Earth Science - iSCI1A24 - Fall Midterm 2026

Age of the solar system

~4.5-4.6 billion years (4.5-4.6 Ga)

Likely trigger for the collapse of the gas and dust cloud

Shockwave from a nearby exploding star (supernova)

Solar nebula

Spinning flattened disk of gas and dust that formed the solar system

What formed at the centre of the solar nebula?

The Sun (hydrogen fused into helium and released energy)

Share of the solar system's matter that is in the Sun

~99%

Planetesimal

Small body (~1 km) built by clumping of dust in the disk - later grows to ~1000 km

Order of planet growth

Dust → planetesimals (~1 km) → larger planetesimals (~1000 km) → planets

What do small leftover pieces of the disk become?

Asteroids / comets / meteoroids / small moons

What do larger objects farther out in the disk become?

Planetesimals / dwarf planets / large moons / planets

Evidence that planets form routinely with stars

Rotating disks of gas and dust seen around new stars (Orion Nebula)

Four inner (terrestrial) planets

Mercury / Venus / Earth / Mars

Four outer planets

Jupiter / Saturn / Uranus / Neptune

Where is the asteroid belt?

Between Mars and Jupiter (2-4.5 AU)

Where is the Kuiper belt?

30-50 AU from the Sun

Where is the Oort cloud?

20000-100000 AU from the Sun

1 AU (astronomical unit)

Average Earth-Sun distance (~150 million km)

Best evidence for the age of the solar system

Meteorites

Meteorite

Fragment of an asteroid - a sample of original solar system material

Age of meteorites

4.53-4.58 Ga (≈4.54 Ga) - the same age for all

Why is the uniform age of meteorites important?

It dates the formation of the solar system to ~4.54 Ga

Main source of meteorites

The asteroid belt (between Mars and Jupiter)

What do many meteorites contain?

Small particles solidified from molten droplets - rich in iron and silicon

Iron meteorite

Made of Fe and Ni - formed in the cores of asteroids

Widmanstatten pattern

Crystal pattern seen in iron meteorites

Stony-iron meteorite

Formed between the core and mantle of asteroids

Chondrite

Rocky (stony) meteorite - the most abundant type

Carbonaceous chondrite

Contains ~5% carbon compounds (amino acids = building blocks of DNA)

Manicouagan Crater

Quebec - 214 Ma - ~5 km meteorite - area 1942 km²

Meteor Crater

Arizona - 50 ka - Canyon Diablo (Holsinger) meteorite

Canada's largest meteorite

Madoc meteorite (Ontario) - 167.8 kg - now in the Geological Survey of Canada museum in Ottawa

Energy conversion during accretion

Gravitational potential → kinetic → thermal energy

Energy conversion during differentiation

Dense materials fall to the core: gravitational potential → thermal energy

Energy conversion from radioactivity

Nuclear energy → thermal energy

Three heat sources of the early Earth

Accretion / differentiation / radioactivity

Differentiation ⭐

Separation of a hot molten planet into layers by density

During differentiation what sinks to the centre?

Dense iron and nickel (Fe-Ni) - forms the core

During differentiation what rises to the surface?

Light materials - form the crust

Result of differentiation (outside → in)

Light rock crust → intermediate-density mantle → liquid outer core → solid Ni-Fe inner core

Differentiated body (definition)

Celestial body separated into distinct internal layers based on the density of its elements

Density of the Moon vs Earth

Moon 3.34 g/cm³ vs Earth 5.52 g/cm³ (Moon is less dense)

Composition of the Moon

No large amount of Fe - basaltic crust - possibly a small iron core - no magnetic field

Giant Impact theory

A proto-planet (~10% of Earth's mass) hit Earth and the ejected debris formed the Moon

Theia

The proto-planet that hit Earth ~4.45 Ga

When did the Moon form?

~4.45 Ga (Giant Impact)

Steps of Moon formation

Impact → disk of debris → debris coalesces into the Moon

Why does the Moon have little iron?

The debris came mostly from rocky outer layers - not from the iron core

Other effect of the giant impact on Earth

Thinned the atmosphere and let Earth's surface cool and solidify

How did the terrestrial planets develop?

As hot rocky masses - all have undergone differentiation

What are the outer planets made of?

Gases: hydrogen / helium / water / methane / ammonia

Interior of Jupiter and Saturn (outside → in)

Molecular hydrogen → metallic hydrogen → rock-ice core

Interior of Uranus and Neptune (outside → in)

H-He-methane gas → mantle of water-ammonia-methane ices → rock-ice core

Mercury

Heavily cratered crust - diameter 4880 km - 0.056 Earth volumes - density 5.427 g/cm³

Venus

Similar in size and mass to Earth - diameter 12104 km - 0.857 Earth volumes - density 5.243 g/cm³

Mars

Has a crust and core - composition similar to Earth

Height of Everest relative to Earth's radius

8 km out of 6378 km (~0.12%) - Earth is nearly smooth at scale

Equatorial vs polar diameter of Earth

12756 km vs 12714 km (slight equatorial bulge)

Layers of Earth by composition (outside → in)

Crust (Si and O) → mantle (O / Si / Fe / Mg) → core (Ni and Fe)

Composition of the core

Nickel and iron (Ni-Fe)

State of the inner core

Solid

State of the outer core

Liquid

Core radius ⭐ (KNOW THIS LENGTH)

~3500 km

Radius of the inner core

~1220 km

Thickness of the outer core

~2250 km

Thickness of the mantle

~2900 km

Behaviour of the mantle

Solid but flows over long amounts of time

Composition of the mantle

Oxygen / silicon / iron / magnesium

Composition of the crust

Silicon and oxygen (Si and O)

Oceanic crust

Thin (5-12 km) - density 3.0 g/cm³

Continental crust

Thicker (up to ~70-75 km) - density 2.7 g/cm³

Which crust is denser?

Oceanic (3.0 vs 2.7 g/cm³) - so it sinks under continents

Lithosphere ⭐

Rigid outer layer = crust + uppermost solid mantle

Asthenosphere ⭐

Soft partially melted mantle layer under the lithosphere - it flows (low-velocity zone)

Moho (Mohorovicic discontinuity) ⭐

Boundary between the crust and the mantle

Discontinuity (in Earth science)

A boundary where chemistry or density or structure changes

Gutenberg discontinuity

Core-mantle boundary (~2900 km deep)

Static model of Earth

Layers defined by chemical composition (crust / mantle / core)

Dynamic model of Earth

Layers defined by mechanical behaviour (lithosphere / asthenosphere / mesosphere / outer core / inner core)

Mantle convection

Slow heat-driven circulation of the mantle - hot rises and cool sinks - moves tectonic plates

Ridge vs trench

Ridge = plates spread apart (new crust) - Trench = plate sinks into the mantle (subduction)

Density ladder (surface → centre)

Crust 2.7-3.3 → upper mantle 3.3 → lower mantle 5.5 → outer core 10-12 → inner core 12-13 g/cm³

Three types of evidence for Earth's internal structure ⭐

Drilling / seismic data / Earth materials

Deepest hole ever drilled ⭐

~12 km

Limitation of drilling

Too shallow - barely scratches the crust

Why is the deep Earth studied indirectly?

It cannot be observed directly - so we rely on indirect evidence (mainly seismic waves)

Seismic waves

Energy waves from earthquakes or explosions that travel through Earth and reveal its layers

Seismic reflection

Some wave energy bounces back from a rock boundary

Seismic refraction

Waves bend when they pass into different material - they bend toward lower-velocity material

P-waves ⭐

Compressional waves - travel through solids AND liquids

S-waves ⭐

Shear waves - travel through solids ONLY

What shows the outer core is liquid?

S-waves do not travel through it

What suggests the inner core is solid?

S-waves appear again in the inner core (lecture diagram)

Low-velocity zone

The asthenosphere (waves travel more slowly)

Materials that give information about the mantle ⭐

Volcanic rocks (lava) and diamonds

What can lava tell us?

Its composition gives clues about the composition of Earth's interior

Problem with using Earth materials as evidence ⭐

Recycled materials - rocks get reworked by tectonics and surface processes

Diamond

High-pressure crystalline form of carbon

Conditions for diamond formation ⭐

Over 1500 °C and over 55 kilobars

Depth where diamonds form ⭐

Over 150 km (in the mantle) - they are only found there

Kimberlite pipe

Volcano-like vent that brings diamonds up - found on the oldest parts of continents

Age of most diamonds

~3.3 Ga