🌍 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.

🌍 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.

🌍 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.

🌍 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.

🌍 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.

🌍 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.)

🌍 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.