1. Summary

After briefly describing how the sun and our solar system came to be, this video shifts focus to the Earth’s layers and their importance. The video explains what the various layers of the Earth are made of and why they are organized the way they are. A focal point of this video is that density plays a huge role in how the Earth’s layers are organized.

2. Why Watch This Video?

• Have you ever wondered what the Earth’s layers are made of?
• Would you like to know why the Earth has an atmosphere, while some planets (i.e. Mercury) don’t?
• Have you ever wondered why some parts of the world are more susceptible to earthquakes than others?

3. Key Terms

• Density – with relation to this topic, density refers to the amount of a particular element populating a certain area or layer of the Earth.
• Atmosphere – the air, the envelope of gas that surrounds the Earth; the Earth’s atmosphere is made up mostly of nitrogen and oxygen with small amounts of argon and carbon dioxide.
• Earthquake – vibrations in the Earth’s crust resulting from the accumulation of elastic strain which causes a sudden rupture and rebound of rocks; earthquakes always originate from one place within the Earth and send seismic waves throughout the Earth, many of which can emerge at the surface of the Earth and be detected by seismographs

4. Loose Ends

Loose end #1: What does “mechanical” and “chemical” organization mean?

Despite showing an image depicting “mechanical” and “chemical” organizations of the Earth’s layers, the video does not explain what these classifications mean. Mechanical layers of the Earth are differentiated by their strength or rigidity. That is to say that they are determined by their physical properties. Mechanical layers of the earth include:

• Lithosphere (outermost, most rigid; 0-100km)
• Asthenosphere (100-350 km)
• Mesosphere (the stiff parts of the upper and lower mantle; 350-2900 km). Note: mesosphere can also refer to a layer in the atmosphere.
• Outer core: (only layer of the earth that is a liquid; 2900-5100 km)
• Inner core (innermost layer; 5100-6370 km)

Chemical (or compositional) layers of the Earth are determined by their components. Chemical layers of the Earth include:

• Crust (outermost layer, rocky and solid; 0-100 km)
• Mantle (rocky, plastic in nature; 100-2900 km)
• Core (outer core is liquid, inner core is solid; outer and inner core are not chemically distinct from each other; mainly composed of nickel and iron; 2900-6370 km)

Loose end #2: What is accretion?

The video briefly mentions the term “accretion” when talking about how the Earth developed but did not explain what this means. Accretion refers to the model that explains Earth’s formation by a building up of materials causing growth. Earth formed first as heavy elements collided and bound to one another. Later the densest materials would sink to the center while lighter materials began to form the crust. Gasses were then pulled in by gravity and formed an early atmosphere.

Loose end #3: The video briefly mentions the concept of “seismic shadows” but does not elaborate on what these are.

Seismic shadows occur in areas of the Earth’s surface where seismographs do not detect an earthquake after its seismic waves have passed through the Earth. Seismic waves (which radiate spherically away from their origin) are refracted or stopped as they pass the outer core of the Earth (due to its liquid nature.)

5. Self-Test Questions