Geology and Earth Structure

Earth science is the most orderly part of General Science. Almost everything here is a set of three or four things with one distinguishing feature each.

Earth's layers

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The inner core is solid and the outer core is liquid, and that is the pairing the test asks. The inner core is hotter, and it is solid anyway because the pressure at that depth is enormous - which is a satisfying piece of physics rather than a fact to memorize.

Convection in the mantle is what moves the plates. Hot material rises, cools, and sinks, and the crust rides on top of that circulation.

Two more names for layers, defined by how the rock behaves rather than what it is made of: the lithosphere is the rigid outer shell - the crust plus the top of the mantle - broken into the plates. The asthenosphere beneath it is the soft, slowly flowing part of the mantle the plates slide over.

Plate tectonics

The crust is broken into plates that move a few centimeters a year - about as fast as fingernails grow.

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Subduction is one plate sliding beneath another at a convergent boundary. The denser oceanic plate goes under the lighter continental one, which is why deep trenches sit offshore and volcanoes sit inland from them.

The transform boundary is the one to know precisely, because it is defined by what it does not do. No new crust is created, none is destroyed, and no volcanoes form. Only earthquakes. The San Andreas fault is the standard example.

Earthquakes occur at all three. Volcanoes occur at convergent and divergent boundaries only. That asymmetry is a question.

Continental drift was the observation that the continents fit together and once formed a single mass, called Pangaea. It was proposed by Alfred Wegener in 1912 and mocked for decades, because he could not say what moved the continents. Plate tectonics, and mantle convection, is the explanation he lacked.

Volcanoes

Runny lava gives gentle eruptions; thick lava traps gas and explodes. A caldera is the huge basin left when a volcano's top collapses into its emptied magma chamber, which is how Crater Lake formed.

Earthquakes

The focus is where the rock actually slips, underground. The epicenter is the point on the surface directly above it.

Epicenter is on the surface, focus is below it. That distinction is asked almost every time earthquakes appear.

A seismograph records the waves. The Richter scale measures magnitude and is logarithmic: each whole number is about ten times the amplitude of the one below, so a magnitude 7 is not slightly worse than a 6.

Seismic waves come in types, and the useful contrast is that P waves are faster and arrive first - P for primary - while S waves are slower and do more damage. P waves travel through liquid and S waves do not, which is how we know the outer core is liquid.

The rock cycle

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Three types, named for the process that made them:

Igneous means fire, sedimentary means settling, metamorphic means changed form. The names are the definitions.

Two facts the test reaches for:

Fossils are found almost exclusively in sedimentary rock, because the heat that makes igneous and metamorphic rock destroys them.

The pairs matter: limestone becomes marble, shale becomes slate, sandstone becomes quartzite. A question giving you one and asking for the other is asking about metamorphism. Many metamorphic rocks show foliation - their minerals squeezed into parallel bands or layers - which is what lets slate split into sheets.

Any rock can become any other. The cycle has no start and no end, which is the point of drawing it as a cycle.

Minerals

A mineral is a naturally occurring, inorganic solid with a definite chemical makeup and an orderly crystal structure. Rocks are made of minerals: granite, for instance, is mostly quartz and feldspar.

Minerals are identified by their properties, and these are the ones asked:

Color is the least reliable property, because small impurities change it. Streak is more reliable, which is why it exists.

The Mohs scale runs from talc (1) to diamond (10). A harder mineral scratches a softer one. Quartz is 7, which is why it scratches glass.

Three minerals worth naming: quartz (hard, glassy, very common), feldspar (the most common mineral in the crust) and calcite (the mineral in limestone and marble, which fizzes in acid).

Reading the rock record

Superposition: in undisturbed layers of sedimentary rock, the lowest layers are the oldest, because each layer was laid down on top of the last. That gives relative age - older or younger - without a number.

Radiometric dating gives the number, by measuring how much of a radioactive element has decayed. Its half-life - the time for half of it to decay - is the clock.

Uniformitarianism is the principle that the processes shaping the Earth now - erosion, deposition, volcanoes - are the same ones that shaped it in the past. It is what lets geologists read ancient rock by watching modern rivers.

Geologic time is split into eras, oldest first: Precambrian, Paleozoic, Mesozoic (the age of dinosaurs) and Cenozoic (the age of mammals, which includes today).

Weathering and erosion

Different words for different steps, and the test separates them.

Weathering breaks rock down in place. Physical weathering breaks it without changing it chemically - frost wedging, plant roots. Chemical weathering alters the minerals - acid rain dissolving limestone, iron rusting.

Erosion moves the pieces away, by water, wind, ice or gravity.

Deposition drops them somewhere else, which is where sedimentary rock begins. A delta is deposition you can see from space: the fan of sediment a river drops where it slows down entering the sea, like the Mississippi's.

Break, move, drop. Weathering, erosion, deposition, in that order.

What you can skip

Geology has a vast vocabulary and the questions use the everyday end of it. Across all 2,104 General Science questions in our bank:

• The named geologic periods - Jurassic, Cretaceous, Devonian - come up once each. Know the four eras in order and which one had the dinosaurs.
• The minerals beyond quartz, feldspar and calcite. Halite, olivine and the rest appear once apiece.
• Unconformities by type - angular, nonconformity, disconformity - come up twice in all, at the hard end. Superposition carries the relative-dating questions.
• Rock textures by name (vesicular, porphyritic) appear once. Crystal size and where the rock cooled is the idea.

Where people lose points

Saying the inner core is liquid. The outer core is.

Putting volcanoes at transform boundaries. Earthquakes only.

Confusing focus and epicenter.

Saying the Richter scale is linear.

Looking for fossils in igneous rock.

Using weathering and erosion interchangeably.

Work one in under a minute

A rock is found with visible layers containing shell fragments. It is later subjected to heat and pressure deep underground. What type was it, and what does it become?

Layers and shell fragments mean sedimentary - specifically limestone, given the shells.

Heat and pressure make it metamorphic, and limestone becomes marble.

Note what the clues did: layers and fossils identify sedimentary, and the process named identifies the destination.

Where this leads

The water that drives weathering and erosion is the subject of the water cycle lesson, and the atmosphere sits on top of all of this.