Geologic Structures and Earth's Interior

Geologic structures

Dynamically-produced arrangements of rocks and sediments resulting from internal Earth forces

Basic types of stress

Compressive, shear, and tensional

Strain in rocks

A change in size or shape in response to stress

Elastic deformation in rocks

The rock returns to its original shape after stress is removed

Anticlines

Upward-arching folds in rocks

Plunging fold

The hinge line being non-horizontal

Basin

Beds dipping toward a central point

Joints in geologic terms

Fractures in bedrock with no movement

Normal fault

A fault caused by tensional stress

Horsts

Fault blocks that are uplifted

Thrust faults

Characterized by dip angles less than 30°

Right-lateral strike-slip fault

The opposite side appears to move to the right

Strike

The compass direction of a line formed by the intersection of an inclined plane with a horizontal plane

Oil or gas pool formation conditions

Porous rocks and traps

Columnar jointing

Occurs due to contraction of a cooling lava flow

Structures indicating compressional stress

Folds (anticlines and synclines)

Geologic cross-section

A vertical slice through a portion of Earth

Overturned fold

A fold where limbs dip in the same direction

Dip-slip fault types

Normal fault, reverse fault, and thrust fault

San Andreas Fault

An example of a strike-slip fault

Studying Earth's deep interior

Indirectly using geophysics and seismic data

Deepest drill hole on Earth

Reaches approximately 12 km

Geophysics

The branch of geology studying Earth's interior using physical principles

Seismic waves

Vibrations caused by earthquakes and artificial sources like nuclear testing

Seismic reflection

The return of seismic waves to the surface after hitting a boundary

Seismic refraction

Waves changing direction as they pass through materials with different wave velocities

Seismic shadow zones

Regions on Earth's surface where seismic waves are not detected

Main zones of Earth's interior

Crust, mantle, and core

Earth's crust composition

Primarily composed of silicate rocks

Lithosphere

The rigid outer shell composed of the crust and upper mantle

Asthenosphere

A plastic layer beneath the lithosphere where seismic wave velocities decrease

Evidence for liquid outer core

S-wave shadow zones

Composition of Earth's core

Iron-nickel alloy with small amounts of lighter elements

Core-mantle boundary feature

D" layer with significant changes in seismic velocity and density

Isostasy

The equilibrium of crustal blocks floating on the mantle

Crustal rebound

The crust rises when a large mass (e.g., an ice sheet) is removed

Gravity

Slightly higher over areas with dense materials below the surface

Paleomagnetism

The study of ancient magnetic fields preserved in rocks

Magnetic reversals

The flipping of Earth's magnetic poles

Geothermal gradient

The rate of temperature increase with depth into the Earth

Earthquake

Trembling or shaking of the ground caused by energy release in rocks

Elastic Rebound Theory

Explains the sudden release of strain in rocks causing earthquakes

Energy storage before an earthquake

In rocks beneath the Earth's surface

Hypocenter (Focus)

The point of initial breakage along a fault

Epicenter

The point directly above the earthquake's focus on the Earth's surface

P waves

The fastest seismic wave

S waves

Seismic wave that can only travel through solids

Surface waves

The type of seismic wave most destructive to buildings

Seismograph

Device that records seismic vibrations

Distance to earthquake's focus

Determined using a travel-time curve and P-S wave time differences

Locating an earthquake's epicenter

Using distances from three stations plotted on a map

Modified Mercalli Scale

Measures earthquake intensity based on observed effects

Moment Magnitude Scale

More accurate for measuring large earthquakes

Richter scale

Uses the amplitude of seismic waves to determine magnitude

Liquefaction

Soil behaving like a liquid during shaking

Tsunamis

Hazard caused by submarine earthquakes

Fires during earthquakes

Caused by broken gas and water mains

Earthquake occurrence

Most occur along tectonic plate boundaries

Active earthquake belts

Circum-Pacific Belt and Mediterranean-Himalayan Belt

Reliable earthquake precursors

Animal behavior, radon emissions, magnetic field changes

Seismic gap

An inactive segment of a fault likely to produce future earthquakes

Reducing earthquake damage

Using early warning systems to shut down infrastructure

Mountain formation

Reflects uplift, deformation, and metamorphism

Mountain belts

Chains of mountain ranges that are thousands of kilometers long

Mountain building process

Orogenesis

Intense deformation cause

Compression leading to folding and faulting

Vertical movement of mountain belts

Isostasy

Factors affecting weathering and erosion

Climate, rock type, and height

Deformation of rocks

They experience displacement, rotation, and distortion

Result of deformation on rock layers

Tilted beds, metamorphic alteration, and faulting

Displacement

Change in the location of the rocks

Deformation shape change

Involves a change in the shape of the rocks

Deformation

A change in the shape of the rocks.

Appalachian mountain range

It is much older than the North American Cordillera.

Precambrian Shield

A large area of exposed metamorphic and igneous rock.

Mountain uplift at convergent plate boundaries

Driven by plate convergence and continental collision.

Oceanic-continental subduction

Characteristic of mountain belts like the Andes, leading to the formation of deep ocean trenches.

Continent-continent convergence

Both continents are too buoyant to be subducted.

Block-faulting

A type of deformation caused by compression at convergent boundaries.

Tension in an uplifting crust

Results in normal faulting and fault-block mountain ranges.

Delamination in mountain building

The thinning of the crust caused by the detachment of the mantle lithosphere.

Result of delamination in the Basin-and-Range province

Stretching and thinning of the crust, producing normal faults.

Mountain belts contribution to continent growth

By accumulating sediment and igneous activity, adding new crust.

Displaced terranes

Geologically continuous areas within mountain belts with unrelated bedrock.

Plate Tectonics

The primary theory that explains the movement of Earth's tectonic plates.

Pangaea

A supercontinent that existed millions of years ago, proposed by Alfred Wegener.

Evidence for continental drift

Puzzle-piece fit of coastlines.

Glacial striations

Evidence of past glaciers in regions that are not glaciated today.

Paleoclimate belts

Revealed that the poles have shifted, and continents were once joined together.

Polar wandering

The Earth's poles remained fixed, but continents moved.

Seafloor spreading

Shows the movement of oceanic plates away from mid-ocean ridges.

Vine-Matthews Hypothesis

Suggests that magnetic anomalies form stripes on the ocean floor.

Types of plate boundaries

Divergent, Convergent, Transform.

Motion of tectonic plates

Caused by convection currents in the mantle.

Earth's lithosphere

It is rigid and floats on the asthenosphere.

Rift valley

A valley formed by divergent boundary activity, caused by tensional forces.

Age of the seafloor

The age pattern of seafloor supports the theory of seafloor spreading.

Exploded view of Earth's tectonic plates

Emphasizes the variety of shapes and sizes of tectonic plates.

Relationship between earthquakes and tectonic plate boundaries

Earthquakes are concentrated in distinct belts along plate boundaries

Difference between active and passive margins of continents

Active margins are associated with tectonic activity, whereas passive margins are tectonically stable