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Earth and space science: Earth's systems, hazards and the solar system

How Earth's air, water, rock and living things connect; the carbon cycle and fossil fuels; the atmosphere and climate; plate tectonics and natural hazards; and the Sun, planets, seasons, tides, eclipses, star life cycles and how scientists date rocks with half-life. Includes six questions with worked answers.

By FinishHigh · Published 2026-10-28 · Updated 2026-10-28

Why this matters on the test

About 20 percent of the GED® science test is Earth and space science. Questions often show a diagram, like a cycle, a cross-section of Earth or the Sun, Earth and Moon in a row, and ask what is happening or what comes next.

Most of the topic is about systems: parts that move matter and energy between each other.

Earth's systems and cycles

Earth has four connected systems: the geosphere (rock and soil), the hydrosphere (water), the atmosphere (air) and the biosphere (living things).

In the water cycle, water evaporates, condenses into clouds, falls as precipitation, and runs off or soaks into the ground. Then it evaporates again.

In the carbon cycle, plants take carbon dioxide from the air during photosynthesis. Living things release it again through respiration and decay. Burning fossil fuels (coal, oil and natural gas) releases carbon that was stored underground for millions of years. Fossil fuels formed from ancient plants and sea life, so they are nonrenewable.

Air, oceans and climate

Dry air is about 78 percent nitrogen and 21 percent oxygen. The lowest layer, the troposphere, is where weather happens. The next layer up, the stratosphere, holds the ozone layer, which absorbs much of the Sun's harmful ultraviolet light.

Greenhouse gases such as carbon dioxide and methane trap heat near Earth's surface. Without any, Earth would be far too cold. Adding more from burning fuels traps more heat, which is the main cause of today's climate change.

Weather is the short-term condition of the air (today's rain). Climate is the average pattern over many years.

About 97 percent of Earth's water is salt water in the oceans. Ocean currents move heat around the planet. Coral reefs are harmed when ocean water gets too warm.

Inside Earth, moving plates and hazards

From the outside in, Earth has a thin rocky crust, a thick hot mantle, a liquid outer core and a solid inner core.

The crust and upper mantle are broken into plates that move a few centimeters a year. At divergent boundaries, plates pull apart and new crust forms, as at mid-ocean ridges. At convergent boundaries, plates push together and build mountains, deep trenches and volcanoes. At transform boundaries, plates slide past each other and cause earthquakes, as along California's San Andreas Fault.

Weathering breaks rock down. Erosion carries the pieces away by water, wind or ice.

People cannot stop natural hazards, but they can reduce the harm. Earthquake building codes and bracing protect buildings. Hurricane warnings, evacuation routes and storm shelters protect people. Levees and raised homes help in floods.

The Sun, planets, seasons, tides and eclipses

Our solar system has the Sun, eight planets, five named dwarf planets, hundreds of moons, and many asteroids and comets. The inner planets (Mercury, Venus, Earth, Mars) are small and rocky. The outer planets (Jupiter, Saturn, Uranus, Neptune) are giants made mostly of gas and ice.

Seasons come from Earth's tilt of about 23.5 degrees, not from Earth's distance to the Sun. When your half of Earth tilts toward the Sun, sunlight hits more directly and days are longer. That is summer.

Tides are caused mainly by the Moon's gravity pulling on the oceans. Most coasts get two high tides and two low tides about every day.

A solar eclipse happens when the Moon passes between the Sun and Earth and blocks the Sun. A lunar eclipse happens when Earth passes between the Sun and the Moon, and Earth's shadow falls on the Moon.

Stars, the universe and the age of Earth

Stars form from clouds of gas and dust (nebulas). A star like the Sun spends most of its life as a main-sequence star, then swells into a red giant, and ends as a small white dwarf. Much more massive stars explode as supernovas and leave a neutron star or a black hole.

The universe is about 13.8 billion years old. Earth is about 4.5 billion years old.

Scientists date rocks with radioactive half-life. A half-life is the time it takes for half of a radioactive substance to change into something else. Worked: start with 80 grams and a half-life of 1,000 years. After 1,000 years, 40 g is left. After 2,000 years, 20 g. After 3,000 years, 10 g.

In undisturbed rock layers, the lowest layers are the oldest. This is the law of superposition. Fossils in lower layers are older than fossils above them.

Where this comes from

GED Testing Service's Assessment Guide for Educators: Science lists Earth and space topics ES.a (cycles of matter, fossil fuels, natural hazards and resources), ES.b (atmosphere, oceans, Earth's systems and plate tectonics) and ES.c (stars, the solar system, tides, eclipses and the age of Earth). It says about 20 percent of the test is Earth and space science. We checked the guide and NASA's solar system pages on October 28, 2026.

The explanations, worked numbers and questions are original FinishHigh practice, not GED® test questions. GED® is a registered trademark of the American Council on Education. FinishHigh is independent and not affiliated with or endorsed by it. This lesson teaches a few skills. It is not a full course or an official readiness result.

Next, try the questions below. For reading science graphs, open the lesson "Read graph axes and units before the numbers."

Try this next: 6 questions ↓

ORIGINAL PRACTICE

Your turn.

Try each question before opening the explanation. These are FinishHigh practice examples, not GED® test questions.

It is summer in the United States. Is Earth closer to the Sun than in winter? What causes summer?
Show the answer and why
Not necessarily. In fact, Earth is closest to the Sun in early January, during the U.S. winter. Summer comes from Earth's tilt: the Northern Hemisphere tilts toward the Sun, so sunlight is more direct and days are longer.
Two plates push into each other, and one slides under the other. Name the boundary type and two features it can make.
Show the answer and why
A convergent boundary. It can form deep ocean trenches, volcanoes and mountains, and it can cause earthquakes.
A rock sample starts with 64 grams of a radioactive substance that has a half-life of 2 million years. How much is left after 6 million years?
Show the answer and why
8 grams. 6 million ÷ 2 million = 3 half-lives. 64 → 32 → 16 → 8.
Which kind of eclipse can happen when the Moon is directly between the Sun and Earth?
Show the answer and why
A solar eclipse. The Moon blocks the Sun's light from reaching part of Earth.
Name one way burning fossil fuels changes the carbon cycle.
Show the answer and why
It moves carbon that was stored underground for millions of years into the air as carbon dioxide, much faster than plants and oceans can take it back. That adds greenhouse gas and traps more heat.
A city on a fault line wants to protect people from the next earthquake. Give two sensible steps.
Show the answer and why
Any two of these: enforce earthquake building codes, brace or retrofit older buildings, hold earthquake drills, and keep emergency water and supplies. These reduce harm. They cannot prevent the earthquake.