Result

jQuery

    //初期は非表示
    $("#back-top").hide();
    
    $(function () {
        $(window).scroll(function () {
            //100pxスクロールしたら
            if ($(this).scrollTop() > 100) {
                //フェードインで表示
                $('#back-top').fadeIn();
            } else {
                $('#back-top').fadeOut();
            }
        });
        //ここからクリックイベント
        $('#back-top a').click(function () {
            $('body,html').animate({
                scrollTop: 0
            }, 500);
            return false;
        });
    });

css

#back-top {
    position: fixed;
    bottom: 30px;
    margin-left: 150px;
}
#back-top a {
    width: 108px;
    height: 108px;
    display: block;
    text-align: center;
    text-decoration: none;
    color: #666;
    background:#eee;
}
#back-top a:hover {
    color: #000;
}

html

   <p id="back-top">
        <a href="#top">Back to Top</a>
    </p>

via

Animated Scroll to Top

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