Earth Science

How earthquakes show us the inside of the Earth

Earth cross section with body wave linesIt is pretty amazing how much we know about the inside of the Earth, given that we can only directly observe a small part of it.

When we learn about the layers of the Earth, we learn that the crust is very thin, but that is only on the scale of the rest of the Earth. Relative to the scale of the tools we use to study the Earth, it is very thick, and conditions become terribly harsh quite close to the surface.

Surface currents, the Ekman spiral, and Ekman transport

As wind moves across the surface of the ocean, friction between moving air and the water surface causes water to begin to move as well. This transfer of energy by friction is how wind causes surface currents. Once water at the surface begins to move, some energy gets transferred to deeper layers allowing water movement to penetrate to depths of 50 to 100 meters.

Thermohaline Circulation

Circulation in the oceans helps transport the solar energy that falls at the equator towards the poles. Like the atmosphere, uneven heating drives this flow of energy poleward. Unlike the atmosphere, the oceans are warmed from the top-down rather than from the bottom up, so heat-driven convection is not enough to cause ocean circulation. For density-driven circulation to mix the oceans at a large enough scale to matter, changes in both temperature and salinity are required.

Pressure Gradient Force

We live at the bottom of an ocean of air. Currents in this ocean move mass

Lunar and Solar Tides

Tides cause daily changes in water levels in many coastal areas. Factors such as local topography and weather contribute to the timing and height of tides, but the primary reason for tides is the gravitational attraction between liquid water on the Earth and the Moon. All objects on Earth experience tidal forces. However, the effect is most pronounced with water because, as a liquid, it is more easily deformed by gravity when compared to solid objects.


The energy that warms Earth’s lower atmosphere comes from the Sun, but sunlight does not warm the lower atmosphere directly. This region of the atmosphere warms from below. Most of the short wavelength, electromagnetic energy from the Sun passes through the atmosphere and is absorbed by the Earth, which warms up as a result. As the Earth warms, it emits some long-wave radiation back out, heating up the lower atmosphere above it. Eventually, this energy radiates from the atmosphere back into space.

The Earth does not absorb all of the electromagnetic energy that hits it. Some reflects back out into space. This is important for the Earth’s energy balance, because only absorbed energy contributes to the temperature of the Earth/atmosphere system. The proportion of the total energy reflected by the Earth (or any object) is called the Albedo.

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