A study published by Rice University geoscientist Ming Tang in 2019 offers a new explanation for the origin of Earth's...

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

• A study published by Rice University geoscientist Ming Tang in 2019
  • offers a new explanation
    • for the origin of Earth's continents (?) geological structures called arcs,
      • towering ridges that form
        • when a dense oceanic plate subducts under a less dense continental plate,
          • melts in the mantle below,
          • and then rises and bursts through the continental crust above.

Understanding the Meaning

Let's start reading from the beginning:

• A study published by Rice University geoscientist Ming Tang in 2019
  • offers a new explanation
  • for the origin of Earth's continents...

This is where we have the blank. Let's look at the choices:

• Choice A: continents geological (no punctuation)
• Choice B: continents: geological (colon)
• Choice C: continents; geological (semicolon)
• Choice D: continents. Geological (period, new sentence)

To see what works here, let's read the rest of the sentence and understand what it's saying!

The sentence continues: "geological structures called arcs, towering ridges that form when a dense oceanic plate subducts under a less dense continental plate, melts in the mantle below, and then rises and bursts through the continental crust above."

Now let's understand what this is telling us:

• "Geological structures called arcs"
  • These are specific geological features
  • They're called "arcs"

• "Towering ridges that form when..."
  • This describes what arcs are and how they form
  • The formation process involves plate tectonics - one plate going under another, melting, then rising back up

So the complete picture is:

• The study explains the origin of something specific about Earth's continents
• That "something" is: geological structures called arcs
• These arcs are towering ridges formed by plate tectonics

What do we notice about the structure here?

• Before the blank: "the origin of Earth's continents"
  • This is part of a complete thought (the whole clause starting with "A study... offers a new explanation for...")

• After the blank: "geological structures called arcs, towering ridges..."
  • This is NOT a complete sentence
  • It's a noun phrase ("geological structures") with descriptors
  • No main verb for this phrase

The relationship:

• "Geological structures called arcs" is specifying what aspect of Earth's continents the study is about
• It's like saying: "the origin of Earth's continents - specifically, these geological structures called arcs"

A colon is perfect for this because:

• A colon can follow a complete thought
• A colon introduces a specification or explanation
• The material after the colon doesn't need to be a complete sentence

So we need Choice B: continents: geological - the colon introduces specific information about what aspect of continents the study explains.

GRAMMAR CONCEPT APPLIED

Using Colons to Introduce Specifications

A colon can follow a complete thought and introduce material that explains, specifies, or elaborates on what came before. The material after the colon doesn't need to be a complete sentence - it can be a word, phrase, or list.

Pattern: Complete thought + colon + specification/explanation

Example 1:

• "The research focused on one critical factor: temperature"
  • Before colon: complete thought
  • After colon: a single noun specifying what factor

Example 2:

• "The museum features artifacts from several periods: ancient pottery, medieval weapons, and Renaissance paintings"
  • Before colon: complete thought
  • After colon: a list (not a complete sentence)

Example 3 (our question):

• "A new explanation for the origin of Earth's continents: geological structures called arcs"
  • Before colon: complete thought (as part of the full clause)
  • After colon: noun phrase specifying what aspect of continents

Key distinction:

• A semicolon requires independent clauses on both sides
• A colon requires only a complete thought before it; what follows can be explanatory material that isn't a complete sentence
• In this question, what follows is just a noun phrase with modifiers, so a colon works but a semicolon doesn't