Unit 3 – Earth’s Internal Structure

These flashcards review key facts, terms, and comparisons from Unit 3: Earth’s Internal Structure, covering the crust, mantle, core, seismic boundaries, internal dynamics, and parallels with other terrestrial planets.

What are the three main compositional layers of Earth?

The crust, the mantle, and the core.

Roughly what fraction of Earth’s volume does the crust occupy?

Less than 1 % of Earth’s total volume.

What is the approximate thickness range of Earth’s crust?

About 5 km (beneath oceans) to ~80 km (beneath mountains).

What is the name of the boundary between Earth’s crust and mantle?

The Mohorovičić discontinuity (Moho).

Earth’s tectonic plates consist of which layer(s)?

The brittle lithosphere, composed of the crust plus the uppermost mantle.

Name the four most abundant elements in Earth’s crust.

Oxygen, silicon, aluminum, and iron.

Which element is most abundant in Earth’s crust and what is its approximate abundance?

Oxygen, about 46.6 % by weight (≈276,900 ppm).

What are the two types of Earth’s crust?

Oceanic crust and continental crust.

What rock type and density characterize oceanic crust?

Dark-colored mafic rocks with a density of ~2.9–3.1 g/cm³.

Compare the thickness of oceanic crust to continental crust.

Oceanic: 5–18 km thick; Continental: averages ~30 km, up to 80 km beneath mountains.

Name the three main regions of oceanic crust.

Continental margins, deep-ocean basins, and ocean ridges.

Which light-colored igneous rock dominates continental crust?

Granodiorite.

Why is continental crust generally older than oceanic crust?

Oceanic crust is continually recycled at subduction zones, whereas continental crust is seldom subducted.

What are mountain belts and how do they form?

Elevated regions of uplifted and deformed rocks formed during past orogenies (mountain-building events).

Define cratons and shields.

Cratons are ancient, stable parts of continental lithosphere; shields are exposed Precambrian crystalline rocks within cratons.

According to Baker and Sofonio, how could early crustal material have formed?

Silicate minerals condensed from a steamy atmosphere and rained back onto Earth’s surface (silicate rain).

Approximately what percentage of Earth’s volume is made up by the mantle?

About 83 % of Earth’s volume.

Which oxide is abundant in the mantle and helps distinguish it from the crust?

Magnesium oxide (MgO).

What ultramafic rock composes much of the upper mantle?

Peridotite.

What mineral dominates the lower mantle (mesosphere)?

Bridgmanite, a high-pressure form of perovskite.

List the three mechanical sublayers of the mantle.

Lithosphere, asthenosphere, and mesosphere (lower mantle).

Describe the lithosphere’s properties.

Rigid, brittle layer (crust + uppermost mantle) about 100 km thick, breaks under stress.

How does the asthenosphere differ from the lithosphere?

It lies beneath the lithosphere, is hotter and plastic, and allows plates to move over it.

What is mantle convection?

Heat transfer from mantle to crust via rising and sinking mantle material that sets up convection currents.

What is the typical geothermal gradient in Earth’s shallow interior?

Roughly 15–30 °C per kilometer, varying by region.

What is the boundary between the mantle and the core called?

The Gutenberg discontinuity.

How thick is Earth’s entire core?

Approximately 3,480 km thick (from 2,900 km depth to Earth’s center at 6,380 km).

What is the Lehmann discontinuity?

The boundary between the liquid outer core and solid inner core.

State the primary elemental composition of Earth’s core.

~85 % iron, ~5 % nickel, and ~10 % lighter elements (e.g., S, O, C, H).

Which part of the core is liquid and what global phenomenon does its motion create?

The outer core; its turbulent iron flow generates Earth’s magnetic field.

Why is the inner core solid despite temperatures near 6,000 °C?

Enormous pressure keeps the iron–nickel alloy in a solid state.

When did the solid inner core begin crystallizing, strengthening Earth’s magnetic field?

Roughly 1–1.5 billion years ago.

Which terrestrial planet’s core makes up about 85 % of its radius?

Mercury.

Give two hypotheses for Mercury’s unusually large core.

(1) Early rapid formation with solar heating that stripped outer layers; (2) Giant impacts that removed much of the original mantle/crust.

Why is Venus thought to have a weak magnetic field?

Its very slow rotation reduces the dynamo action in its presumably liquid core.

What feature of Mars’ crust indicates a lack of plate tectonics?

Its crust is effectively ‘one piece,’ allowing stationary volcanic hotspots like Olympus Mons to grow huge.

Why does Mars currently lack a global magnetic field?

Its core is believed to be solid and no longer convecting, so no dynamo action occurs.

What internal layers are common to all terrestrial planets?

A crust, a mantle, and a core.

Which is the smallest terrestrial planet in our solar system?

Mercury.

Which planet hosts Olympus Mons and why is it so large?

Mars; fixed hotspots and lack of plate motion let volcanic eruptions build an enormous shield volcano.

Define the Mohorovičić discontinuity.

Seismic boundary separating the crust from the mantle.

Define the Gutenberg discontinuity.

Seismic boundary between the mantle and the outer core.

Compare the density of oceanic crust to continental crust.

Oceanic: ~2.9–3.1 g/cm³; Continental: ~2.6–2.9 g/cm³ (thus less dense).

What are ocean ridges and where are they found?

2-km-high mountain belts of newly formed oceanic crust located in the middle of oceans, encircling the globe.

What does the term “geothermal gradient” mean?

The rate of temperature increase with depth inside Earth, typically 15–30 °C per kilometer near the surface.