CAT SC

[Cobra]

SC#1
Geologists once thought that the molten rock known as lava was an underground remnant of Earth's earliest days, sporadically erupting through volcanoes, but they now know that it is continuously created by the heat of the radioactivity deep inside the planet.

SC#2

Until Antoine Lavoisier proved otherwise in the eighteenth century, many scientists had believed that combustion released phlogiston, an imaginary substance whose properties were not fully understood.

In SC1, the verb tense used is simple past whereas in SC2 the verb tense is past perfect.

past perfect is used in SC2 is b/c of 2 Actions ( the earlier action ' scientist believing' and the later action ' proved ')
I got this logic. But shouldn't the same be applied to SC1

2 Actions Lava had been a part ( earlier action) and Geologists thought (later action)

I get pretty confused w verb tense inspite of MGMAT SC prep. :(
Any help is ofcourse appreciated

[RonPurewal]

subtle indeed.

SC#1
Geologists once thought that the molten rock known as lava was an underground remnant of Earth's earliest days, sporadically erupting through volcanoes, but they now know that it is continuously created by the heat of the radioactivity deep inside the planet.

...


2 Actions Lava had been a part ( earlier action) and Geologists thought (later action)

nope.
the problem is that the statement about the lava isn't an "earlier action"; it's a statement of truth about the nature of the lava, which is concurrent with the scientists' thinking.
translate this into the present tense:
geologists think that the lava is (NOT 'was') a remnant...
because, well, it's still a remnant. 'was' would falsely imply that the lava is no longer a remnant.
translating this sentence back into the past tense renders both of these verbs into the simple past tense, because both of them are currently in the present tense.

now, if the verb were truly to represent an earlier action (rather than a state of being that extends into the present), then you'd be right.
example:
geologists think that the lava was formed by...
therefore
geologists thought that the lava had been formed by...
...just like you thought.
but the problem is that the sentence in question simply doesn't work this way, because the thing you're describing as the 'earlier action' isn't an action at all.