A force that strong had tremendous effects on the area around the volcano. 

【翻译】如此强大的力量对火山周围地区产生了巨大的影响。

that strong = so strong ，that 是程度副词，修饰形容词strong，意思相当于so。

这是形容词短语作后置定语。等于：

A force which was so strong had tremendous effects on the area around the volcano. 

that 表示程度的例句：

Try and cheer up a bit; life isn't that bad!  想办法高兴点儿, 生活并不是那么糟！

No kidding, Ginny, you look good. I wish I looked that good. 不是开玩笑, 琪妮, 你看上去还很神气, 但愿我看上去也能像你那样神气就好了。

赞同刘老师和曹老师的解答，that long为形容词短语作后置定语。

that表示“那么，那样”，是程度副词作状语，修饰形容词或副词，相当于so。另外，this也可以用作程度副词，表示“这么，这样”。例如：

He isn't that clever.他没有那么聪明。

He didn’t look that bad. 他看上去没那么坏。

I didn't think it would be this hard.没想到它会这么难。

I've never been out this late before.我从来没在外面待到这么晚。

A force that strong had tremendous effects on the area around the volcano. 

参考译文：对火山周围地区产生巨大影响的一股力量/威力。

stong

(1)adj.  强的; 坚强的; 强烈的; 强壮的

(2)adv.  强劲地; 猛烈地; 大大地; 坚强地

that strong had tremendous effects on the area around the volcano为关系代词that引导定语从句修饰先行词“a force”,that代替先行词“a force”并在定语从句作主语成分，strong为副词用法，作状语修饰谓语动词“had”

have+形容词+ effect on sth 对…产生影响

The austerity measures will have little immediate adverse effect on the average Moroccan. 

财政紧缩政策不会对普通的摩洛哥人产生直接的负面影响。