Earth History

Human recorded history dates back only a few thousand years on a planet that has existed for about 4.5 billion years.  The answer is that this history is recorded in rocks and fossils, relics of organisms that lived and died long ago.  reading this history is not as easy as reading words on a page, but with proper scientific tools it can be read just as reliably.  What is required is an understanding of how fossils and rocks are made, why the are found in distinct geological layers, and how we can determine their ages through rediometric dating and other techniques.  Rocks and fossils tell us about the Earth's past in a variety of ways.  For example, a rock's structure and composition depend on the conditions under which it formed, which may offer clues to the climate at the time.  Fossils may also tell us about past environmental conditions and allow us to reconstruct the evolutionary history of life.  Reading the history recorded in rocks and fossils--often called the rock record and fossil record, respectively--requres knowing a little about how rocks and fossils are formed.

Geologist classify rocks into three basic types according to how they are made.  Igneous rock is made from molten rock that cools and solidifies.  Sedimentary rock is made by the gradual compression of sediments, such as sand and silt at the bottoms of seas and swamps.  Metamorphie rock is rock that has been structurally transformed by high pressure or heat that is not quite high enough to melt it.

Note that rock can change from one type to another.  Igneous rock is often made from sedimentary or metamorphic rock that has been carried deep underground and melts under high heat and pressure.  This molten rock cools and solidifies into igneous rock as it rises toward the surface or erupts from a volcano.  Sedimentary rock may be made as erosion breaks up existing rock into sediments that are carried to the sea.  Metamorphic rock is made from igneous or sedimentary rock that is transformed by high pressure or heat.  Because rock can be recycled among the three types, the rock type is not directly related to its composition.  In fact, individual rocks of any of the three types usually contain a mixture of different crystals in close contact.  Each individual crystal represents a particular mineral, which is the word used to describe a crystal of a particular chemical composition and structure.  Thus, a rock's type (igneous, metamorphic, or sedimentary) tells us how it was made, while its mineral composition tells us what it is made of.

If you study geology in greater depth, you'll learn a variety of other terms used by geologists to sub-classify rocks by their type and mineral composition.  Igneous rocks may differ substantially from one another depending on exactly what melted to make them, and geologists have special names for different types of igneous rocks.  For example, a type of dark, dense igneous rock commonly produced by undersea volcanoes is called basalt; another common type of igneous rock is granite, which is named for its grainy appearance and is often found in mountain ranges.  We generally find recognizable fossils only in sedimentary rocks, because the high heat and pressure involved in forming metamorphic or igneous rocks tends to destroy fossils.  Although we tend to think of fossils as "remains" of living organisms, most fossils contain little or no organic matter.  In general, when an organism dies and gets buried in sediments, minerals dissolved in groundwater gradually replace organic material.  Mineral-rich portions of organisms, such as bones, teeth, and shells, may be left behind, becoming fossils like those of the dinosaur bones displayed in many museums.  In some cases, the mineral replacement is complete and organisms literally turn to stone; the "stone trees" of Arizona's Petrified Forest formed in this way.  In many other cases, the organisms themselves decay, but in doing so they leave an empty mold that fills with minerals dissolved in water.  The minerals may then make a cast in the shape of the dead organism.  More rarely, some of the organic material from a dead organism may be preserved well enough to allow at least some study.  Some fossil plant leaves are still green and well enough preserved for their cells to be studied with microscopes, even though they died millions of years ago.  In other rare cases, whole organisms may be preserved in tree resin or frozen in ice.  One of the most interesting types of fossil is left not by a dead organism but by the activity of an organism while it was alive.  For example, "coprolites" are rocks that consist of petrified excrement, which can allow us to learn about an animal's diet.  In other cases, scientists have found fossilized dinosaur footprints, made when mineral processes preserved impressions left by dinosaur as it walked through soft soil or mud.  Such fossil tracks provide clues about how dinosaurs walked and can help scientists hypothesize about dinosaur behavior.

Very few living organisms leave fossils behind.  For example, scientist have discovered only a small number of complete dinosaur skeletons despite the huge number of dinosaurs that once must have roamed the Earth.  Fossils are rare because most dead organisms decay becoming food for living organisms in the soil--long before any mineral replacement can occur.  Nevertheless, over millions and billions of years, enough dead organisms have become fossilized to leave a substantial fossil record.