed

Dust particles in the disk collided and stuck together (accretion), forming larger clumps called planetesimals. Planetesimals collided and merged into protoplanets, which continued to grow through further collisions and gravitational accumulation of material until they became planets.

Order of the planets

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

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Three types of rocks and how they form

• Igneous – form from the cooling and solidification (crystallization) of molten rock (magma or lava).

• Sedimentary – form from the compaction and cementation (lithification) of sediment, or by precipitation from solution.

• Metamorphic – form when existing rock is transformed by heat, pressure, and/or chemically active fluids without melting.

Three compositional layers of the Earth

• Crust – thin, outermost layer; oceanic crust is basaltic (mafic), continental crust is more granitic (felsic).

• Mantle – thick middle layer, ultramafic composition rich in iron and magnesium silicates (e.g., peridotite).

• Core – innermost layer, composed mainly of iron and nickel.

Inner vs. outer core

Both are composed of iron-nickel alloy. The outer core is liquid due to high temperature and lower pressure, and its convective flow generates Earth's magnetic field. The inner core is solid because, despite even higher temperatures, the pressure is great enough to keep the iron-nickel alloy in a solid state.

Continental vs. oceanic crust

Oceanic crust is thinner, denser, and basaltic (mafic) in composition, formed at mid-ocean ridges. Continental crust is thicker, less dense, and more granitic (felsic) in composition. The main component of both is silicate minerals (rock built from silicon and oxygen).

Lithosphere and asthenosphere

The lithosphere is the rigid outer shell of the Earth, comprising the crust and the uppermost, rigid part of the mantle; it is broken into tectonic plates. The asthenosphere is the weaker, plastic/ductile layer of the upper mantle beneath the lithosphere, capable of slow flow, over which the lithospheric plates move.

Three types of plate boundaries and plate motion

• Divergent – plates move apart from each other (e.g., mid-ocean ridges).

• Convergent – plates move toward each other and collide (subduction or continental collision).

• Transform – plates slide horizontally past one another

Conditions for subduction zones vs. continental collision zones

A subduction zone develops where oceanic lithosphere (denser) converges with either oceanic or continental lithosphere; the denser oceanic plate sinks beneath the other plate into the mantle. A continental collision zone develops where two continental plates converge; since continental crust is too buoyant to subduct, the crust crumples and thickens, forming large mountain ranges (e.g., the Himalayas).

Five characteristics that define minerals

A mineral is (1) crystaline, (2) inorganic, (3) solid, (4) has a definite chemical composition, and (5) natural.

Physical properties used to identify minerals

Color, streak, luster, hardness, cleavage/fracture, crystal form/habit, specific gravity (density), and special properties such as magnetism, reaction to acid, or taste.

Fracture vs. cleavage

Minerals break by cleavage along smooth, flat planes of weakness that correspond to weaker bonds in the crystal structure. Minerals break by fracture (uneven or irregular breakage) when bond strength is roughly equal in all directions, so there's no preferred plane of weakness.

Element

a substance that cannot be broken down into simpler substances by chemical means; defined by its number of protons.

Atom

the smallest unit of an element that retains the properties of that element.

Isotope

atoms of the same element with the same number of protons but a different number of neutrons.

Ion

an atom (or group of atoms) that has gained or lost electrons, giving it a net electrical charge.

Compound

a substance formed from two or more elements chemically bonded in a fixed ratio.

Molecule

two or more atoms bonded together; the smallest unit of a compound that retains its chemical properties.

The number of _______ never varies in an atom

Protons (the number of protons defines the element/its atomic number).

Three types of chemical bonds

Ionic, covalent, and metallic bonds (van der Waals is sometimes added as a weaker bonding force).

Eight most common elements in Earth's crust; two most abundant

Oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. The two most abundant are oxygen and silicon.

Most abundant mineral group in the crust Silicates

are the most abundant mineral group. Their fundamental building block is the silica tetrahedron (one silicon atom covalently bonded to four oxygen atoms). These tetrahedra link together via shared oxygen atoms, forming covalent bonds between them.

Physical property influenced by silicate structure Cleavage

the way silicate minerals break) is strongly influenced by how the silica tetrahedra are linked (isolated, chains, sheets, or frameworks).

Extrusive (volcanic)

igneous rocks form when magma solidifies at the Earth's surface.

Intrusive (plutonic)

igneous rocks form when magma solidifies below the Earth's surface.

Texture and mineral composition

are used to classify igneous rocks.

Igneous rock texture is determined mainly by:

cooling rate of the magma (which controls crystal size — and also by silica/gas content for glassy or vesicular textures).

A phaneritic texture means

the crystals are large enough to see with the naked eye. Phaneritic texture indicates slow cooling of magma. Phaneritic texture indicates an intrusive origin.

Aphanitic

crystals too small to see without magnification; indicates fast cooling; extrusive origin

Porphyritic

a mix of large crystals (phenocrysts) in a fine-grained groundmass; indicates a two-stage cooling history (slow, then fast) — magma began cooling slowly at depth, then erupted/intruded shallower and cooled quickly.

Glassy

no crystals form at all; indicates extremely rapid (near-instant) cooling; extrusive.

Vesicular

full of gas bubble holes (vesicles); indicates rapid cooling with gas escaping; extrusive.

Pyroclastic

• fragmented rock formed from explosive volcanic eruptions; extrusive.

The primary component of all magmas is

silica (SiO₂).

Felsic magmas have the

highest silica content.

Mafic magmas have the

lowest silica content.

Mafic

basalt (extrusive), gabbro (intrusive)

Intermediate

andesite (extrusive), diorite (intrusive)

Felsic

rhyolite (extrusive), granite (intrusive)

Granite vs. rhyolite

same (felsic) composition, but granite is intrusive/phaneritic (coarse-grained, cooled slowly) while rhyolite is extrusive/aphanitic (fine-grained, cooled quickly).

Granite vs. diorite:

both intrusive/phaneritic, but they differ in composition — granite is felsic (high silica, light-colored, quartz + feldspar rich) while diorite is intermediate (less silica, darker, more plagioclase + amphibole).

Common igneous rocks with aphanitic texture

rhyolite, andesite, basalt.

Geothermal gradient

The rate at which temperature increases with depth in the Earth (roughly 25–30°C per km near the surface).

Decompression melting

Melting that occurs when hot mantle rock rises and the pressure on it decreases, without a significant loss of heat — lowering the pressure lowers the rock's melting point enough that it begins to melt even though temperature stayed about the same. Common at mid-ocean ridges and mantle plumes.

Effect of pressure on melting temperature of dry rock

Increasing pressure raises the melting temperature of dry rock (it takes a higher temperature to melt rock at greater depth/pressure).

Effect of pressure on melting temperature of wet rock

Adding water lowers the melting temperature of rock (water acts as a flux, allowing rock to melt at a lower temperature than it would if dry) — this effect is especially important at subduction zones.

Processes that create magmas of different composition, Partial melting

only part of a rock melts (usually the lower-melting-point minerals), producing magma with a different (typically more felsic) composition than the source rock.

Fractional crystallization

as magma cools, minerals crystallize out in a sequence (Bowen's Reaction Series); removing early-formed crystals changes the composition of the remaining melt, driving it toward more felsic compositions.

Magma mixing

two magmas of different composition combine to form a hybrid magma.

Assimilation

magma melts and incorporates surrounding country rock, changing its composition.

Common intrusive igneous rock structures Batholith

very large, irregular intrusive body.

Stock

smaller version of a batholith.

Dike

tabular intrusion that cuts across existing rock layers.

Sill

tabular intrusion parallel to existing rock layers.

Laccolith

lens-shaped intrusion that domes up overlying rock layers.

Mid-ocean ridges (divergent boundaries) –

decompression melting of rising mantle peridotite produces mafic (basaltic) magma.

Subduction zones (convergent boundaries)

water released from the sub-ducting oceanic plate lowers the melting point of the overlying mantle wedge (flux melting), producing magma that rises and interacts with continental crust, typically yielding intermediate to felsic (andesitic) magma.

Mantle plumes / hotspots

localized upwelling of very hot mantle material causes decompression melting, producing mafic (basaltic) magma (e.g., Hawaii).

Continental rift zones

similar to mid-ocean ridges but under continental crust; decompression melting of rising mantle, sometimes with crustal melting/assimilation, can produce a range from mafic to felsic magmas.

A'a'

blocky, rough, jagged-surfaced lava flow.

Pahoehoe

smooth, ropy-textured lava flow.

Lava tube

a tunnel formed when the surface of a lava flow cools and hardens while molten lava continues to flow beneath it, eventually draining out and leaving a hollow tube.

Lava dome

a steep-sided mound formed by the slow extrusion of highly viscous (usually felsic) lava that piles up near the vent rather than flowing away.

Fissure eruption

eruption of lava from a long crack (fissure) in the ground rather than a single central vent

Lava plateau / flood basalt

• a broad, flat landform created by huge volumes of low-viscosity basaltic lava erupting from fissures and spreading over a large area.