Geology

• 1. Introduction

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  Geology is the branch of natural science concerned with the Earth, the rocks of which it is composed, and the processes by which it changes over time. The word itself was first used by Ulisse Aldrovandi in 1603, then by Jean-Andre Deluc in 1778, before Horace-Benedict de Saussure fixed it as a term in 1779. It comes from the Greek ge, meaning earth, and logos, meaning speech. So how do scientists read the story of a planet that is roughly 4.5 billion years old? How can a single rock reveal whether older layers slid on top of younger ones, or whether continents once drifted across the surface? The answers run from a porcelain plate scratched with a mineral, to seismic waves imaging the Earth's core like a CT scan, to a memoir written by a man who crossed the Allegheny Mountains some 50 times. This is the science of reading the Earth itself.

• 2. Reading The Rocks

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  Minerals are naturally occurring elements and compounds with a definite homogeneous chemical composition and an ordered atomic arrangement. To identify one, a geologist runs a battery of simple tests. Scratching a sample across a porcelain plate produces a streak whose color can name the mineral. Dripping hydrochloric acid checks for effervescence, the telltale fizz. A magnet tests for magnetism, and taste itself can be diagnostic, since halite tastes like table salt. Hardness, luster, specific gravity, and the way a sample breaks all narrow the answer further.

  A rock is any naturally occurring solid mass or aggregate of minerals, and there are three major types. When rock solidifies from melt, whether magma or lava, it is igneous. Rock that is weathered, eroded, redeposited, and lithified becomes sedimentary, divided into sandstone, shale, carbonate, and evaporite. Heat and pressure then transform igneous and sedimentary rock into metamorphic rock, with its characteristic fabric. All three can melt again, and the cycle begins anew. Organic matter such as coal, bitumen, oil, and natural gas is linked mainly to organic-rich sedimentary rocks.

  Above the bedrock lie unlithified materials called superficial deposits. Studying them is often called Quaternary geology, named for the Quaternary period, the most recent stretch of geologic time.

• 3. The Grand Unifying Theory

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  In the 1960s came a discovery that gave geology a single unifying framework. The Earth's lithosphere, which includes the crust and the rigid uppermost upper mantle, is broken into tectonic plates. These plates ride across the plastically deforming asthenosphere below. Seafloor spreading and the global distribution of mountain terrain and seismicity all support the idea.

  The plates are intimately coupled to the convection of the mantle, the slow heat transfer carried by ductile mantle rock. Oceanic lithosphere is really the rigid upper thermal boundary layer of that convecting mantle, so it moves in lockstep with the currents beneath it. Long linear features mark the plate boundaries. Mid-ocean ridges, where hydrothermal vents and volcanoes sit, are divergent boundaries where two plates pull apart. Arcs of volcanoes and earthquakes mark convergent boundaries, where one plate subducts beneath another. The San Andreas Fault system is a transform boundary, where plates slide horizontally past one another.

  Alfred Wegener had proposed continental drift, the idea that continents move across the surface over geological time. Plate tectonics finally supplied the mechanism. The theory's power is its ability to fold seafloor spreading, mountain building, earthquakes, and crustal deformation into one account of how the lithosphere moves over the convecting mantle, a grand unifying theory of geology.

• 4. Imaging The Interior

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  Seismologists read the interior of the Earth using the arrival times of seismic waves. Early work revealed a liquid outer core, where shear waves could not propagate, wrapped around a dense solid inner core. This produced a layered model: a lithosphere on top, a mantle below split by seismic discontinuities at 410 and 660 kilometers, and the outer and inner cores beneath.

  Starting in the 1970s, a technique called seismic full-waveform inversion let scientists image wave speeds inside the Earth much as a doctor images a body in a CT scan. The detailed pictures replaced the simple layered model with a far more dynamic one. Mineralogists then took the pressure and temperature data from those studies and recreated the conditions in the laboratory, measuring how crystal structures change. Their experiments explain the chemical shifts tied to the mantle's seismic discontinuities and predict the crystallographic structures expected in the inner core.

• 5. The Earth's Calendar

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  The geological time scale begins with the first Solar System material at 4.567 billion years ago and the formation of the Earth at 4.54 billion years, the start of the Hadean eon. A proposed Moon-forming impact follows at about 4.5 billion years. Around 4 billion years ago, the Late Heavy Bombardment ends and the first life appears, with photosynthesis starting near 3.5 billion years.

  At roughly 2.3 billion years ago came an oxygenated atmosphere and the first snowball Earth. Supercontinents rise and fall: Columbia at 1.8 billion years, Rodinia at 1100 million years, Pannotia at 650 million. At 541 million years the Cambrian explosion multiplies hard-bodied life and leaves the first abundant fossils, opening the Paleozoic. The Permian-Triassic extinction at 250 million years kills 90 percent of all land animals, and the Cretaceous-Paleogene extinction at 66 million years ends the dinosaurs.

  The later milestones turn toward our own lineage. The Himalayas form around 45 million years ago. First hominins appear near 7 million years ago, the first Australopithecus at 3.9 million years, and the first modern Homo sapiens in East Africa around 200 thousand years ago. The Moon and Mars keep their own scales: lunar epochs are dated by major impacts, so the Imbrian is named for the Mare Imbrium basin.

• 6. Telling Time In Stone

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  James Hutton, an 18th-century Scottish physician and geologist, gave geology a guiding rule: the present is the key to the past. In his words, the past history of our globe must be explained by what can be seen to be happening now. This is the principle of uniformitarianism, that processes shaping the crust today have worked much the same way across geological time.

  A set of relative-dating principles followed from that foundation. Intrusive relationships hold that an igneous intrusion cutting across sedimentary rock is younger than the rock it cuts. Cross-cutting relationships say faults are younger than the rocks they break. The principle of inclusions states that clasts found within a formation are older than the formation, just as xenoliths are older than the rock that carries them. Original horizontality says sediments are laid down as horizontal beds, and superposition says a layer is younger than the one beneath it and older than the one above. Faunal succession, drawn from William Smith's work nearly a hundred years before Charles Darwin published his theory of evolution, uses fossils to give the relative age of strata.

  At the start of the 20th century, radioactive isotopes opened the door to absolute dating. Geologists could now assign true ages to rock units instead of only ordering them. Closure temperature, the point at which radiometric isotopes stop diffusing through a crystal lattice, anchors the method. The most suitable systems include uranium-lead, rubidium-strontium, and potassium-argon, while uranium-thorium dating handles calcium carbonate. Optically stimulated luminescence and cosmogenic radionuclide dating date surfaces and erosion rates, dendrochronology dates landscapes, and radiocarbon dating handles geologically young material that contains organic carbon.

• 7. Forces That Shape Land

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  The geology of an area changes as rock units are deposited, intruded, and then deformed. Deformation comes in three flavors: horizontal shortening, horizontal extension, and side-to-side strike-slip motion, broadly matching convergent, divergent, and transform plate boundaries. Under horizontal compression, rock shortens and thickens through faulting and folding. Thrust faults can drive older rock on top of younger, inverting the order set by superposition.

  Deeper in the Earth, rock behaves plastically and folds rather than breaks. Folds that buckle upward make antiforms, those that buckle downward make synforms, and when the tops still point up they are anticlines and synclines. Extension does the opposite, stretching and thinning rock through normal faulting. At one spot in the Maria Fold and Thrust Belt, the entire sedimentary sequence of the Grand Canyon appears over a length of less than a meter. Stretched rocks can pinch into lenses called boudins, after the French word for sausage.

  Not every area carries the same story. The Hawaiian Islands consist almost entirely of layered basaltic lava flows. The Grand Canyon holds nearly undeformed stacks of sedimentary rock that have stayed in place since Cambrian time. By contrast, the Acasta gneiss of the Slave craton in northwestern Canada, the oldest known rock in the world, has been metamorphosed so thoroughly that its origin cannot be read without laboratory analysis.

• 8. From Theophrastus To Tectonics

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  Theophrastus, who lived from 372 to 287 BCE, wrote Peri Lithon, or On Stones, one of the earliest works on the physical material of the Earth. In the 4th century BCE, Aristotle observed that the Earth changes too slowly to be seen within a single lifetime. The Persian geologist Abu al-Rayhan al-Biruni, from 973 to 1048 CE, hypothesized that the Indian subcontinent was once a sea. Ibn Sina, known as Avicenna and living from 981 to 1037, explained the formation of mountains and the origin of earthquakes. In China, the polymath Shen Kuo, from 1031 to 1095, inferred from fossil shells in a mountain hundreds of miles from the ocean that land forms through erosion and deposition of silt.

  Georgius Agricola, who lived from 1494 to 1555, published De Natura Fossilium in 1546 and is seen as the founder of geology as a scientific discipline. Nicolas Steno, from 1638 to 1686, is credited with the law of superposition and the principles of original horizontality and lateral continuity. Followers of Hutton were called Plutonists, holding that some rocks form from volcanic lava, against the Neptunists led by Abraham Werner, who believed all rocks settled out of a shrinking ocean. The first geological map of the United States came in 1809 from William Maclure, who began his survey in 1807 and crossed the Allegheny Mountains some 50 times. His map antedated William Smith's map of England by six years.

  Sir Charles Lyell, from 1797 to 1875, published Principles of Geology in 1830, a book that influenced Charles Darwin and promoted uniformitarianism against catastrophism. Much of 19th-century geology turned on the Earth's age, with estimates ranging from a few hundred thousand to billions of years. By the early 20th century, radiometric dating set the figure near two billion years. Today the Earth is known to be approximately 4.5 billion years old, the same patient record that planetary geologists now read on Mars and the Moon, where the Phoenix lander once analyzed Martian polar soil for traces of biological processes.