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Earth Structure, Seismic Waves, and Plate Tectonics

Explore Earth's internal layers, seismic wave behavior, and the fundamentals of plate tectonics, including key boundaries and seafloor spreading.

12 cards~4 min
Earth Structure, Seismic Waves, and Plate Tectonics — Qwi
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1What is the density of the continental crust?
Answer

2.7 g/cm³

Continental crust, composed of granite and gneiss, has a density of 2.7 grams per cubic centimetre.
2The mantle covers ________ of Earth’s volume.
Answer

84%

3The outer core is solid iron.
Answer

False

The outer core consists of liquid iron that generates Earth's magnetic field.
4How do the asthenosphere and mesosphere differ?
Answer

Asthenosphere: upper mantle, easily deformed | Mesosphere: lower mantle, flows slowly

The asthenosphere is the upper mantle where rocks deform easily; the mesosphere is the lower mantle where rocks behave plastically and flow much more slowly.
5Which seismic wave can travel through solids, liquids, and gases?
Answer

P-waves

Primary (P) waves are compressional and can propagate through all states of matter.
6The discontinuity between the lower mantle and outer core is the ________ Discontinuity.
Answer

Gutenberg

7Seismic S-waves can travel through both solids and gases.
Answer

False

S-waves are shear waves that can only travel through solid materials.
8How does seafloor spreading differ from continental drift?
Answer

Seafloor spreading: creates new oceanic crust | Continental drift: movement of continents

Seafloor spreading forms new oceanic crust at mid‑ocean ridges, while continental drift describes the horizontal movement of continental plates.
9What instrument records earthquake vibrations as a seismogram?
Answer

Seismograph

A seismograph captures ground motion and produces a seismogram for analysis.
10The inner core is ________ iron.
Answer

solid

11All tectonic plates move at the same speed.
Answer

False

Plate velocities vary; for example, the North American plate moves about 2.5 cm/year while the average is around 10 cm/year.
12What are the three main types of plate boundaries?
Answer

Divergent: spreading | Convergent: subduction | Transform: lateral sliding

Divergent boundaries create new lithosphere, convergent boundaries destroy lithosphere via subduction, and transform boundaries involve lateral sliding without creation or destruction.

Earth Structure

The Earth is composed of several concentric layers, each with distinct physical properties and compositions. Understanding these layers provides the foundation for interpreting seismic data and plate movements.

Continental Crust

The continental crust is primarily made of granite and gneiss. Its average density is 2.7 g/cm³, which is lighter than the underlying mantle. This lower density contributes to the buoyancy of continents above the mantle.

Mantle

The mantle occupies the majority of Earth's volume, covering 84 % of the planet. It is divided into two main regions:

  • Asthenosphere: The upper mantle where rocks are hot enough to deform easily, allowing for the slow flow that drives plate motion.
  • Mesosphere: The lower mantle where rocks behave plastically and flow much more slowly.

Core

The core consists of two parts with contrasting physical states:

  • Outer Core: A liquid layer composed of iron and nickel. Its fluid nature generates Earth's magnetic field.
  • Inner Core: A solid sphere of iron, despite the extreme temperatures, due to the immense pressure at Earth's center.

Seismic Waves

Seismic waves are the primary tools geologists use to probe Earth's interior. They are generated by earthquakes and travel through the planet, revealing information about the materials they traverse.

Body Waves

Two main types of body waves propagate through the interior:

  • P‑waves (Primary waves): Compressional waves that can travel through solids, liquids, and gases. Their ability to move through all states of matter makes them the first arrivals on seismograms.
  • S‑waves (Secondary waves): Shear waves that can only travel through solid materials. They are unable to propagate through liquids or gases, which is why they do not pass through the outer core.

Discontinuities

Sharp changes in seismic velocity mark boundaries between layers. The most notable is the Gutenberg Discontinuity, separating the lower mantle from the outer core. This discontinuity reflects the transition from solid silicate mantle to liquid iron alloy.

Instruments

Earthquake vibrations are recorded by a seismograph, which produces a visual record called a seismogram. Analyzing seismograms allows scientists to determine wave types, travel times, and the internal structure of the Earth.

Plate Tectonics

Plate tectonics describes the movement of rigid lithospheric plates atop the more ductile asthenosphere. These motions shape the planet's surface over geological time.

Plate Motions

Not all plates move at the same speed. For example, the North American plate advances about 2.5 cm per year, whereas the global average is closer to 10 cm per year. This variation leads to diverse geological phenomena.

Types of Plate Boundaries

There are three principal boundary types, each associated with characteristic processes:

  1. Divergent boundaries: Plates move apart, creating new lithosphere as magma rises and solidifies. This process is evident at mid‑ocean ridges.
  2. Convergent boundaries: Plates collide, and one plate is forced beneath another in a process called subduction, destroying lithosphere.
  3. Transform boundaries: Plates slide laterally past one another, causing earthquakes without creating or destroying crust.

Seafloor Spreading vs. Continental Drift

Seafloor spreading and continental drift are related but distinct concepts:

  • Seafloor spreading generates new oceanic crust at mid‑ocean ridges, pushing plates apart.
  • Continental drift refers to the horizontal movement of existing continental plates across the Earth's surface.

Both mechanisms contribute to the dynamic reshaping of Earth's surface.

Summary

By integrating knowledge of Earth's layered structure, the behavior of seismic waves, and the principles of plate tectonics, we gain a comprehensive view of the forces that drive geological change. The density of the continental crust, the extensive mantle, the liquid outer core, and the solid inner core each play crucial roles in how seismic energy travels. Understanding the differences between P‑waves and S‑waves, as well as recognizing key discontinuities like the Gutenberg Discontinuity, enables accurate interpretation of seismograms. Finally, recognizing the diversity of plate motions and boundary types clarifies why continents shift, oceans expand, and earthquakes occur.