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  • Writer's pictureFrank DiGiovanni

The World Beneath Your Feet

Updated: Feb 16, 2023

Have you ever really thought about the world under your feet? Yes, the world under your feet is one of fascinating diversity, danger and beauty. So, let’s take a quick trip down, under the surfaces we drive, walk and play on.

Dig but a few inches and you’ll see what’s known as “top soil”, the layer we affectionately call “dirt”. This unobtrusive layer of earth might be considered the most important one to humans! Mineral rich, full of living and dead organisms, water and air, this layer feeds the United States and the world! Pick a vegetable or fruit and that plants has its roots in this 8-10-inch layer. But while we enjoy the fruits (sorry for the pun) of the plants labor, we humans can also destroy it (the top soil) and once it’s gone, it’s gone for a LONG time, far longer than the average human life. Top soil is considered to be both a renewable and non-renewable; it’s invaluable regardless. An inch of topsoil can take anywhere between a few hundred to 1,000 years or more, depending on where that soil is located! There are five factors that contribute to the type of soil as well: parent material (what kind of rock the soil started as), topography (shape, elevation, direction), climate, organisms (living and dead) and time. Because of the length of time needed to create just one inch of soil, it should be referred to as a non-renewable resource. According to The Food and Agriculture Organization (FAO), an agency of the United Nations, soil is a finite resource, meaning its loss or degradation is not recoverable within a human lifespan. So let’s do our best to make sure it’s here and healthy for generations to come. Just like every state has its own flower (rose) or insect (Nine-Spotted Ladybug), each state even has its own “official soil”! New York’s is called Honeoye “Ho-knee-o-yay”.

Digging deeper you find the bedrock, the thing rock upon which most of the most well-known sky scrapers need to be anchored into to stand tall and strong. This impressive layer can be made from any of the three types of rock: sedimentary, metamorphic or igneous. This rock extends hundreds of feet further beneath our feet. Past the subways and sewer tunnels, past the power lines and other man-made things, this layer of rock also hides valuable things. Aquifers, underground pockets of water, exist in porous bedrock formations, such as sandstone. Deposits of petroleum and natural gas can also be found and accessed by drilling through bedrock. Vast caves, such as Howes Cavern in New York, exist over 150 feet

Bedrock outcrop, NYC Central Park. Image from Wikimedia Commons.

deep into this layer of Limestone which were formed when the Atlantic Ocean was roughly 160 miles further inland! The Hang Son Doong cave- translated to “cave of the mountain river”- in Vietnam is the largest known cave in the world and could fit an entire New York City block inside of it with room to spare! This cave stayed hidden until 1991 when it found accidentally by someone looking for agarwood, a valuable timber. Giant crystal caves exist in bedrock in places (Naica, Mexico) where mineral-rich water sat for millions of years, allowing crystals to grow to mammoth sizes, weighing thousands of tons!

Further down, things start to get hot and more dangerous. In our “trip” we’ve not even left the crust, the life sustaining layer on top of the molten sphere of magma called Earth. Although we think this layer is solid, thick and stable, this layer is comparatively thinner than a (chicken) egg shell, 3.1-43.5 miles is nothing to the 7,900+ mile diameter of Earth. Earthquakes happens as the massive slabs of crust grind and slide past each other as pressure is generated and released, escaping gasses and magma boil up to the surface as volcanoes.

Earth’s upper layers. Image from Wikimedia Commons.

From the depths of the mantle, a roughly 1,800-mile-thick layer of molten rock, magma rises to find any crack in the bedrock to create a volcano on the crust. Down here, the boundary between solid and liquid rock begins to blur at a place called the Mohorovičić (MOH-haw-ROH-vuh-chich) discontinuity or Moho. As tectonic plates subduct (go under) another plate, carbon atoms start to reorganize under the intense heat and pressure to form the hardest known mineral, diamond, before being carried to the surface in volcanic eruptions. At the surface, your body is used to one atmosphere of pressure or 14.7 pounds per square inch. Down here the, pressure increases to a staggering 1.4 million times what it is at the surface or 20,574,328.3 pounds per square inch! Huge convection cells of rock rise towards the surface, cool and sink back down in a never-ending cycle, transporting heat and magma towards the surface.

The last stop, at roughly 3,200 miles below your feet, is the place where geologists can only infer EXACTLY what is going on and what’s down there. Here in the core exist two layers, the inner and outer core. For as long as humans have been studying the Earth’s interior, it was only in 1936 that the inner core was known to exist! Inge Lehmann, was a Danish seismologist and geophysicist who deduced that the core must have two layers.

Inge Lehmann, 1932. Image from Wikimedia Commons

She did this by analyzing earthquake waves as they passed through earth. The core is generally believed to be composed primarily of iron and some nickel; but this is a hypothesis as no direct measurements can be made. Since this layer is able to transmit shear waves, it must be solid, Lehmann stated. Because of her contribution to geological science, in 1997, the American Geophysical Union established the annual Inge Lehmann Medal to honor "outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth's mantle and core." Suggestions from recent research say that gold, platinum and other iron-loving minerals may be present in vast amounts! This outer core, of molten metal, and heated largely by the radioactive decay of the elements uranium and thorium spinning around, has given Earth an invisible shield called a “magnetosphere” which has help shield the Earth from the sun’s harmful cosmic rays, allowing life to flourish for billions of years!

The ground beneath you isn’t entirely solid, rather constantly moving and shifting, sometimes only millimeters a year (the speed your fingernails grow) but the incredible nature below your feet is something to stop and consider the next time you’ve got your hands dirty from digging in the dirt.

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