HOW THE EARTH WORKS

Igneous Rock

Rock formed from the cooling and solidification of molten rock (magma or lava).

Intrusive (Plutonic) Igneous Rock

Igneous rock that cools slowly underground, producing large crystals.

Extrusive (Volcanic) Igneous Rock

Igneous rock that cools rapidly at Earth’s surface, producing small crystals or glassy texture.

Cooling Rate–Texture Relationship

The speed at which magma cools determines crystal size in igneous rocks.

Slow Cooling

Allows large crystals to grow, producing coarse-grained rocks.

Rapid Cooling

Produces fine-grained or glassy textures because crystals do not have time to grow.

Obsidian

A volcanic glass formed when lava cools extremely quickly.

Bowen’s Reaction Series

The order in which minerals crystallize from cooling magma.

Mafic Minerals

Dark colored minerals with low silica content that crystallize at high temperatures.

Felsic Minerals

Light colored minerals with high silica content that crystallize at lower temperatures.

Silica (SiO₂)

A major component of magma that controls viscosity and eruption style.

High Silica Magma

Thick, sticky magma that traps gas and causes explosive eruptions.

Low Silica Magma

Runny magma that allows gas to escape and produces gentle eruptions.

Pyroclastic Flow

Fast-moving avalanche of hot ash, gas, and volcanic debris; the deadliest volcanic hazard.

Lahar

A volcanic mudflow made of ash, water, and debris that can bury towns.

Volcanic Aerosols

Tiny particles injected into the atmosphere during eruptions that reflect sunlight.

Global Cooling from Volcanoes

Large eruptions can temporarily cool Earth’s climate by reflecting sunlight.

Xenolith

A fragment of surrounding rock trapped inside magma.

What Xenoliths Tell Us

Xenoliths show that magma breaks through and carries pieces of surrounding rock as it rises.

Why Cooling Rate Is Important for Classification

Cooling rate determines crystal size and texture, which geologists use to classify igneous rocks.

Orogenic Cycle

The mountain-building process caused by continental collision.

Continental Collision

Occurs when two buoyant continental plates collide and neither subducts.

Crustal Thickening

When continental crust is compressed and stacked during collision.

Stress

A force applied to rock.

Three Types of Stress

Compression, tension, and shear.

Strain

The deformation or change in shape of rock caused by stress.

Brittle Deformation

Rocks break or fracture, forming faults; occurs near the surface.

Ductile Deformation

Rocks bend or flow without breaking; occurs deeper in the crust.

Fault

A fracture in rock where movement has occurred.

Fold

Bending of rock layers due to ductile deformation.

Anticline

An arch-shaped fold where the oldest rocks are in the center.

Syncline

A trough-shaped fold where the youngest rocks are in the center.

Dome

A circular fold where the oldest rocks are exposed in the center.

Basin

A circular fold where the youngest rocks are exposed in the center.

Suture Zone

A major fault zone marking where two continental plates have joined.

Why Orogeny Produces Metamorphism

High heat and pressure during mountain building alter rocks into metamorphic rocks.

Why Orogeny Causes Melting

Extreme pressure and heat during crustal thickening can partially melt rocks to form magma.

Slickensides

Linear grooves on fault surfaces that indicate the direction of movement.

Importance of Slickensides

They help geologists identify and confirm the presence of faults.

Tectonic Sandwich (Himalayas)

A stack of different rock types and crustal fragments pushed together during collision.

Earthquake

A sudden release of stored energy in Earth’s crust that produces seismic waves.

Elastic Rebound Theory

The idea that rocks bend elastically until stress exceeds strength, then snap and release energy.

Focus (Hypocenter)

The point underground where an earthquake begins.

Epicenter

The point on Earth’s surface directly above the focus.

Seismic Waves

Energy waves that travel through Earth during an earthquake.

Magnitude

A measure of the total energy released by an earthquake.

Intensity

A measure of the shaking and damage at a specific location.

Deep Earthquakes

Earthquakes occurring 300–700 km below the surface.

Why Deep Earthquakes Cause Less Damage

Seismic energy spreads out before reaching the surface.

Induced Seismicity

Earthquakes caused by human activities such as wastewater injection.

Wastewater Injection

Pumping fluids underground that increases pore pressure and lubricates faults.

Intraplate Earthquakes

Earthquakes that occur far from plate boundaries.

New Madrid Seismic Zone

A major intraplate earthquake zone in the central United States.

Seismology

The study of earthquake waves to understand Earth’s structure.

P-Waves (Primary Waves)

The fastest seismic waves that travel through solids and liquids.

S-Waves (Secondary Waves)

Seismic waves that travel only through solids.

Triangulation

A method used to determine the epicenter of an earthquake using three seismic stations.

Lag Time

The time difference between the arrival of P-waves and S-waves.

Seismograph

The instrument that records seismic waves.

Seismogram

The recorded output produced by a seismograph.

S-Wave Shadow Zone

A region where S-waves are not detected because they cannot travel through liquid.

Evidence for Liquid Outer Core

The absence of S-waves beyond the shadow zone shows the outer core is liquid.

P-Wave Refraction in the Core

P-waves bend sharply when entering the dense core.

What P-Wave Bending Indicates

A major change in density between Earth’s layers.

Wave Amplification

When seismic waves increase in height as they slow down in soft sediments.

Why Soft Sediments Are Dangerous During Earthquakes

They amplify shaking and increase building damage.

Resonance

During earthquakes, energy can build up in soft ground and increase shaking intensity.

Tsunami

A series of large ocean waves caused by underwater earthquakes, landslides, or volcanic eruptions.

Why Tsunamis Are Multiple Waves

Tsunamis travel as a wave train, so several waves arrive over time rather than just one.