The pale dot in the center is our home. Taken by
Voyager, this is the most distant image of the Earth.
Credit: Voyager 1, NASA
The image above reminds me of a book I read as a child called Horton Hears a Who by Dr. Seuss. In this story, Horton (the elephant) discovers an entire civilization living on a tiny speck of dust which sits on a clover. Our home planet is no different. It is a tiny speck orbiting one of a few hundred billion stars in a galaxy which is just one of many billions of galaxies in the visible universe. But as Dorothy said in the Wizard of Oz, "There's no place like home". Of all the planets in our solar system, we are blessed with all the conditions necessary for life (as we know it) to thrive. Are we alone? Is there intelligent life somewhere else?
Our planet is constantly changing. It is active at all levels - the interior, the oceans, and the atmosphere. We need to understand the nature of these changes, investigate the forces that drive these changes, .... as well as some of the effects of these changes. We can start with the interior of the earth.
The Interior - The deepest anyone has actually gone down into the earth itself is only a few miles (deep wells), yet we actually know quite a bit about the interior of the earth by indirect means. We know, for example, that there are 4 well defined layers that make up the interior of the earth- crust, mantle, outer core, and inner core. Volcanoes deliver material to the surface, but only from the upper mantle. We know that the crust and mantle is composed of rocky material (mostly silicate type rocks) and the core is mostly iron. We know the outer core is liquid, and the inner core is solid. Pretty bold claims? Not really.
Geologist draw these conclusions based on many different clues. Probably the best data comes from seismic activity. When an earthquake goes off, it sends waves (which geologists distinguish as S and P) deep into the earth which serve as probes. In a way, it works like radar, ... where a wave is sent out, ... it interacts with the medium it moves through (gets reflected, refracted, etc), ... and then gets picked up by detectors. Bats navigate the darkness using this technique and it is used in technologies such as ultrasound, depth gauges, fish finders, and even the Magellan space probe to Venus. Geologists have known about the existence of the solid inner and liquid outer cores for many years using seismic waves.
Seismic waves give us clues about the earth's interior.
But why do geologists believe the core is mostly iron? Several more clues are available. Here are a few:
The overall density of the earth is 5.5 g/cm3. Yet the material from the crust and upper mantle have much lower densities. Something with a much higher overall density is needed in the core to balance the equation. Iron fits the bill.
The earth has a strong magnetic field. Physicists believe that churning hot liquid iron in the outer core acts as a dynamo which is able to produce and sustain this magnetic field.
We are constantly being pelted by material entering our atmosphere. When we study meteorites (pieces that get to the ground), we find that they come in two main groups - stony (silicates) and iron. It is reasonable to assume that the material hitting the earth now is fairly similar to material that have hit the earth in the past. These materials become the building blocks of the planets. If that is the case, there must be lots of iron around. But how did it all end up in the core?
(animation)
Radioactivity - A certain percent of each element is radioactive (unstable atomically). Each radioactive isotope has its own timetable by which it stabilizes itself (referred to as the half-life). When it does, the radioactive atom (parent) will decay into a stable daughter product. During this process, heat is released to the environment. No matter what radioactive substance you choose, the decay follows an exponential decay ... which means that the level of radioactivity falls off rapidly as time progresses. This also means that the level of radioactivity was much higher in the past than it is now.
Age of the Earth - By comparing the ratio of parent atoms to daughter atoms, it is possible to estimate the time of formation of a rock. This technique of radiometric dating can be cross referenced with the isotopes of many different elements to give fairly accurate results. When geologists determine the age of most all meteorites falling to earth now, they reach a date of 4.6 billion years (actually 4.55 billion years). This is typically taken as the age of the earth since it was from this material the earth formed. (The oldest moon rock dated to 4.5 billion years so it is believed that the Earth was formed from debris orbiting the sun in a rather short time).
Now we have a very young earth formed from rocky and iron minerals ... but it is extremely radioactive and generating a lot of heat in the interior. Enough heat to melt the entire planet! This allows the materials to separate by density. The dense iron sinks to the interior, and the less dense rocky materials rise to the top forming the mantle and crust. The level of radioactivity has since diminished significantly, ... allowing the earth to re-solidify. However, there is still a lot of heat down there, and it is still being generated. You will soon see that it is this heat that is responsible for a lot of changes we see at the surface of the earth.
The Changing Surface - The crust is a very thin layer which is constantly under attack from various forces. The top two are erosion and plate tectonics.
Weathering/Erosion - The sun heats our atmosphere/oceans and evaporates water, ... which initiates changes in the oceans and atmosphere. Winds blow, ocean currents flow, snow, and rain fall, and glaciers form. As they do, the surface of the earth takes a beating. Over the course of time, entire mountain ranges have been washed to the sea. Just 12,000 years ago, a thick glacier created the Kettle-Moraine area in Wisconsin. The next time you witness a river flow (or overflow its bank), you are witnessing this slow and steady power of running water.
Plate Tectonics - The crust of the earth is hard and brittle. It is easily broken into sections which geologists call plates. In places, these plates pull apart, or push together or even rub across each other. In the process, they create huge changes on the surface which often are witnessed in the form of an earthquake or volcano.
The Atlantic Ocean was formed when the super continent, Pangaea, broke apart about 180 million years ago.
The Himalayan Mountains formed when the Indian Plate crashed into the Eurasian Plate.
The San Andres fault is where the Pacific Plate is rubbing across the North American Plate.
Credit: NASA
Mountains are made when plates collide (animations)
But what drives the plates to move in the first place? The heat that is constantly trying to escape from the interior has a profound effect on the surface. Although the mantle is considered a solid (with isolated packets of molten rock), it is able to move and circulate much like taffy is able to move. Heat from the interior creates convection cells in the mantle (areas of circular motion), which eventually exert horizontal and vertical forces on the crust. The crust is much more brittle and tends to break rather than bend.
Other agents for change -
Life - All living creatures interact with their environment. The surface of the earth changes as a result. Humans build roads and homes which alter the reflectivity of the surface. We cut down rain forests which effects the carbon cycle and greenhouse effect. Trees have root systems which can crack bedrock. Lichen secrete weak acids which erode surface rocks. Termites and ants are constantly overturning the soil. You get the idea.
Most of the oxygen, O2 , in our atmosphere was produced by photosynthesis. That is, plants take in sunlight and carbon dioxide, CO2, and produce the oxygen we need to breath.
Impacts from Space - You don't usually worry about this kind of thing, but every once in a while, a large rock or comet crashes into the earth and can make a dramatic change. About 50,000 years ago, a rock hit the Arizona desert and made a mile diameter hole I was able to image from an airplane.
Meteor Crater in Arizona
In the past, we got hit with much bigger rocks (and comets) and the results were much more dramatic. It should be obvious that when large impacts occur, they can really alter the surface of the earth and the life forms living on it. Can this happen again?
The point is ... our planet is extremely active, both geographically and biologically. This also presents a problem, namely, it erases most of the clues about the distant past. The Earth is a poor place to start if you want to investigate details about the first billion years of the solar system. These clues have to be found somewhere else.
If you want to learn much more about our planet, I suggest you enroll in NatSci 232 - Earth Science.
ŠJim Mihal 2004, 2014- all rights reserved
