Earth and Space Science

🌍 Earth and Space Science

Part 1 of 7 — Earth and Space Science on the ASVAB

Topic Overview

Where This Fits on the Test

General Science (GS) is the first subtest of the CAT-ASVAB, so it sets the tone for your whole session.

On the CAT that works out to about 48 seconds per question. Some questions may be unscored tryout items, and you can't tell which ones, so treat every question as if it counts.

The official description of GS lists earth and space science as one of its three areas, alongside life science and physical science. It covers four fields:

Key Fact: GS questions are short. Most are a single fact ("Which layer...?") or a single cause-and-effect step ("What causes...?"). You win this subtest by knowing a compact set of facts cold, and this lesson gives you that set in tables you can memorize.

How Earth and Space Questions Are Asked

Almost every earth and space item is one of four kinds:

The short lists worth memorizing

You will meet each of these again in Parts 2 through 5:

• The three rock types: igneous, sedimentary, metamorphic
• The layers of Earth from the outside in: crust, mantle, outer core, inner core
• The layers of the atmosphere from the ground up: troposphere, stratosphere, mesosphere, thermosphere, exosphere
• The eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

Word roots that unlock answers

Key Fact: A missing fact is not a lost question. If you know two of the four choices belong to the wrong field, you are down to a coin flip in a few seconds.

Warm-up. 🎯 Three quick questions to see what you already know. Don't worry if you miss some; each one is taught later in the lesson.

Key Takeaways — Part 1

• GS is the first CAT-ASVAB subtest: 15 scored questions in 12 minutes, about 48 seconds each
• Earth and space science covers geology, meteorology, oceanography and astronomy
• Most items are a single fact or a single cause-and-effect step, so a compact set of memorized lists goes a long way
• Learn four ordered lists: rock types, Earth's layers, atmosphere layers, planets
• Word roots (geo-, hydro-, seism-, astro-, meteor-) help you eliminate wrong choices fast

Up Next: Part 2 — rocks, minerals and the rock cycle.

🌍 Earth and Space Science

Part 2 of 7 — Rocks, Minerals and the Rock Cycle

Minerals Versus Rocks

A mineral is a naturally occurring, inorganic solid with a definite chemical makeup and an orderly crystal structure. Quartz, feldspar, calcite and halite (rock salt) are minerals.

A rock is a mixture of one or more minerals (or bits of other rocks). Granite, for example, is a rock made of quartz, feldspar and mica crystals.

Key Fact: Minerals are the ingredients; rocks are the recipes. If an answer choice names a single substance with one chemical formula, it is a mineral.

Telling minerals apart

The Mohs hardness scale

Hardness is ranked from 1 (softest) to 10 (hardest). A mineral scratches anything lower on the scale and is scratched by anything higher.

Mnemonic, in order 1 to 10: "Tired Giraffes Can Find Apples For Quick Tasty Crunchy Dinners."

Key Fact: The scale is a ranking, not a ruler. A mineral of hardness 6 scratches one of hardness 5; you don't need any more than that.

The Three Rock Types

Igneous: where it cooled sets the crystal size

Magma is molten rock below the surface; once it erupts it is called lava. Pumice is lava so full of gas bubbles that it can float.

Metamorphic rocks and their parents

The rock cycle

Any rock can become any other type:

• Weathering and erosion break rock into sediment → compaction and cementation make sedimentary rock
• Deep burial with heat and pressure → metamorphic rock
• Melting makes magma → cooling makes igneous rock

Breaking rock down and moving it

Soil is weathered rock mixed with humus, the dark decayed remains of plants and animals.

Key Fact: Weathering breaks, erosion takes, deposition drops. Fossils almost always appear in sedimentary rock, because heat destroys them in igneous and metamorphic rock.

Worked Examples

Example 1. A rock has large pink and white crystals you can see without a magnifier. How did it form?

Large crystals mean the crystals had a long time to grow, which happens when magma cools slowly underground. The rock is intrusive igneous rock, such as granite.

Example 2. A mineral scratches calcite but is scratched by apatite. What is its hardness?

Calcite is 3 and apatite is 5. The mineral scratches 3 and is scratched by 5, so its hardness is between them: 4, which is fluorite.

Example 3. Water seeps into a crack in a rock, freezes and expands, and splits the rock. Is this weathering or erosion?

The rock broke apart where it sits, so this is mechanical weathering (called ice wedging or frost wedging). If a stream then carried the pieces away, that would be erosion.

Rocks and minerals. 🎯

Classify each rock. 🔽

Use the Mohs scale. ✍️

1) The hardness of diamond:

2) A mineral scratches apatite but is scratched by quartz. What whole-number hardness does it have?

Key Takeaways — Part 2

• A mineral is one natural inorganic crystalline substance; a rock is a mixture of minerals
• Mohs scale 1 to 10: talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, diamond ("Tired Giraffes Can Find Apples For Quick Tasty Crunchy Dinners")
• Igneous = cooled molten rock (slow underground → big crystals; fast at surface → tiny crystals or glass)
• Sedimentary = compacted, cemented sediment, in layers, often with fossils
• Metamorphic = changed by heat and pressure without melting: limestone → marble, shale → slate, sandstone → quartzite
• Weathering breaks rock in place, erosion carries it away, deposition drops it

Up Next: Part 3 — Earth's layers, plate tectonics and earthquakes.

🌍 Earth and Space Science

Part 3 of 7 — Earth's Layers, Plate Tectonics and Earthquakes

Earth's Layers, Outside In

Mnemonic: "Cats Make Odd Inventions" → Crust, Mantle, Outer core, Inner core.

Two more names you will see

• Lithosphere ("rock sphere"): the crust plus the rigid top of the mantle. It is broken into large pieces called tectonic plates.
• Asthenosphere: the soft, slowly flowing layer of the upper mantle that the plates ride on.

Key Fact: The outer core is the only liquid layer. Temperature rises all the way to the center, so if a question asks why the inner core is solid even though it is hotter, the answer is pressure.

Read the cross-section. 🔽 Match each description to a numbered layer in the figure.

Plate Tectonics

About 300 million years ago the continents were joined in one supercontinent, Pangaea. Alfred Wegener proposed continental drift after noticing that coastlines (like South America and Africa) fit together and that matching fossils and rocks appear on both sides of oceans. Today we know the continents ride on moving plates, a few centimeters per year.

The three kinds of plate boundaries

Memory hook: Divergent = divide, convergent = collide, transform = slide.

When a dense ocean plate meets a lighter plate, the ocean plate sinks beneath it: this is subduction. The sinking plate melts, and that magma feeds volcanoes.

Earthquakes and Volcanoes

When rocks along a fault suddenly slip, the energy travels outward as seismic waves.

• Focus (hypocenter): the point underground where the slip starts
• Epicenter: the point on the surface directly above the focus

Seismic waves

S waves cannot pass through the outer core. That "shadow" is how scientists learned the outer core is liquid.

Measuring size

A seismograph records the shaking. On the magnitude scale (the Richter scale and its modern replacement), each whole number up means 10 times the ground-shaking amplitude. Two steps up is $10 \times 10 = 100$ times.

Volcanoes

• Most volcanoes sit on plate boundaries, especially the Ring of Fire around the Pacific Ocean.
• Some form over a hot spot, a plume of hot mantle under the middle of a plate (Hawaii).
• An undersea earthquake can push up the water above it and start a tsunami.

Plates and earthquakes. 🎯

Magnitude math. ✍️ Each whole step up the magnitude scale multiplies the ground-shaking amplitude by 10.

1) How many times greater is the shaking amplitude of a magnitude 6 earthquake than a magnitude 4?

2) How many times greater is the shaking amplitude of a magnitude 7 earthquake than a magnitude 6?

Key Takeaways — Part 3

• Layers outside in: crust (thinnest), mantle (thickest), outer core (the only liquid layer), inner core (solid because of pressure)
• Lithosphere = rigid plates; asthenosphere = the soft mantle layer they ride on
• Divergent = divide (ridges, new crust); convergent = collide (trenches, volcanoes, folded mountains); transform = slide (faults, earthquakes)
• Focus is underground; epicenter is the surface point above it
• P waves are fastest and pass through anything; S waves need solids; surface waves do the most damage
• Each magnitude step = 10 times the shaking amplitude

Up Next: Part 4 — water, the atmosphere and weather.

🌍 Earth and Space Science

Part 4 of 7 — Water, Atmosphere and Weather

The Water Cycle

The Sun's energy drives water in a never-ending loop:

The oceans

• About 71% of Earth's surface is covered by water.
• About 97% of Earth's water is salty ocean water. Most of the fresh water is frozen in ice caps and glaciers; much of the rest is groundwater.
• Seawater averages about 3.5% salt (about 35 parts per thousand), mostly sodium chloride.
• Large ocean currents move heat around the planet; the Gulf Stream carries warm water north along the U.S. East Coast.

Key Fact: Condensation is the step that makes clouds. Evaporation goes from liquid to gas; condensation goes from gas back to liquid.

The Atmosphere

What air is made of (dry air)

Water vapor varies from almost none to a few percent depending on the weather.

Key Fact: The most common gas in the air is nitrogen, not oxygen. This is one of the most-asked GS facts.

The layers, from the ground up

Mnemonic: "The Strong Man's Triceps Explode" → Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

Air pressure is the weight of the air above you, so it drops as you go higher. The air on a mountaintop is still about 21% oxygen, but it is thinner, so each breath holds less of it.

Weather

Pressure systems and wind

Wind blows from areas of high pressure toward areas of low pressure.

Fronts (where two air masses meet)

Clouds

Humidity and dew point

Humidity is the water vapor in the air. Relative humidity compares how much there is to the most the air could hold at that temperature. The dew point is the temperature at which the air becomes saturated and vapor starts condensing into dew, fog or clouds.

Weather instruments

Weather, climate and the greenhouse effect

• Weather is the condition of the air right now or this week. Climate is the average pattern over many years.
• Greenhouse effect: sunlight warms the ground; the ground gives off heat (infrared energy); gases such as carbon dioxide, water vapor and methane absorb some of that heat and keep the planet warm. More of these gases traps more heat.
• The ozone layer is a different thing: it blocks incoming UV light.

Water and weather. 🎯

What does each instrument measure? 🔽

The air you breathe. ✍️ Give each answer as a whole-number percent.

1) About what percent of dry air is nitrogen?

2) About what percent of dry air is oxygen?

Key Takeaways — Part 4

• Water cycle: evaporation, transpiration, condensation (clouds), precipitation, runoff, infiltration (groundwater)
• About 97% of Earth's water is salty; most fresh water is frozen in ice caps and glaciers
• Air is about 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide
• Layers: troposphere (weather), stratosphere (ozone), mesosphere (meteors), thermosphere (auroras), exosphere — "The Strong Man's Triceps Explode"
• High pressure = fair; low pressure = clouds and storms; wind blows from high to low
• Cold front = quick storms then cooler; warm front = long steady drizzle
• Barometer = pressure, anemometer = wind speed, hygrometer = humidity
• Greenhouse gases trap heat; the ozone layer blocks UV — different things

Up Next: Part 5 — the solar system, stars and seasons.

🌍 Earth and Space Science

Part 5 of 7 — The Solar System, Stars and Seasons

The Eight Planets

Mnemonic: "My Very Educated Mother Just Served Us Nachos."

• The four inner planets are small, rocky and dense. The four outer planets are huge, made mostly of gas and ice, and all have rings and many moons.
• Pluto was reclassified as a dwarf planet in 2006.
• A comet is a ball of ice and dust; near the Sun it grows a tail that points away from the Sun.
• A meteoroid is a space rock; a meteor is the streak of light when it burns in our air; a meteorite is a piece that reaches the ground.

The Sun and Other Stars

• The Sun is a star of average size and temperature (a yellow star). It shines by nuclear fusion: hydrogen nuclei fuse into helium, releasing energy.
• Sunlight takes about 8 minutes to reach Earth.
• A star's color shows its surface temperature: blue stars are hottest, then white, yellow, orange, and red stars are coolest.

The life of a star

Distances in space

A light-year is the distance light travels in one year, about 9.5 trillion kilometers (about 6 trillion miles). Despite the word "year," it measures distance, not time. The nearest star after the Sun is about 4.2 light-years away. Our Sun is one of billions of stars in the Milky Way, a spiral galaxy.

Earth, the Moon and the Seasons

Rotation and revolution

Earth turns 360° in $24$ hours, which is $360 \div 24 = 15$ degrees of longitude per hour. That is why the world uses 24 time zones, each about 15° wide.

Seasons come from the tilt

Earth's axis is tilted about 23.5°. When your hemisphere leans toward the Sun, sunlight hits it more directly and days are longer: summer. When it leans away: winter.

Key Fact: Seasons are not caused by Earth's distance from the Sun. Earth is actually closest to the Sun in early January, during the Northern Hemisphere's winter. The hemispheres have opposite seasons.

The Moon

• The Moon orbits Earth about once a month and always shows us the same side.
• Phases happen because we see different amounts of the Moon's sunlit half as it orbits, not because of Earth's shadow. Order: new → waxing crescent → first quarter → waxing gibbous → full → waning gibbous → third quarter → waning crescent → new (about 29.5 days).

Eclipses

The name tells you what gets blocked: in a solar eclipse the Sun is blocked; in a lunar eclipse the Moon is darkened.

Tides

Tides are caused mainly by the Moon's gravity pulling on the oceans (the Sun adds a smaller pull). Most coasts get two high tides and two low tides about every day.

"Spring" here has nothing to do with the season; think of the water "springing up."

Read the line-up. 🎯 Use the figure above.

Read the tilt. 🎯 Use the figure above.

Space facts. 🎯

Time zones. ✍️ Earth turns 360° in $24$ hours.

1) How many degrees of longitude does Earth turn through in one hour?

2) Two cities are 45° of longitude apart. About how many hours apart are their clocks (solar time)?

Key Takeaways — Part 5

• Planets in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune ("My Very Educated Mother Just Served Us Nachos")
• Mercury closest and smallest; Venus hottest; Mars red; Jupiter largest; Saturn's rings; Neptune farthest
• The Sun is an average star powered by fusion of hydrogen into helium; blue stars are hottest, red coolest
• Sun-like stars end as white dwarfs; massive stars explode as supernovas
• A light-year is a distance
• Rotation (24 h) = day and night; revolution (365¼ days) = year; 15° per hour = time zones
• Seasons come from the 23.5° tilt, not distance
• Solar eclipse = Moon in the middle (new moon); lunar eclipse = Earth in the middle (full moon)
• Tides come mainly from the Moon; spring tides (new/full) are biggest, neap tides (quarters) smallest

Up Next: Part 6 — test strategy and the traps GS questions set.

🌍 Earth and Space Science

Part 6 of 7 — Test Strategy and Common Traps

Your Clock on GS

Remaining time and remaining questions show in the lower-right corner of the CAT screen. Most people finish each subtest early: the official materials describe the time limits as generous.

How to spend those 48 seconds

1. Read the whole question. Words like "NOT," "most likely" and "mainly" change the answer.
2. Answer in your head first, then look for your answer among the choices.
3. If you don't know it, eliminate. Cross out choices from the wrong field (a weather word in a rock question) and choices that state a famous myth.
4. Commit and move on. On the CAT you cannot go back to a question once you answer it, and you must answer to move on, so don't second-guess.

Key Fact: If time is running out on the CAT, keep reading and genuinely answering. The official guidance warns that the test applies a relatively large penalty when several wrong answers are given toward the end of a subtest, so a rapid string of random guesses hurts. Unfinished questions are scored as if you had guessed at random. (On the paper test, there is no penalty for guessing: answer everything.)

The Traps Earth and Space Questions Set

Each wrong answer below is popular because it sounds right.

Spotting the field from a single word

Dodge the trap. 🎯

Pace check. ✍️ On the CAT, GS gives you about 48 seconds per question.

1) At that pace, how many minutes do 5 questions take?

2) At that pace, how many minutes do all 15 questions take?

Key Takeaways — Part 6

• CAT GS: 15 questions, 12 minutes, about 48 seconds each; you can't go back, so commit
• Answer in your head first, then find it among the choices; eliminate by field and by myth
• Near the time limit, keep genuinely answering: a burst of wrong guesses at the end is penalized
• The big myths: seasons from distance, phases from Earth's shadow, light-year as time, meteorology as meteors, Mercury hottest, oxygen most common, ozone as warming, liquid inner core

Up Next: Part 7 — a mixed mastery set at ASVAB difficulty.

🌍 Earth and Space Science

Part 7 of 7 — Mixed Mastery Set

How This Set Works

Twelve questions mixed from every part of the lesson, at the difficulty you will see on the CAT-ASVAB: some quick recall, some one-step reasoning, a few harder ones.

Give yourself about 48 seconds per question, the real GS pace. Answer in your head first, eliminate, commit.

Mastery set, questions 1 to 5. 🎯

Mastery set, questions 6 to 9. 🎯

Mastery set, questions 10 to 12. 🔽

Key Takeaways — Earth and Space Science

• Rocks: igneous (cooled melt), sedimentary (layers, fossils), metamorphic (heat and pressure); Mohs 1 talc to 10 diamond
• Interior: crust, mantle, liquid outer core, solid inner core; divide / collide / slide; P waves first, S waves need solids
• Air and water: 78% nitrogen; weather in the troposphere, ozone in the stratosphere; low pressure = storms; condensation makes clouds
• Space: My Very Educated Mother Just Served Us Nachos; Venus hottest, Jupiter largest; light-year = distance
• Earth and Moon: seasons from tilt; solar eclipse at new moon, lunar at full; spring tides biggest

Missed one? The exit quiz will send you back to the part that teaches it.