Geologists are historians of the Earth. A number of important tools have been available to determine the sequence of geologic events. These allow us to understand relative geologic time. Other methods that were developed only in this century have allowed us to study absolute geologic time, to determine the number of years (or millions of years) before present that certain events occurred.
Sequence of Geologic Events - Relative Geologic Time
Steno's principles
Nicolaus Steno (1669) formulated principles that told how
sedimentary rock layers formed. Steno recognized that sediments were
deposited in horizontal layers (original
horizontality) that were continuous throughout the
basin of deposition (original
continuity), with the bottom layers deposited first
and the upper layers afterwards
(superposition). Therefore tilted strata
must have been disturbed, and gorges and stream valleys, etc., must
have formed by erosion after the strata were formed. The oldest
strata lie on the bottom of a sequence (at least if it hasn't been
tilted to the point of being overturned).
Crosscutting relations
Any feature that cuts across another geologic feature must be
younger than the thing that is cut ("the cutter is younger than the
cuttee"). For example, a dike that intrudes through sedimentary
strata, or a fault that offsets strata, must be younger than the
strata that are intruded or offset.
baked contacts
When an igneous intrusion occurs, the rock that was intruded
is often heated around the intrusion to the point that it becomes
contact metamorphosed. Baked contact zones are particularly helpful
where an igneous rock has intruded another igneous rock. The rock
that was metamorphosed must be the older.
inclusions (components)
Inclusions are often found in basal conglomerates at
unconformities. In the case of an unconformity within sedimentary
strata the sequence of events is not a problem because we can use
superposition. But in the case of an igneous intrusion inclusions can
be a big help. Inclusions of the granite in a basal conglomerate at
the base of an overlying sedimentary sequence would show that the
granite was older. However, rafts (blocks) of un-melted sedimentary
rock found within the body of the granite would indicate that the
sediments were older and the granite intruded them.
fossil succession
William Smith ("Strata Smith" 1796 ->) surveyed for canal
excavations and also studied strata in coal mines. He recognized
strata over long distances from the fossils in them. He published a
geologic map in 1815. Georges Cuvier with Alexandre
Brongniart published a geologic map of the Paris Basin based on
fossil correlations in 1811. By the turn of the nineteenth century
these men recognized that there had been a progression of organisms,
preserved as fossils. There are distinct fossils in each sequence of
strata that can be used to identify the relative age of the
strata.
The Geologic Timescale
Adam Sedgewick began the systematic description of geologic systems. He studied strata in Wales that rested on unfossiliferous rock. He named his strata the Cambrian system after the Roman's name for Wales. His friend Roderick Murchison described the fossils in strata overlying his friend Sedgewick's Cambrian strata in Wales and southwestern England. He named these strata the Silurian system after the Silures, an ancient Welsh tribe. They published their results in 1835. The two friends came to argue because they could not agree on a suitable boundary between the two systems. Later, in 1879, Charles Lapworth showed that there were three distinctive systems of fossils within the sequence and subdivided these strata as Cambrian, Ordovician, and Silurian systems, thus ending the dispute. In Devonshire, England, other strata overlay strata containing Silurian fossils. These younger strata and fossils were called the Devonian system. In central England coal-bearing strata overlay Devonian strata. These strata were described as the Carboniferous system. Strata in the Perm Basin in Russia were found to overlay the Carboniferous strata. These were called the Permian system. In eastern Europe strata that overlay Permian strata were found with three major divisions. These layers were called the Triassic system. Other strata overlying Triassic strata in the Jura mountains were called the Jurassic system. Chalk-bearing strata that overlay Jurassic strata in northwestern Europe and southeastern England were called the Cretaceous system.
By the middle to late nineteenth century most of the periods and epochs of the geologic timescale were defined. The rock systems described in the field could be thought of in terms of geologic time as geologic periods. So the Cambrian system of rocks implied a Cambrian Period of time. However, there was no clear idea as to how much time the geologic timescale represented, especially for the older rocks that the Cambrian system lay on. Those pre- Cambrian rocks were the most difficult to deal with because they were without fossils and were mostly metamorphosed.
How Old is the Earth? - Absolute Geologic Time
Coming into the modern era there were few modern ideas about the age of the Earth. Many scientists simply accepted the Biblical account in Genesis.
Bishop Ussher (1654) determined from the genealogies in the Bible that the Earth was created October 23, 4004 B.C. In effect, the Earth was less than 6000 years old.
A.G. Werner (1787) in his theory known as Neptunism he thought that mountains and most rocks were formed as chemical precipitates from a primeval sea that once covered the whole Earth (the book of Genesis in the Bible states that when the Earth was first formed waters covered its surface). The various rocks were precipitated as the primeval sea receded. This view of the Earth as forming very quickly is known as catastrophism.
James Hutton (1785) "the first
modern geologist"
Hutton was a great observer. He recognized that through the
deposition of sediments like sand, silt, and clay, sedimentary rocks
resembling ancient rocks are presently forming in the sea. He
believed that ancient rocks had been formed in the same way that
modern rocks are being formed. His view of an Earth that evolved
gradually over a long period of time by way of the same processes
that are acting today is known as uniformitarianism.
Uniformitarian gradualism contrasts with various theories of
catastrophism in which the Earth's features were thought to have come
about in sudden events. Charles Lyell (1830) was a
champion of uniformitarianism. He wrote the first standard textbook
of geology "The Principles of Geology". In it, he codified the law
of crosscutting relations to add to Steno's Principles and
Hutton's angular unconformities as tools for deciphering geologic
time.
Hutton, Lyell, and their followers of uniformitarianism understood
that geologic processes are very slow. Therefore the Earth must be
very old (millions of years?).
Late 19th century geologists added up the maximum known thicknesses of Cambrian through recent sedimentary strata and studied modern sediment accumulation rates. From these two factors they determined that it must have taken at least 75 to perhaps 100 million years to account for all of these sediments.
Joly (1899) an Irish scientist, following a proposal by Sir Edmund Halley of comet fame (1715), estimated the age of the Earth as around 100 million years from determining the delivery rate of salts to the oceans from the world's rivers and from calculations of the total amount of salt in the oceans.
Lord Kelvin (late 19th century) studied heat flow in common materials. He assumed the Earth was initially molten and has been cooling to its present conditions. He estimated that the Earth has been cooling for 20 to 40 or perhaps 100 million years.
Marie Curie in the Paris laboratory of Henri Becquerel (1896) discovered radioactivity. Certain elements, or isotopes of elements, decay spontaneously into other elements or isotopes. This decay process releases particles and energy (gamma radiation).
Ernest Rutherford noted that radioactive decay and the resultant production of heat from the energy released in the Earth's interior would invalidate Lord Kelvin's estimate of the age of the Earth.
Rutherford and Soddy (1902) determined that the rate of radioactive decay was proportional to the number of radioactive atoms in a sample and that the number of radioactive atoms should decrease over time while producing increasing numbers of daughter products. The ratio of unstable (radioactive) parent atoms to stable daughter atoms should therefore increase with time since the formation of a rock or mineral that incorporated the unstable parent. Rutherford suggested that this could form the foundation for dating natural Earth materials.
B.B. Boltwood (1907) was the "first man to date a rock". Following Rutherford's suggestion, he determined the rate of decay of uranium into lead. He then measured the ratio of radioactive uranium atoms (parent atoms) to lead atoms (daughter atoms) in several rocks from various places on the Earth and found that the rocks ranged in age from 410 million to 2.2 billion years, far exceeding previous estimates.
These weren't even the oldest rocks on Earth. Through the twentieth century geologists have refined radiometric dating techniques and applied them to dating the Earth and the divisions of the geologic timescale.
The oldest rocks formed on Earth have been dated radiometrically at 4 billion years.
The oldest minerals formed on Earth have been dated radiometrically at 4.3 to 4.4 billion years.
The oldest pieces of rock in the solar system are moon rocks and meteorites that have been dated at 4.5 to 4.6 billion years.