Question

in this text, rachel m. barker discusses different types of rocks. geology is the study of rocks and other material that make up the earths surface.
(1) geologists classify rocks in three groups, according to the major earth processes that formed them. the three rock groups are igneous, sedimentary, and metamorphic rocks. anyone who wishes to collect rocks should become familiar with the characteristics of these three rock groups. knowing how a geologist classifies rocks is important. if you want to transform a random group of rock specimens into a true collection.
igneous rocks are formed from melted rockthat has cooled and solidified. when rocks are buried deep within the earth, they melt because of the high pressure and temperature, the molten rock (called magma) can then flow upward or even erupt from a volcano onto the earths surface. when magma cools slowly, usually at depths of thousands of feet, crystals grow from the molten liquid, and a coarse - grained rock forms. when magma cools rapidly, usually at or near the earths surface, the crystals are extremely small, and a fine - grained rock results. a wide variety of rocks are formed due to different cooling rates and different chemical makeups of the original magma. obsidian, granite, basalt, and andesite porphyry are four of the many types of igneous rocks.
question 18
what is the central idea of the text?
a. there are three main layers of rock covering the whole earth.
b. there are three main types of rocks that are grouped by how they form.
c. there are three main types of rocks that are grouped by where they are found.
d. there are many different types of rocks and geologists focus on the three most valuable.

Explanation:

The text explains that geologists classify rocks into three groups (igneous, sedimentary, metamorphic) based on the Earth processes that form them. Option B states "There are three main types of rocks that are grouped by how they form," which matches the central idea. Option A is incorrect as it refers to layers, not groups. Option C is incorrect as it focuses on location, not formation. Option D is incorrect as the text doesn't say geologists focus on three "most valuable" types.

Answer:

B. There are three main types of rocks that are grouped by how they form.