In my last column, I mentioned journeys to the center of the Earth, both fictional and using geological models.
How can science explore what lies far below, especially since no humans or drills have ever penetrated beneath the crust, the outer shell of the Earth (analogous to the crust of an egg)?
The answer is: a global network of seismometers (Earthquake measuring devices), the laws of physics and extreme high pressure devices (to simulate conditions far below the surface of Earth).
China is a country prone to earthquakes, a few of which have killed hundreds of thousands of their residents.
The first seismometer was developed in second century China using eight metallic dragons from which bronze balls would roll out of their mouths, indicating the direction of the earthquake.
In the late 1880’s British geologists were sent to Japan (also a land of earthquakes) to study earthquakes there. The basic design of a seismometer came out of their research.
You have a large mass suspended by a spring that is attached to a frame, anchored to the Earth.
At the end of the large mass is a stylus whose motion shows earthquakes. The large mass because of its mass (or inertia) remains stationary while the frame moves.
Modern seismometers use electronic sensors, amplifiers and digital recorders. The latest seismometers use fiber optics cable whose lengths change as the seismic waves pass. You need a global network of seismometers to record the times that a particular earthquake occurs.
Then you can learn the speed of the earthquake waves as they travel through the Earth. There are three kinds of earthquake waves: P waves (compression), S waves (transverse) and surface waves.
P waves can travel all the way through the Earth, but change their speed as the density of material varies. S waves are absorbed by liquids. The very destructive Surface waves are confined to the surface.
By careful analysis of many thousands of earthquake waves, geologists have developed a model of our planet’s interior. High pressure devices have pinned down the likely composition of these layers.
At the center of the Earth is solid iron nickel core of about 763 miles in radius. Above is the molten iron-nickel core that is about 1,465 miles thick. Currents in this liquid outer core generates the Earth’s magnetic field.
The 1,333-mile thick lower mantle consists of molten magma, which often emerges to the Earth’s surface spewed out of volcanoes.
The next 400 miles is the asthenosphere where convection currents from beneath power the motion of crustal plates. The last 200 miles is the shallow mantle. On top is the lithosphere and crust.
Finally, we reach the Earth’s surface, where the plates collide, scrape by each other, go under one another, and spread apart in the mid oceanic trenches.
SKY SIGHTS FOR THE COMING WEEK: Dawn begins about 6:10 a.m., sunset at 7:12 a.m., mid-day at 12:12 p.m., sunset at 4:53 p.m., dusk ends at 5:56 p.m. Daily sunlight lasts nine hours, 41 minutes, shrinking by about a minute a day.
The sun now appears in Scorpius in this last week of November.
At about 5:30 p.m. in midweek, you may be able to see low in the west a slender crescent moon, the brilliant planet Venus and the bright planet Jupiter.
Bob Doyle, professor emeritus at Frostburg State University, invites any readers comments and questions. E-mail him at firstname.lastname@example.org. He is available as a speaker on his column topics.