Geology

What geology studies

Geology is the study of the Earth.^[7] Early geology assumed that change came suddenly, through a series of catastrophes.^[7] In 1785 the Scottish geologist James Hutton proposed the opposite idea, usually summed up as “the present is the key to the past”.^[7]

Inside the Earth

The ancient Greeks already knew Earth’s size, about 12,750 km across, but only around 1900 did scientists find that the planet has three main layers: crust, mantle and core.^[1]

• Crust: a thin, brittle skin. Under the oceans it is usually only about 5 km thick; under the continents it averages about 30 km, and beneath big mountain ranges such as the Alps it can reach 100 km.^[1]
• Mantle: a dense, hot layer of semi-solid rock about 2,900 km thick.^[1]
• Core: nearly twice as dense as the mantle because it is metal (an iron-nickel alloy), with a liquid outer core about 2,200 km thick around a solid inner core about 1,250 km thick.^[1]

The crust and the cooler, stiffer top of the mantle together form a rigid shell called the lithosphere.^[1] Below it, scientists believe, lies the asthenosphere, a zone of hot, semi-solid rock that can slowly flow over geologic time, and the lithosphere is thought to move about on top of it.^[1]

How earthquake waves reveal the inside

Earthquakes send two kinds of waves through solid rock. P (compressional) waves vibrate along their direction of travel and arrive first; S (shear) waves vibrate across it and travel at about 60 percent of P-wave speed.^[3] The outer core is presumed to be liquid because S waves do not pass through it and P waves slow sharply inside it.^[2]

The core was the first inner layer to be identified: R.D. Oldham found it in 1906 from his study of earthquake records.^[2] Per the USGS timeline, Beno Gutenberg later worked out the core’s radius and the depth of its boundary with the mantle, and the Danish seismologist Inge Lehmann discovered the inner core.^[4] The boundary between crust and mantle, where earthquake waves suddenly speed up, is called the Moho after the seismologist who discovered it, Andrija Mohorovičić.^[2]

Plate tectonics

A tectonic plate is a huge, irregular slab of solid rock, usually made of both continental and oceanic lithosphere.^[5] USGS describes about twelve plates moving around Earth’s surface through much of geologic time, at a few centimetres a year.^[2] Earthquakes and volcanoes cluster near their edges.^[5] Besides some broad, poorly defined boundary belts, there are three main kinds of boundary:^[6]

• Divergent: plates pull apart and new crust forms, as along the mid-ocean ridges.^{[6, 9]} NOAA gives the ridge system’s length as nearly 65,000 km, more than 90 percent of it underwater; an older USGS text says more than 50,000 km.^{[9, 8]} NOAA puts spreading at 2 to 5 cm a year on the Mid-Atlantic Ridge and 6 to 16 cm a year on the East Pacific Rise.^[9]
• Convergent: crust is destroyed as one plate dives under another, which is called subduction.^[6] Off South America, the Nazca Plate is sinking under the South American Plate and lifting the Andes.^[6]
• Transform: plates slide past each other sideways, and crust is neither made nor destroyed.^[6]

From drifting continents to moving plates

The Dutch mapmaker Abraham Ortelius suggested moving continents in 1596, but the idea became a full theory, continental drift, only in 1912, with the German meteorologist Alfred Wegener.^[7] His fatal weakness was that he could not satisfactorily explain what forces could move whole continents.^[7]

Evidence from the ocean floor revived his idea after his death.^[7] When lava cools, magnetic grains in it lock in the direction of Earth’s magnetic field, and the sea floor turned out to carry stripes of alternating polarity on both sides of the ridges, with the youngest rock at the crest: a record of the field flipping many times.^[8] The geologist Harry Hess proposed that new ocean crust spreads away from the ridges like a conveyor belt, and Robert Dietz coined the term “seafloor spreading”.^[8]

What moves the plates?

The plates are driven by real but unseen forces that scientists can neither precisely describe nor fully understand.^[10] Most scientists now give the main role to “slab pull”, the sinking of a cold, dense plate edge that drags the rest of the plate along.^[10] Still, no proposed mechanism explains every aspect of plate movement, and none can be tested directly.^[10]

Hotspots

Hawaii formed more than 3,200 km from the nearest plate boundary.^[11] In 1963 the Canadian geophysicist J. Tuzo Wilson explained this with “hotspots”: small, long-lasting, very hot regions under the plates, over which the Pacific Plate carries one island after another.^[11] A note added to that 1996 USGS text says newer studies suggest hotspots are neither deep nor fixed in position, as the popular plume model assumed.^[11]

Minerals, rocks and the rock cycle

A mineral is a naturally occurring inorganic element or compound with an orderly internal structure and its own chemical make-up; quartz, feldspar and calcite are common ones.^[12] A rock is an aggregate of one or more minerals, such as granite, basalt, limestone or sandstone.^[12]

Geologists sort rocks by how they formed.^[13]

• Igneous rocks form when melted rock cools; slow cooling deep underground grows big crystals, fast cooling at the surface makes tiny ones.^[13]
• Sedimentary rocks form at the surface from layers of rock fragments, minerals or plant and animal remains.^[13]
• Metamorphic rocks form when intense heat or pressure transforms buried rock without melting it.^[13] Limestone, for example, becomes marble.^[14]

Wind and water wear rocks down, and the particles settle in lakes or oceans and harden into new rock, so material keeps cycling.^[13]

Reading Earth’s history

In a stack of layered rocks, each layer is older than the ones above it.^[18] Fossils, the remains or traces of past life, help date rocks: the same kinds of fossils in different places show that the rocks are the same age.^[18]

Reliable dates came only in the mid-1900s, with radiometric dating, which uses radioactive parent atoms that turn into stable daughter atoms at a constant rate.^{[17, 18]} The US National Park Service dates the eras this way: the Precambrian before 541 million years ago, the Paleozoic 541 to 252, the Mesozoic 252 to 66, and the Cenozoic from 66 million years ago to today.^[17]

USGS gives the best age for the Earth as 4.54 billion years, with an uncertainty of under 1 percent, worked out from meteorites and lead isotopes.^[16] Earth’s own oldest rocks cannot fix it, because plate tectonics has recycled them; the oldest found so far are the Acasta Gneisses in northwestern Canada, at 4.03 billion years.^[16]

Common misconceptions

“Earth is only thousands of years old.” Many people traditionally believed this.^[15] Radiometric dating shows an age of billions of years.^[16]

“Continents stay put.” Scientists once firmly believed this, but the plates have moved and are still moving.^{[7, 10]}

What this page does not cover

Earthquake and volcano hazards belong to Natural Hazards. Surface processes and the ocean floor are covered more fully in Landscapes and Oceans.

Going further

See Magnetism for Earth’s magnetic field, Atoms & Chemistry for radioactive isotopes and Evolution for the life recorded in fossils.