Earth's Layers, Plate Tectonics, and Hot Spot Geology

Earth's three compositional layers

Crust, mantle, and core.

Crust

Earth's thin, solid, outermost compositional layer.

Mantle

The thick layer of mostly solid, slowly deforming silicate rock between the crust and core.

Core

Earth's innermost layer, made primarily of iron and nickel.

Earth's five physical layers

Lithosphere, asthenosphere, mesosphere, outer core, and inner core.

Lithosphere

Rigid outer layer consisting of the crust and uppermost mantle; broken into tectonic plates.

Asthenosphere

Weak, plastic-like region of the upper mantle that slowly flows beneath the lithosphere.

Mesosphere

The stronger, mostly solid lower mantle.

Outer core

Liquid iron-nickel layer that helps generate Earth's magnetic field.

Inner core

Solid iron-nickel center of Earth.

Continental crust

Thicker, less dense, generally granite-rich crust that forms continents.

Oceanic crust

Thinner, denser, generally basalt-rich crust that forms ocean floors.

Why are continents higher than ocean basins?

Continental crust is generally thicker and less dense, so it floats higher on the mantle.

Continental shelf

Shallow, gently sloping submerged edge of a continent.

Continental slope

Steep descent from the continental shelf toward the deep ocean.

Continental rise

Sediment-covered region at the base of many continental slopes.

Abyssal plain

Broad, relatively flat area of the deep ocean floor.

Mid-ocean ridge

Underwater mountain chain where plates diverge and new oceanic crust forms.

Ocean trench

Long, deep depression commonly formed where one plate subducts beneath another.

Seamount

Underwater volcanic mountain.

Island arc

Curved chain of volcanic islands commonly formed above an ocean-ocean subduction zone.

Continental drift

Wegener's idea that continents were once joined and have moved to their present positions.

Alfred Wegener

Scientist who proposed continental drift and the former supercontinent Pangaea.

Pangaea

Ancient supercontinent that contained nearly all Earth's continents.

Evidence for continental drift

Matching continental coastlines, fossils, rock formations, mountain belts, and ancient climate evidence.

Why was Wegener's theory initially rejected?

He could not provide a convincing mechanism for moving continents.

Plate tectonics

Theory that Earth's lithosphere is divided into moving plates that interact at their boundaries.

Three types of plate boundaries

Divergent, convergent, and transform.

Divergent boundary

A boundary where plates move apart.

Divergent boundary features

Mid-ocean ridges, continental rifts, new crust, shallow earthquakes, and volcanic activity.

Convergent boundary

A boundary where plates move toward each other.

Convergent boundary features

Subduction zones, trenches, volcanic arcs, mountain ranges, and earthquakes.

Transform boundary

A boundary where plates slide horizontally past each other.

Transform boundary features

Strike-slip faults and usually shallow earthquakes; volcanoes are generally absent.

Oceanic-continental convergence

Dense oceanic lithosphere subducts beneath continental lithosphere, forming a trench and volcanic mountain chain.

Oceanic-oceanic convergence

One oceanic plate subducts beneath another, commonly forming a trench and volcanic island arc.

Continental-continental convergence

Continents collide, producing large mountain ranges and earthquakes, generally without a volcanic arc.

Subduction

The process in which one lithospheric plate sinks beneath another and enters the mantle.

Continental rift

A region where continental lithosphere stretches, thins, and may split apart.

Oceanic divergent boundary

A plate boundary where seafloor spreads apart and new oceanic crust forms, usually at a mid-ocean ridge.

Transform fault

An active plate-boundary fault where plates slide horizontally past each other.

Fracture zone

A seafloor feature extending beyond an active transform fault, usually with little earthquake activity away from the boundary.

Magnetic polarity reversal

A change in the direction of Earth's magnetic field.

How do magnetic stripes support seafloor spreading?

Symmetrical magnetic stripes on either side of mid-ocean ridges record reversals as new crust moves outward.

Hot spot

A location where magma rises through a tectonic plate and produces volcanic activity, often away from plate boundaries.

Mantle plume

A proposed rising column of unusually hot mantle material that can produce hot-spot volcanism.

How are hot spots recognized?

By volcanic chains showing an age progression, often with the youngest volcano near the active hot spot.

How do hot spots show plate direction?

The plate generally moves from the youngest volcano toward older volcanoes in the chain.

How is plate speed calculated from a hot-spot chain?

Distance between volcanoes divided by the age difference between them.

Slab pull

A sinking, cold, dense plate pulls the rest of the plate toward a subduction zone.

Ridge push

Gravity causes elevated lithosphere near a mid-ocean ridge to slide away from the ridge.

Mantle convection

Slow movement of mantle material as heat is transferred; hot material tends to rise and cooler material tends to sink.

Mineral

A naturally occurring, inorganic solid with a definite chemical composition and an orderly crystalline structure.

Rock

A naturally occurring solid aggregate of one or more minerals or mineral-like materials.

Atom

The basic building block of an element.

Proton

Positively charged particle found in an atom's nucleus.

Neutron

Particle with no electrical charge found in an atom's nucleus.

Electron

Negatively charged particle found in the electron cloud surrounding the nucleus.

Atomic number

The number of protons in an atom's nucleus.

Mass number

The total number of protons and neutrons in an atom's nucleus.

Ion

An atom or group of atoms with a net electrical charge.

Chemical bond

An attraction that holds atoms or ions together.

Hardness

A mineral's resistance to scratching.

Mohs hardness scale

A scale ranking mineral hardness from 1, talc, to 10, diamond.

Streak

The color of a mineral's powdered form.

Cleavage

A mineral's tendency to break along flat planes of weakness.

Fracture

The way a mineral breaks when it does not split along cleavage planes.

Luster

The way a mineral reflects light.

Specific gravity

A measure of a mineral's density relative to water.

Acid test

A mineral identification test; calcite fizzes when exposed to dilute hydrochloric acid.

Magnetism test

A mineral identification test that checks whether a mineral is attracted to a magnet.

Talc hardness

1 on the Mohs scale; the softest mineral on the scale.

Gypsum hardness

2 on the Mohs scale.

Calcite hardness

3 on the Mohs scale.

Fluorite hardness

4 on the Mohs scale.

Apatite hardness

5 on the Mohs scale.

Orthoclase feldspar hardness

6 on the Mohs scale.

Quartz hardness

7 on the Mohs scale.

Topaz hardness

8 on the Mohs scale.

Corundum hardness

9 on the Mohs scale.

Diamond hardness

10 on the Mohs scale; the hardest mineral on the Mohs scale.

Internal crystal structure

The orderly, repeating arrangement of atoms, ions, or molecules within a mineral.

Polymorphs

Minerals with the same chemical composition but different internal crystal structures.

Diamond and graphite

Polymorphs made of carbon; their different atomic structures give them different properties.

Silicate minerals

Minerals built from silicon-oxygen tetrahedra; their basic unit is SiO₄⁴⁻.

Carbonate minerals

Minerals containing the carbonate ion, CO₃²⁻; examples include calcite and dolomite.

Oxide minerals

Minerals containing oxygen combined with another element; examples include hematite and magnetite.

Sulfide minerals

Minerals containing sulfur combined with a metal or another element; examples include pyrite and galena.

Sulfate minerals

Minerals containing the sulfate ion, SO₄²⁻; examples include gypsum and anhydrite.

Halide minerals

Minerals containing halogen ions such as chloride or fluoride; examples include halite and fluorite.

Native element minerals

Minerals made of a single chemical element; examples include gold, copper, sulfur, and diamond.

Igneous rock

Rock formed when magma or lava cools and solidifies.

Intrusive igneous rock

Igneous rock formed when magma cools underground, usually slowly and with large crystals.

Extrusive igneous rock

Igneous rock formed when lava cools at or near Earth's surface, usually quickly.

Magma

Molten or partially molten rock beneath Earth's surface, often containing crystals and dissolved gases.

Lava

Magma that reaches Earth's surface.

Volatiles

Dissolved gases in magma, especially water vapor, carbon dioxide, and sulfur-bearing gases.

Igneous texture

The size, shape, and arrangement of crystals or fragments in an igneous rock.

Phaneritic texture

Coarse-grained texture with crystals visible to the naked eye; usually forms through slow underground cooling.

Aphanitic texture

Fine-grained texture with crystals too small to see easily; forms through relatively rapid cooling.